diff --git a/README.md b/README.md
index cd9f989..8cb1b19 100644
--- a/README.md
+++ b/README.md
@@ -479,6 +479,20 @@ zarr-python (`test/fixtures/structure`, made by
 `lev`, then `lvl`, `t` (unfold), `ex` (stack), `pick` (x,y in the CRS),
 `tiles`, `pix`, `coast`, `grat`, `cam`.
 
+## Rectilinear chunks
+
+`rectilinear.html` compares a regular Zarr chunk grid with a rectilinear one
+(the `rectilinear` chunk grid of zarr-extensions, after ZEP0003): one list of
+chunk lengths per axis, cut across the whole array. Cases: the overhanging last
+chunk of a regular grid, days in months and years, years of files each chunked
+inside (a short last chunk per file, exact only if the file did not pad it),
+and mosaics of uneven, staggered and overlapping tiles (a grid is a partition,
+so overlap cannot be expressed). A read along time shows the chunks it touches
+and the values decoded against those wanted, for both grids. Paste a zarr.json
+or read one from a URL to draw its grid. `src/core/chunkgrid.js`, checked in
+`test/check-chunkgrid.mjs`. Hash keys: `case`, `grid` (`reg`, `rect`), `c`,
+`a`, `w`, `where` (`map`, `pixel`), `ex`, `cam`.
+
 ## Rasterize
 
 `rasterize.html` puts one polygon on one grid two ways, side by side. On the
@@ -842,6 +856,7 @@ h3.html, js/h3.js                      H3 hexagons on the icosahedron
 grids.html, js/grids-page.js           model grids page
 indexspace.html, js/indexspace.js      index space page
 structure.html, js/structure-page.js   tiles and overviews page
+rectilinear.html, js/rectilinear.js    rectilinear chunks: regular and rectilinear Zarr chunk grids, calendars, files, mosaics
 regular.html, js/regular-page.js       regular in what? storyline: geostationary, lon/lat arrays
 rasterize.html, js/rasterize-page.js   rasterize page
 affine.html, js/affine.js              Affine: the geotransform, world file, registering an image by two points
@@ -889,6 +904,7 @@ src/core/icosa.js                 icosahedron from 12 vertices, gnomonic face fr
 src/core/roms.js                  ROMS s-coordinate, synthetic grids and data, Zarr grid reader
 src/core/regular.js               which CRS makes a lon/lat array regular; example grids; geos scan angles
 src/core/structure.js             grid, tile and file structure of COG, Zarr, WMTS, ArcGIS, XYZ and specs
+src/core/chunkgrid.js             regular and rectilinear chunk grids: lookup, read cost, zarr.json chunk_grid, calendars, mosaics
 src/core/rasterize.js             centre-in scanline fill, exact coverage fractions, runs, tiles and bytes
 src/core/grids.js                 rotated pole, tripolar, cubed sphere and Yin-Yang grids
 src/core/homage-geo.js            Robinson, Winkel tripel, Van der Grinten, cea; Dymaxion icosahedron and net
diff --git a/aatgrid.html b/aatgrid.html
index b90d804..23062d5 100644
--- a/aatgrid.html
+++ b/aatgrid.html
@@ -28,7 +28,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html" aria-current="page">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html" aria-current="page">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/affine.html b/affine.html
index c9c44ce..db9c794 100644
--- a/affine.html
+++ b/affine.html
@@ -25,7 +25,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html" aria-current="page">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html" aria-current="page">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section>
diff --git a/bins.html b/bins.html
index 46faace..26031e1 100644
--- a/bins.html
+++ b/bins.html
@@ -28,7 +28,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html" aria-current="page">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html" aria-current="page">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/catastrophes.html b/catastrophes.html
index 50f6afa..965cce6 100644
--- a/catastrophes.html
+++ b/catastrophes.html
@@ -29,7 +29,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html" aria-current="page">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html" aria-current="page">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/cells.html b/cells.html
index 66209a3..4153287 100644
--- a/cells.html
+++ b/cells.html
@@ -28,7 +28,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html" aria-current="page">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html" aria-current="page">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/cones.html b/cones.html
index 5b01645..70167df 100644
--- a/cones.html
+++ b/cones.html
@@ -29,7 +29,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html" aria-current="page">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html" aria-current="page">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/css/app.css b/css/app.css
index 4623695..a363a09 100644
--- a/css/app.css
+++ b/css/app.css
@@ -288,3 +288,8 @@ table.gcps input { width: 72px; font: inherit; color: var(--text); background: #
 }
 /* ortho.html: the four images take the whole lower half on a phone */
 @media (max-width: 820px) { #map-label { display: none; } }
+/* rectilinear.html: the cuts along an axis, and the chunk_grid */
+canvas.axis { position: absolute; inset: 0; width: 100%; height: 100%; display: block; cursor: col-resize; touch-action: none; }
+.warn { color: var(--orange); }
+#url { display: block; width: 100%; margin-top: 4px; }
+#note3d:empty, #custom-note:empty { display: none; }
diff --git a/ellipsoid.html b/ellipsoid.html
index 938a6f6..9578b9c 100644
--- a/ellipsoid.html
+++ b/ellipsoid.html
@@ -33,7 +33,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html" aria-current="page">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html" aria-current="page">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
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diff --git a/example.html b/example.html
index dcf3b5d..2e11285 100644
--- a/example.html
+++ b/example.html
@@ -29,7 +29,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html" aria-current="page">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html" aria-current="page">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
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diff --git a/geoid.html b/geoid.html
index 32d92e4..c4bb1d3 100644
--- a/geoid.html
+++ b/geoid.html
@@ -34,7 +34,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html" aria-current="page">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html" aria-current="page">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
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diff --git a/globals.html b/globals.html
index 90daa2c..a756887 100644
--- a/globals.html
+++ b/globals.html
@@ -28,7 +28,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html" aria-current="page">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html" aria-current="page">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
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diff --git a/grids.html b/grids.html
index 24c1247..d5b7674 100644
--- a/grids.html
+++ b/grids.html
@@ -28,7 +28,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html" aria-current="page">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html" aria-current="page">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
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diff --git a/h3.html b/h3.html
index cd87611..8caff3d 100644
--- a/h3.html
+++ b/h3.html
@@ -29,7 +29,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html" aria-current="page">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html" aria-current="page">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
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diff --git a/homage.html b/homage.html
index 4a8ade4..05d54f3 100644
--- a/homage.html
+++ b/homage.html
@@ -29,7 +29,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html" aria-current="page">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html" aria-current="page">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
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     <section class="embed-keep">
diff --git a/hull.html b/hull.html
index 4b817c4..ab46885 100644
--- a/hull.html
+++ b/hull.html
@@ -33,7 +33,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html" aria-current="page">Convex hull</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html" aria-current="page">Convex hull</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section>
diff --git a/index.html b/index.html
index fc715a9..85f8553 100644
--- a/index.html
+++ b/index.html
@@ -29,7 +29,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./" aria-current="page">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./" aria-current="page">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/indexspace.html b/indexspace.html
index 3a0afcb..e44dfa8 100644
--- a/indexspace.html
+++ b/indexspace.html
@@ -27,7 +27,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html" aria-current="page">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html" aria-current="page">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/js/rectilinear.js b/js/rectilinear.js
new file mode 100644
index 0000000..6ce189c
--- /dev/null
+++ b/js/rectilinear.js
@@ -0,0 +1,665 @@
+// playproj: rectilinear chunks
+//
+// An array cut into chunks (left, in 3D) and the cuts along its first axis
+// or across a mosaic (right, in 2D), for a regular chunk grid and a
+// rectilinear one: the last chunk of a regular grid, days in months, days
+// in years of files, and mosaics of uneven, staggered or overlapping
+// tiles. A selection shows which chunks a read touches. The grids are
+// src/core/chunkgrid.js.
+
+import * as THREE from "three";
+import * as G from "../src/core/chunkgrid.js";
+import { makeStage, readParams, hashWriter, shareLinks, oneOf } from "../src/scene/index.js";
+
+const $ = (id) => document.getElementById(id);
+
+const C = {
+  bg: 0x05070d, a: 0x2b5487, b: 0x3d6ea8, hit: 0xffd34d, want: 0xffb347, pad: 0x8a97ab, file: 0xff7a59, lap: 0xe0566b, hole: 0x1a2236,
+  aCss: "#2b5487", bCss: "#3d6ea8", hitCss: "#ffd34d", wantCss: "rgba(255,179,71,0.9)", padCss: "rgba(138,151,171,0.35)",
+  fileCss: "#ff7a59", lapCss: "#e0566b", ink: "#dfe7f2", muted: "#8a97ab", line: "#1f2a40",
+  tiles: ["#4f8fd6", "#6ab787", "#c98bd8", "#e0a458", "#5fc4c0", "#d6736b", "#9aa7ff", "#c2c96a"]
+};
+
+// ---------------------------------------------------------------- cases
+
+// Time cases: an array of days x rows x columns, its time axis cut by a
+// regular chunk size or by the calendar (or by the files it came from).
+const SPACE = [60, 120], SPACE_CHUNK = [30, 60];
+function timeCase({ y0, y1, by, label, regular }) {
+  const cal = by === "month" ? G.months(y0, y1) : G.years(y0, y1);
+  const n = cal.sizes.reduce((a, b) => a + b, 0);
+  const filesLabel = by === "month" ? "Files, one a month" : "Files, one a year";
+  return { kind: "time", label, y0, files: cal, filesLabel, n, rect: () => cal.sizes, regular, unit: "day" };
+}
+const CASES = {
+  edge: { kind: "time", label: "The last chunk", n: 100, files: null, rect: (c) => G.regularAxis(100, c, { trim: true }), regular: 30, unit: "step" },
+  months: timeCase({ y0: 2023, y1: 2024, by: "month", label: "Days in months", regular: 30 }),
+  years: timeCase({ y0: 2019, y1: 2024, by: "year", label: "Years of daily files", regular: 365 }),
+  // each file chunked inside (netCDF-4, say) every c days: the last chunk of
+  // each file is short, and a reference to it can only be exact if the file
+  // did not pad it out to c on disk
+  native: (() => {
+    const k = timeCase({ y0: 2022, y1: 2024, by: "year", label: "Files with their own chunks", regular: 100 });
+    k.filesLabel = "Files, one a year, each chunked inside";
+    k.rect = (c) => k.files.sizes.flatMap((n) => G.regularAxis(n, c, { trim: true }));
+    k.native = true;
+    return k;
+  })(),
+  uneven: {
+    kind: "mosaic", label: "Uneven tiles",
+    rects: grid2([0, 100, 160, 240], [0, 50, 80])
+  },
+  brick: {
+    kind: "mosaic", label: "Staggered tiles",
+    rects: [[0, 100, 0, 40], [100, 200, 0, 40], [0, 50, 40, 80], [50, 150, 40, 80], [150, 200, 40, 80], [0, 100, 80, 120], [100, 200, 80, 120]].map(rc)
+  },
+  overlap: {
+    kind: "mosaic", label: "Overlapping scenes",
+    rects: [[0, 110, 0, 66], [96, 206, 0, 66], [0, 110, 58, 124], [96, 206, 58, 124]].map(rc)
+  },
+  custom: null
+};
+function rc([x0, x1, y0, y1]) { return { x0, x1, y0, y1 }; }
+function grid2(xs, ys) {
+  const out = [];
+  for (let j = 0; j + 1 < ys.length; j++) for (let i = 0; i + 1 < xs.length; i++) out.push(rc([xs[i], xs[i + 1], ys[j], ys[j + 1]]));
+  return out;
+}
+const CASE_KEYS = Object.keys(CASES);
+
+// ---------------------------------------------------------------- state
+
+const defaults = { case: "months", grid: "rect", c: 30, a: 396, w: 29, where: "map", ex: 0.35, cam: null };
+const state = { ...defaults };
+function readHash() {
+  readParams(state, defaults, {
+    case: oneOf(CASE_KEYS, "months"), grid: oneOf(["reg", "rect"], "rect"), where: oneOf(["map", "pixel"], "map")
+  });
+  if (state.case === "custom" && !CASES.custom) state.case = "months";
+  state.ex = Math.max(0, Math.min(1, state.ex));
+  clampSel();
+}
+const cur = () => CASES[state.case];
+function clampSel() {
+  const k = CASES[state.case];
+  if (!k || k.kind !== "time") return;
+  state.c = Math.round(Math.max(1, Math.min(k.n, state.c)));
+  state.w = Math.round(Math.max(1, Math.min(k.n, state.w)));
+  state.a = Math.round(Math.max(0, Math.min(k.n - state.w, state.a)));
+}
+readHash();
+const writeHash = hashWriter(state, defaults, { digits: 3 });
+
+// The two grids of a time case: regular c along time, the case's own
+// sizes for rectilinear, and the same spatial chunks for both.
+function timeGrids(k) {
+  const sp = k.space || SPACE, spc = k.spaceChunk || SPACE_CHUNK;
+  const shape = [k.n, ...sp];
+  const spaceAxes = k.spaceAxes || sp.map((n, d) => G.regularAxis(n, spc[d]));
+  return {
+    reg: G.grid(shape, [G.regularAxis(k.n, state.c), ...spaceAxes]),
+    rect: G.grid(shape, [k.rect(state.c), ...spaceAxes])
+  };
+}
+let grids = null, report = null;
+function solve() {
+  const k = cur();
+  if (k.kind === "time") { grids = timeGrids(k); report = null; }
+  else { grids = null; report = G.mosaicReport(k.rects); }
+}
+
+// ---------------------------------------------------------------- 3D
+
+const stage = $("three");
+const { renderer, scene, camera, controls, setCamera, resize } = makeStage(stage, {
+  bg: C.bg, fov: 34, far: 200,
+  onCamera: (c) => { state.cam = c; writeHash(); }
+});
+controls.minDistance = 2;
+controls.maxDistance = 60;
+scene.add(new THREE.AmbientLight(0xffffff, 0.6));
+const sun = new THREE.DirectionalLight(0xffffff, 1.5);
+sun.position.set(1, 2, 1.5);
+camera.add(sun);
+scene.add(camera);
+
+const group = new THREE.Group();
+scene.add(group);
+const BOX = new THREE.BoxGeometry(1, 1, 1);
+const EDGES = new THREE.EdgesGeometry(BOX);
+const mats = new Map();
+function mat(color, opacity = 1) {
+  const key = `${color}/${opacity}`;
+  if (!mats.has(key)) mats.set(key, new THREE.MeshLambertMaterial({ color, transparent: opacity < 1, opacity, depthWrite: opacity >= 1 }));
+  return mats.get(key);
+}
+const lineMats = new Map();
+function lineMat(color, opacity = 1) {
+  const key = `${color}/${opacity}`;
+  if (!lineMats.has(key)) lineMats.set(key, new THREE.LineBasicMaterial({ color, transparent: opacity < 1, opacity }));
+  return lineMats.get(key);
+}
+function box(x0, x1, y0, y1, z0, z1, m, edge) {
+  const mesh = new THREE.Mesh(BOX, m);
+  mesh.scale.set(Math.max(1e-4, x1 - x0), Math.max(1e-4, y1 - y0), Math.max(1e-4, z1 - z0));
+  mesh.position.set((x0 + x1) / 2, (y0 + y1) / 2, (z0 + z1) / 2);
+  group.add(mesh);
+  if (edge) {
+    const l = new THREE.LineSegments(EDGES, edge);
+    l.scale.copy(mesh.scale); l.position.copy(mesh.position);
+    group.add(l);
+  }
+  return mesh;
+}
+function clearGroup() {
+  for (const o of [...group.children]) group.remove(o);
+}
+
+// world size of the drawing: time along x, rows up (y), columns along z
+let world = { X: 9, Y: 2.2, Z: 4.4 };
+const MAX_BOXES = 6000;
+
+function buildTime() {
+  const k = cur(), g = state.grid === "reg" ? grids.reg : grids.rect;
+  const L = [world.X, world.Y, world.Z];
+  const gap = state.ex * 0.25;
+  const n = g.shape, offs = g.off;
+  const sc = L.map((l, d) => l / n[d]);
+  // where index i of axis d sits: its own place plus a gap for each chunk before it
+  const at = (d, kk, i) => i * sc[d] + kk * gap;
+  const span = (d) => at(d, g.axes[d].length - 1, offs[d][offs[d].length - 1]);
+  const ox = -span(0) / 2, oy = 0, oz = -span(2) / 2;
+  const o = [ox, oy, oz];
+  const sel = selection(g);
+  const cost = G.readCost(g, sel);
+  const count = G.chunkCount(g);
+
+  if (count > MAX_BOXES) {
+    $("note3d").textContent = `${count} chunks: too many to draw one by one, so only the outline is shown.`;
+    box(ox, ox + span(0), oy, oy + span(1), oz, oz + span(2), mat(C.a, 0.4), lineMat(0x8a97ab));
+    return;
+  }
+  $("note3d").textContent = "";
+  // a chunk's data along an axis, and the overhang past the end of the array
+  const seg = (d, kk) => {
+    const p0 = o[d] + at(d, kk, offs[d][kk]);
+    return [p0, p0 + G.filled(offs[d], kk, n[d]) * sc[d], p0 + g.axes[d][kk] * sc[d]];
+  };
+  const inSpan = (d, kk) => cost.spans && kk >= cost.spans[d][0] && kk <= cost.spans[d][1];
+  for (let i = 0; i < g.axes[0].length; i++) for (let j = 0; j < g.axes[1].length; j++) for (let l = 0; l < g.axes[2].length; l++) {
+    const [x0, x1, xe] = seg(0, i), [y0, y1, ye] = seg(1, j), [z0, z1, ze] = seg(2, l);
+    const hit = inSpan(0, i) && inSpan(1, j) && inSpan(2, l);
+    const base = (i + j + l) % 2 ? C.b : C.a;
+    if (x1 > x0 && y1 > y0 && z1 > z0) box(x0, x1, y0, y1, z0, z1, hit ? mat(C.hit, 0.55) : mat(base, 0.92), lineMat(hit ? C.hit : 0x05070d, hit ? 1 : 0.6));
+    // the overhang: stored, as fill, but not part of the array
+    if (xe > x1) box(x1, xe, y0, ye, z0, ze, mat(C.pad, 0.12), lineMat(C.pad, 0.5));
+    else if (ye > y1) box(x0, xe, y1, ye, z0, ze, mat(C.pad, 0.12), lineMat(C.pad, 0.5));
+    else if (ze > z1) box(x0, xe, y0, ye, z1, ze, mat(C.pad, 0.12), lineMat(C.pad, 0.5));
+  }
+  // the part of the touched chunks the read wants
+  if (cost.spans) {
+    const part = (d, kk) => {
+      const [va, vb] = sel[d];
+      const a = Math.max(va, offs[d][kk]), b = Math.min(vb, offs[d][kk + 1], n[d]);
+      return [o[d] + at(d, kk, a), o[d] + at(d, kk, b)];
+    };
+    const e = 0.004;
+    for (let i = cost.spans[0][0]; i <= cost.spans[0][1]; i++) for (let j = cost.spans[1][0]; j <= cost.spans[1][1]; j++) for (let l = cost.spans[2][0]; l <= cost.spans[2][1]; l++) {
+      const [x0, x1] = part(0, i), [y0, y1] = part(1, j), [z0, z1] = part(2, l);
+      box(x0 - e, x1 + e, y0 - e, y1 + e, z0 - e, z1 + e, mat(C.want, 1), null);
+    }
+  }
+  // file boundaries, where the case has files: in the gap when a chunk starts there
+  if (k.files) {
+    const pts = [];
+    for (const f of G.offsets(k.files.sizes)) {
+      const kk = f >= n[0] ? g.axes[0].length - 1 : G.locate(offs[0], f);
+      let x = ox + at(0, kk, f);
+      if (f > 0 && f < n[0] && offs[0][kk] === f) x -= gap / 2;
+      const y0 = oy - 0.02, y1 = oy + span(1) + 0.02, z0 = oz - 0.02, z1 = oz + span(2) + 0.02;
+      pts.push(x, y0, z0, x, y1, z0, x, y1, z0, x, y1, z1, x, y1, z1, x, y0, z1, x, y0, z1, x, y0, z0);
+    }
+    const geo = new THREE.BufferGeometry();
+    geo.setAttribute("position", new THREE.Float32BufferAttribute(pts, 3));
+    group.add(new THREE.LineSegments(geo, lineMat(C.file, 0.9)));
+  }
+}
+
+function buildMosaic() {
+  const k = cur(), m = report.grid;
+  const W = m.xs[m.xs.length - 1] - m.xs[0], H = m.ys[m.ys.length - 1] - m.ys[0];
+  const s = 8 / Math.max(W, H);
+  const gap = state.ex * 0.25;
+  const ncol = m.cols.length, nrow = m.rows.length;
+  const ox = -(W * s + (ncol - 1) * gap) / 2, oz = -(H * s + (nrow - 1) * gap) / 2;
+  const xAt = (x) => { const i = Math.max(0, m.xs.indexOf(x)); return ox + (x - m.xs[0]) * s + Math.min(i, ncol) * gap - (i > 0 && i <= ncol - 1 ? gap / 2 : 0) - (i === ncol ? gap : 0); };
+  const zAt = (y) => { const j = Math.max(0, m.ys.indexOf(y)); return oz + (y - m.ys[0]) * s + Math.min(j, nrow) * gap - (j > 0 && j <= nrow - 1 ? gap / 2 : 0) - (j === nrow ? gap : 0); };
+  const tileColor = (t) => new THREE.Color(C.tiles[t % C.tiles.length]).getHex();
+  if (state.grid === "rect") {
+    // the rectilinear grid of every edge: one box per chunk, coloured by the tile that fills it
+    for (let j = 0; j < nrow; j++) for (let i = 0; i < ncol; i++) {
+      const own = report.owners[j * ncol + i];
+      const x0 = ox + (m.xs[i] - m.xs[0]) * s + i * gap, x1 = x0 + m.cols[i] * s;
+      const z0 = oz + (m.ys[j] - m.ys[0]) * s + j * gap, z1 = z0 + m.rows[j] * s;
+      const h = own.length > 1 ? 0.5 : own.length ? 0.25 : 0.04;
+      const col = own.length > 1 ? C.lap : own.length ? tileColor(own[0]) : C.hole;
+      box(x0, x1, 0, h, z0, z1, mat(col, own.length ? 0.95 : 0.5), lineMat(0x05070d, 0.7));
+    }
+  } else {
+    // the files: each tile a slab, later ones stacked on earlier where they overlap
+    k.rects.forEach((r, t) => {
+      const y0 = t * 0.08 * (report.overlapCells ? 1 : 0), y1 = y0 + 0.25;
+      box(xAt(r.x0), xAt(r.x1), y0, y1, zAt(r.y0), zAt(r.y1), mat(tileColor(t), 0.85), lineMat(0xdfe7f2, 0.8));
+    });
+  }
+}
+
+// a half-open selection on each axis of a time grid
+function selection(g) {
+  const sel = [[state.a, state.a + state.w]];
+  for (let d = 1; d < g.shape.length; d++) {
+    if (state.where === "map") sel.push([0, g.shape[d]]);
+    else { const p = Math.floor(g.shape[d] * 0.62); sel.push([p, p + 1]); }
+  }
+  return sel;
+}
+
+function build() {
+  clearGroup();
+  if (cur().kind === "time") buildTime(); else { $("note3d").textContent = ""; buildMosaic(); }
+}
+
+function defaultCam() {
+  // far enough back to see the whole drawing, from above and to one side
+  const b = new THREE.Box3().setFromObject(group), c = b.getCenter(new THREE.Vector3());
+  const R = 0.5 * b.getSize(new THREE.Vector3()).length() || 5;
+  const vf = (camera.fov * Math.PI) / 180, hf = 2 * Math.atan(Math.tan(vf / 2) * camera.aspect);
+  const d = R / Math.sin(Math.min(vf, hf) / 2) * 1.12;
+  const v = cur().kind === "time" ? new THREE.Vector3(0.35, 0.55, 0.76) : new THREE.Vector3(0.12, 0.85, 0.52);
+  v.normalize().multiplyScalar(d).add(c);
+  return [v.x, v.y, v.z, c.x, c.y, c.z];
+}
+
+// Chunks of the rectilinear axis shorter than the files' own chunk, not at
+// the end: a file's last chunk, which HDF5 and tiled TIFF store padded.
+function shortInterior(g) {
+  const a = g.axes[0], out = new Set();
+  if (!cur().native) return out;
+  for (let i = 0; i + 1 < a.length; i++) if (a[i] < state.c) out.add(i);
+  return out;
+}
+
+// ---------------------------------------------------------------- 2D
+
+const cv = $("axis");
+let frame = null;
+function sizeCanvas() {
+  const dpr = Math.min(window.devicePixelRatio || 1, 2), r = cv.getBoundingClientRect();
+  const w = Math.max(10, r.width), h = Math.max(10, r.height);
+  if (cv.width !== Math.round(w * dpr) || cv.height !== Math.round(h * dpr)) { cv.width = Math.round(w * dpr); cv.height = Math.round(h * dpr); }
+  return { w, h, dpr };
+}
+
+function draw2d() {
+  const f = sizeCanvas(), g = cv.getContext("2d");
+  g.setTransform(f.dpr, 0, 0, f.dpr, 0, 0);
+  g.fillStyle = "#05070d"; g.fillRect(0, 0, f.w, f.h);
+  g.font = "12px system-ui, sans-serif"; g.textBaseline = "middle";
+  if (cur().kind === "time") drawAxes(g, f); else drawMosaic(g, f);
+}
+
+// three bands along time: the files or calendar, the regular cuts and the rectilinear ones
+function drawAxes(g, f) {
+  const k = cur(), n = k.n;
+  const ends = [grids.reg, grids.rect].map((gg) => gg.off[0][gg.off[0].length - 1]);
+  const left = 16, right = f.w - 16;
+  const sx = (right - left) / Math.max(n, ...ends);
+  const X = (i) => left + i * sx;
+  const bandH = Math.min(46, (f.h - 120) / 3.6);
+  const top = Math.max(40, f.h / 2 - bandH * 2.2);
+  const rows = [
+    { y: top, label: k.filesLabel || `The array, ${n} ${k.unit}s` },
+    { y: top + bandH * 1.5, label: `Regular, every ${state.c} ${k.unit}s`, grid: grids.reg, reg: true },
+    { y: top + bandH * 3, label: k === CASES.edge ? "Rectilinear, the last chunk cut to what is left" : k.native ? `Rectilinear, each file's own ${state.c}-day chunks` : k.files ? "Rectilinear, one chunk per file" : "Rectilinear, as given", grid: grids.rect }
+  ];
+  frame = { X, sx, left, n, rows, bandH, kind: "time" };
+  // selection
+  const sa = X(state.a), sb = X(state.a + state.w);
+  g.fillStyle = "rgba(255,211,77,0.08)"; g.fillRect(sa, top - 22, sb - sa, bandH * 4.3);
+  g.strokeStyle = "rgba(255,211,77,0.6)"; g.lineWidth = 1;
+  g.beginPath(); g.moveTo(sa + 0.5, top - 22); g.lineTo(sa + 0.5, top + bandH * 4.1); g.moveTo(sb - 0.5, top - 22); g.lineTo(sb - 0.5, top + bandH * 4.1); g.stroke();
+  g.fillStyle = C.hitCss; g.textAlign = "center";
+  g.fillText(`read ${selLabel()}`, Math.min(right - 70, Math.max(left + 70, (sa + sb) / 2)), top - 30);
+
+  for (const r of rows) {
+    g.fillStyle = C.muted; g.textAlign = "left";
+    g.fillText(r.label, left, r.y - 9);
+    if (!r.grid) {
+      const sizes = k.files ? k.files.sizes : [n], labels = k.files ? k.files.labels : [""];
+      const off = G.offsets(sizes);
+      sizes.forEach((s, i) => {
+        g.fillStyle = i % 2 ? "#1d2a44" : "#16203a";
+        g.fillRect(X(off[i]), r.y, s * sx, bandH);
+        g.strokeStyle = C.fileCss; g.lineWidth = 1;
+        g.beginPath(); g.moveTo(Math.round(X(off[i])) + 0.5, r.y); g.lineTo(Math.round(X(off[i])) + 0.5, r.y + bandH); g.stroke();
+        const lab = sizes.length > 12 ? `${labels[i].slice(0, 1)}` : labels[i];
+        const t = sizes.length > 12 ? String(s) : `${lab} ${s}`;
+        if (s * sx > g.measureText(t).width + 4) { g.fillStyle = C.ink; g.textAlign = "center"; g.fillText(t, X(off[i] + s / 2), r.y + bandH / 2); }
+        else if (s * sx > 12) { g.fillStyle = C.ink; g.textAlign = "center"; g.fillText(String(s), X(off[i] + s / 2), r.y + bandH / 2); }
+      });
+      g.strokeStyle = C.fileCss; g.beginPath(); g.moveTo(Math.round(X(n)) - 0.5, r.y); g.lineTo(Math.round(X(n)) - 0.5, r.y + bandH); g.stroke();
+      continue;
+    }
+    const a = r.grid.axes[0], off = r.grid.off[0];
+    const sp = G.span(off, state.a, state.a + state.w, n);
+    const fileOff = k.files ? new Set(G.offsets(k.files.sizes)) : null;
+    const short = r.reg ? new Set() : shortInterior(r.grid);
+    a.forEach((edge, i) => {
+      const s = G.filled(off, i, n);
+      const hit = sp && i >= sp[0] && i <= sp[1];
+      const x0 = X(off[i]), w = s * sx;
+      // past the end of the array: the overhang, stored as fill
+      if (edge > s) {
+        const xp = X(off[i] + s), wp = (edge - s) * sx;
+        g.fillStyle = C.padCss; g.fillRect(xp, r.y, wp, bandH);
+        g.strokeStyle = C.muted; g.setLineDash([3, 3]); g.strokeRect(xp + 0.5, r.y + 0.5, wp - 1, bandH - 1); g.setLineDash([]);
+        if (wp > 40) { g.fillStyle = C.ink; g.textAlign = "center"; g.fillText("fill", xp + wp / 2, r.y + bandH / 2); }
+      }
+      if (!s) return;
+      g.fillStyle = hit ? "rgba(255,211,77,0.55)" : i % 2 ? C.bCss : C.aCss;
+      g.fillRect(x0 + (w > 3 ? 0.5 : 0), r.y, Math.max(0.5, w - (w > 3 ? 1 : 0)), bandH);
+      // a chunk that straddles a file boundary would need bytes from two files
+      if (fileOff) {
+        let straddle = false;
+        for (const fo of fileOff) if (fo > off[i] && fo < off[i + 1] && fo < n) { straddle = true; break; }
+        if (straddle) { g.fillStyle = C.lapCss; g.fillRect(x0, r.y + bandH - 4, w, 4); }
+      }
+      // a file's short last chunk, padded to the full chunk in HDF5 or a tiled TIFF
+      if (short.has(i)) {
+        g.fillStyle = "rgba(255,122,89,0.35)"; g.fillRect(x0, r.y, Math.max(1, w), bandH);
+        g.fillStyle = C.fileCss; g.fillRect(x0, r.y, Math.max(1, w), 4);
+      }
+      const t = String(s);
+      if (w > g.measureText(t).width + 2) { g.fillStyle = hit ? "#05070d" : C.ink; g.textAlign = "center"; g.fillText(t, x0 + w / 2, r.y + bandH / 2); }
+    });
+  }
+  // the end of the array
+  g.strokeStyle = C.ink; g.setLineDash([2, 4]);
+  g.beginPath(); g.moveTo(Math.round(X(n)) + 0.5, top - 22); g.lineTo(Math.round(X(n)) + 0.5, top + bandH * 4.1); g.stroke(); g.setLineDash([]);
+  g.fillStyle = C.muted; g.textAlign = "right";
+  g.fillText(`shape ${n}`, Math.min(right, X(n) - 4), top + bandH * 4.1 + 10);
+  // pointer
+  if (pick != null) {
+    g.strokeStyle = "rgba(223,231,242,0.8)";
+    g.beginPath(); g.moveTo(X(pick + 0.5), top - 14); g.lineTo(X(pick + 0.5), top + bandH * 4.1); g.stroke();
+  }
+}
+
+function drawMosaic(g, f) {
+  const k = cur(), m = report.grid;
+  const W = m.xs[m.xs.length - 1] - m.xs[0], H = m.ys[m.ys.length - 1] - m.ys[0];
+  const pad = 36, s = Math.min((f.w - 2 * pad) / W, (f.h - 2 * pad - 30) / H);
+  const ox = (f.w - W * s) / 2, oy = (f.h - H * s) / 2 + 10;
+  const X = (x) => ox + (x - m.xs[0]) * s, Y = (y) => oy + (y - m.ys[0]) * s;
+  frame = { kind: "mosaic", X, Y, s, ox, oy };
+  const ncol = m.cols.length;
+  // the cells of the grid of all edges
+  for (let j = 0; j < m.rows.length; j++) for (let i = 0; i < ncol; i++) {
+    const own = report.owners[j * ncol + i];
+    if (own.length > 1) { g.fillStyle = "rgba(224,86,107,0.45)"; g.fillRect(X(m.xs[i]), Y(m.ys[j]), m.cols[i] * s, m.rows[j] * s); }
+    else if (!own.length) { g.fillStyle = "#101727"; g.fillRect(X(m.xs[i]), Y(m.ys[j]), m.cols[i] * s, m.rows[j] * s); }
+  }
+  // tiles
+  k.rects.forEach((r, t) => {
+    const col = C.tiles[t % C.tiles.length];
+    g.fillStyle = col + "33"; g.fillRect(X(r.x0), Y(r.y0), (r.x1 - r.x0) * s, (r.y1 - r.y0) * s);
+    g.strokeStyle = col; g.lineWidth = 2; g.strokeRect(X(r.x0) + 1, Y(r.y0) + 1, (r.x1 - r.x0) * s - 2, (r.y1 - r.y0) * s - 2);
+    const p = report.per[t];
+    g.fillStyle = C.ink; g.textAlign = "center";
+    g.fillText(`${r.x1 - r.x0} x ${r.y1 - r.y0}`, X((r.x0 + r.x1) / 2), Y((r.y0 + r.y1) / 2) - 7);
+    g.fillStyle = p.cells > 1 ? C.lapCss : C.muted;
+    g.fillText(p.cells > 1 ? `${p.cells} chunks` : "1 chunk", X((r.x0 + r.x1) / 2), Y((r.y0 + r.y1) / 2) + 9);
+  });
+  // the cuts, every one running the whole way across
+  if (state.grid === "rect") {
+    g.strokeStyle = "rgba(255,211,77,0.85)"; g.lineWidth = 1; g.setLineDash([5, 4]);
+    g.beginPath();
+    for (const x of m.xs) { g.moveTo(Math.round(X(x)) + 0.5, Y(m.ys[0]) - 8); g.lineTo(Math.round(X(x)) + 0.5, Y(m.ys[m.ys.length - 1]) + 8); }
+    for (const y of m.ys) { g.moveTo(X(m.xs[0]) - 8, Math.round(Y(y)) + 0.5); g.lineTo(X(m.xs[m.xs.length - 1]) + 8, Math.round(Y(y)) + 0.5); }
+    g.stroke(); g.setLineDash([]);
+    g.fillStyle = C.hitCss;
+    m.cols.forEach((c, i) => { if (c * s > 18) { g.textAlign = "center"; g.fillText(String(c), X(m.xs[i] + c / 2), Y(m.ys[0]) - 14); } });
+    m.rows.forEach((r, j) => { if (r * s > 12) { g.textAlign = "right"; g.fillText(String(r), X(m.xs[0]) - 10, Y(m.ys[j] + r / 2)); } });
+  }
+}
+
+// ---------------------------------------------------------------- readout
+
+function selLabel() {
+  const k = cur();
+  if (k.files && k.y0) {
+    const d0 = dateOf(state.a), d1 = dateOf(state.a + state.w - 1);
+    return state.w === 1 ? d0 : `${d0} to ${d1} (${state.w} days)`;
+  }
+  return `${state.a} to ${state.a + state.w - 1} (${state.w})`;
+}
+function dateOf(i) {
+  const k = cur();
+  let y = k.y0;
+  while (i >= (G.isLeap(y) ? 366 : 365)) { i -= G.isLeap(y) ? 366 : 365; y++; }
+  let m = 0;
+  while (i >= G.daysInMonth(y, m)) { i -= G.daysInMonth(y, m); m++; }
+  return `${i + 1} ${G.MONTH_NAMES[m]} ${y}`;
+}
+const fmt = (v) => v.toLocaleString("en-US");
+
+// JSON with objects indented and lists of numbers on one line
+function compactJson(v, ind = "") {
+  const flat = (x) => !(x && typeof x === "object") || (Array.isArray(x) && x.every(flat));
+  if (flat(v)) return JSON.stringify(v).replace(/,/g, ", ");
+  const pad = ind + "  ";
+  if (Array.isArray(v)) return "[\n" + v.map((x) => pad + compactJson(x, pad)).join(",\n") + "\n" + ind + "]";
+  return "{\n" + Object.entries(v).map(([k, x]) => `${pad}${JSON.stringify(k)}: ${compactJson(x, pad)}`).join(",\n") + "\n" + ind + "}";
+}
+
+function readout() {
+  const k = cur(), rows = [];
+  const pre = $("json");
+  if (k.kind === "time") {
+    rows.push(["Shape", grids.reg.shape.join(" x ")]);
+    for (const [name, g] of [["Regular", grids.reg], ["Rectilinear", grids.rect]]) {
+      const c = G.readCost(g, selection(g));
+      const amp = c.want ? c.read / c.want : 0;
+      rows.push([`<b>${name}</b>`, `${fmt(G.chunkCount(g))} chunks`]);
+      rows.push(["Chunks read", fmt(c.chunks)]);
+      rows.push(["Values decoded / wanted", `${fmt(c.read)} / ${fmt(c.want)} (${amp.toFixed(2)}x)`]);
+      if (k.files) {
+        const fo = G.offsets(k.files.sizes);
+        const off = g.off[0];
+        let straddle = 0;
+        for (let i = 0; i + 1 < off.length; i++) if (fo.some((f) => f > off[i] && f < off[i + 1] && f < g.shape[0])) straddle++;
+        rows.push(["Chunks across a file edge", straddle ? `<span class="warn">${straddle}</span>` : "none"]);
+      }
+      if (name === "Rectilinear" && k.native) {
+        const sh = shortInterior(g).size;
+        rows.push(["Short chunks inside the array", sh ? `<span class="warn">${sh}</span>, exact only if the files did not pad them` : "none"]);
+      }
+      const last = g.axes[0].length - 1, fill = G.filled(g.off[0], last, g.shape[0]);
+      if (fill < g.axes[0][last]) rows.push(["Last chunk", `${fill} of ${g.axes[0][last]} filled`]);
+    }
+    const g = state.grid === "reg" ? grids.reg : grids.rect;
+    pre.textContent = compactJson({ shape: g.shape, chunk_grid: G.chunkGridJson(g) });
+  } else {
+    const m = report.grid;
+    const shape = [m.ys[m.ys.length - 1] - m.ys[0], m.xs[m.xs.length - 1] - m.xs[0]];
+    const max = Math.max(...report.per.map((p) => p.cells));
+    rows.push(["Tiles", k.rects.length], ["Shape (rows x columns)", shape.join(" x ")],
+      ["Grid of every edge", `${m.rows.length} x ${m.cols.length} = ${m.rows.length * m.cols.length} chunks`],
+      ["Tiles that are one chunk", `${report.per.filter((p) => p.cells === 1).length} of ${k.rects.length}`],
+      ["Most chunks in one tile", max],
+      ["Chunks two tiles claim", report.overlapCells ? `<span class="warn">${report.overlapCells}</span>` : "none"],
+      ["Chunks no tile fills", report.holes || "none"]);
+    const g = G.grid(shape, [m.rows, m.cols]);
+    pre.textContent = compactJson({ shape, chunk_grid: G.chunkGridJson(g, { force: "rectilinear" }) });
+  }
+  $("out").innerHTML = rows.map(([a, b]) => `<tr><td>${a}</td><td>${b}</td></tr>`).join("");
+  $("pick-info").innerHTML = pickText();
+}
+
+// where the pointer is on the time axis: one division for the regular grid, a search for the rectilinear one
+let pick = null;
+function pickText() {
+  const k = cur();
+  if (k.kind !== "time" || pick == null) return k.kind === "time" ? "Point at the bands for the chunk that holds a step." : "";
+  const i = pick;
+  const kr = G.locateRegular(state.c, i), kc = G.locate(grids.rect.off[0], i);
+  const when = k.y0 ? dateOf(i) : `step ${i}`;
+  return `${when} (index ${i}): regular chunk floor(${i} / ${state.c}) = ${kr}, key <code>${G.chunkKey([kr, 0, 0])}</code>; ` +
+    `rectilinear chunk ${kc} by search of ${grids.rect.off[0].length} offsets, key <code>${G.chunkKey([kc, 0, 0])}</code>, which starts at ${grids.rect.off[0][kc]}.`;
+}
+
+// ---------------------------------------------------------------- controls
+
+const PRESETS = [
+  ["The last chunk", { case: "edge", grid: "reg", c: 30, a: 90, w: 10, where: "map" }],
+  ["Days in months", { case: "months", grid: "rect", c: 30, a: G.dayOf(2023, 2024, 1), w: 29, where: "map" }],
+  ["Years of daily files", { case: "years", grid: "reg", c: 365, a: G.dayOf(2019, 2020), w: 366, where: "map" }],
+  ["Files with their own chunks", { case: "native", grid: "rect", c: 100, a: G.dayOf(2022, 2022, 11, 1), w: 62, where: "map" }],
+  ["A time series", { case: "months", grid: "rect", c: 30, a: 0, w: 731, where: "pixel" }],
+  ["Uneven tiles", { case: "uneven", grid: "rect" }],
+  ["Staggered tiles", { case: "brick", grid: "rect" }],
+  ["Overlapping scenes", { case: "overlap", grid: "reg" }]
+];
+for (const [name, p] of PRESETS) {
+  const bt = document.createElement("button");
+  bt.type = "button"; bt.textContent = name;
+  bt.addEventListener("click", () => {
+    Object.assign(state, { ex: defaults.ex }, p);
+    if (CASES[state.case].kind === "time" && p.c == null) state.c = CASES[state.case].regular;
+    clampSel(); pick = null;
+    reset();
+  });
+  $("presets").appendChild(bt);
+}
+
+$("grid-reg").addEventListener("click", () => { state.grid = "reg"; update(); });
+$("grid-rect").addEventListener("click", () => { state.grid = "rect"; update(); });
+$("c").addEventListener("input", () => { state.c = +$("c").value; clampSel(); solve(); update(); });
+$("a").addEventListener("input", () => { state.a = +$("a").value; clampSel(); update(); });
+$("w").addEventListener("input", () => { state.w = +$("w").value; clampSel(); update(); });
+$("where").addEventListener("change", () => { state.where = $("where").value; update(); });
+$("ex").addEventListener("input", () => { state.ex = +$("ex").value; build(); sync(); writeHash(); });
+$("reset-view").addEventListener("click", () => { state.cam = null; setCamera(defaultCam()); writeHash(); });
+
+// move the selection along the bands: press to start it, drag to size it
+let dragFrom = null;
+cv.addEventListener("pointerdown", (e) => {
+  if (!frame || frame.kind !== "time") return;
+  const i = indexAt(e);
+  if (i == null) return;
+  dragFrom = i; state.a = i; state.w = 1; clampSel(); update();
+  cv.setPointerCapture(e.pointerId);
+});
+cv.addEventListener("pointermove", (e) => {
+  if (!frame || frame.kind !== "time") return;
+  const i = indexAt(e);
+  pick = i;
+  if (dragFrom != null && i != null) { state.a = Math.min(dragFrom, i); state.w = Math.abs(i - dragFrom) + 1; clampSel(); update(); }
+  else { draw2d(); $("pick-info").innerHTML = pickText(); }
+});
+cv.addEventListener("pointerup", () => { dragFrom = null; });
+cv.addEventListener("pointerleave", () => { if (dragFrom == null) { pick = null; draw2d(); $("pick-info").innerHTML = pickText(); } });
+function indexAt(e) {
+  const r = cv.getBoundingClientRect();
+  const i = Math.floor((e.clientX - r.left - frame.left) / frame.sx);
+  return i >= 0 && i < frame.n ? i : null;
+}
+
+// a chunk_grid of your own: paste zarr.json, or read one from a URL
+function useMeta(meta, where) {
+  const p = G.parseChunkGrid(meta, meta.shape);
+  const g = p.grid;
+  if (g.shape.length < 1) throw new Error("a scalar has no chunks");
+  // time along the first axis, the next two across; missing ones are 1
+  const sp = [g.shape[1] || 1, g.shape[2] || 1], spAxes = [g.axes[1] || [1], g.axes[2] || [1]];
+  const first = g.axes[0];
+  CASES.custom = {
+    kind: "time", label: where, n: g.shape[0], files: null, unit: "step",
+    rect: () => first, regular: Math.max(...first),
+    space: sp, spaceAxes: spAxes
+  };
+  state.case = "custom"; state.grid = p.kind === "rectilinear" ? "rect" : "reg";
+  state.c = G.regularSize(first, g.shape[0]) || CASES.custom.regular; state.a = 0; state.w = Math.min(g.shape[0], first[0]);
+  clampSel();
+  $("custom-note").textContent = `${where}: ${p.kind}, shape ${g.shape.join(" x ")}, ${fmt(G.chunkCount(g))} chunks` +
+    (g.shape.length > 3 ? ` (the first three axes are drawn)` : "") + ". The regular grid beside it cuts the first axis by the slider.";
+  reset();
+}
+$("draw-json").addEventListener("click", () => {
+  try { useMeta(JSON.parse($("paste").value), "Pasted"); }
+  catch (e) { $("custom-note").textContent = `Could not use that: ${e.message}`; }
+});
+$("read-url").addEventListener("click", async () => {
+  let u = $("url").value.trim();
+  if (!u) return;
+  if (!/zarr\.json$|\.zarray$/.test(u)) u = u.replace(/\/$/, "") + "/zarr.json";
+  $("custom-note").textContent = `Reading ${u}`;
+  try {
+    const r = await fetch(u);
+    if (!r.ok) throw new Error(`HTTP ${r.status}`);
+    const j = await r.json();
+    if (j.node_type === "group") throw new Error("that is a group; give the URL of one array in it");
+    useMeta(j, u.split("/").slice(-2, -1)[0] || "From the URL");
+  } catch (e) { $("custom-note").textContent = `Could not read ${u}: ${e.message}`; }
+});
+
+function sync() {
+  const k = cur(), time = k.kind === "time";
+  $("grid-reg").classList.toggle("active", state.grid === "reg");
+  $("grid-rect").classList.toggle("active", state.grid === "rect");
+  $("grid-reg").textContent = time ? "Regular" : "The files";
+  $("grid-rect").textContent = time ? "Rectilinear" : "Rectilinear grid";
+  $("time-ctl").hidden = !time;
+  if (time) {
+    $("c-label").textContent = k.native ? "Chunk inside each file" : "Regular chunk";
+    $("c").max = k.n; $("c").value = state.c; $("c-out").textContent = `${state.c} ${k.unit}s`;
+    $("a").max = k.n - 1; $("a").value = state.a; $("a-out").textContent = k.y0 ? dateOf(state.a) : state.a;
+    $("w").max = k.n; $("w").value = state.w; $("w-out").textContent = `${state.w} ${k.unit}s`;
+    $("where").value = state.where;
+  }
+  $("ex").value = state.ex; $("ex-out").textContent = state.ex.toFixed(2);
+  $("case-label").textContent = k.label;
+  $("axis-label").textContent = time ? "Cuts along the first axis; drag to choose what to read" : "The tiles, and the grid every edge makes";
+  for (const [name, el] of [["edge", "about-edge"], ["months", "about-cal"], ["years", "about-cal"], ["native", "about-native"], ["uneven", "about-mosaic"], ["brick", "about-mosaic"], ["overlap", "about-mosaic"]]) {
+    if (name === state.case) for (const d of document.querySelectorAll(".about details")) if (d.id === el) d.open = true;
+  }
+}
+
+function update() {
+  build(); draw2d(); readout(); sync(); writeHash();
+}
+function reset({ keepCam = false } = {}) {
+  solve(); update();
+  if (!keepCam) { state.cam = null; setCamera(defaultCam()); }
+  writeHash();
+}
+
+window.addEventListener("hashchange", () => {
+  readHash();
+  reset({ keepCam: true });
+  if (state.cam) setCamera(state.cam);
+});
+
+const main = $("stage");
+new ResizeObserver(() => { main.classList.toggle("stacked", main.clientHeight > main.clientWidth); draw2d(); }).observe(main);
+shareLinks("playproj: rectilinear chunks", { height: 520 });
+
+// ---------------------------------------------------------------- go
+
+solve(); build(); sync(); readout();
+resize();
+setCamera(state.cam || defaultCam());
+draw2d();
+renderer.setAnimationLoop(() => { controls.update(); renderer.render(scene, camera); });
+// for tests
+window.__chunks = () => ({ state: { ...state }, grids, report });
diff --git a/nuyina.html b/nuyina.html
index 3a97dae..1d62c04 100644
--- a/nuyina.html
+++ b/nuyina.html
@@ -31,7 +31,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html" aria-current="page">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html" aria-current="page">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/ortho.html b/ortho.html
index 45edb80..5e0d7b1 100644
--- a/ortho.html
+++ b/ortho.html
@@ -34,7 +34,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html" aria-current="page">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html" aria-current="page">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/package.json b/package.json
index fd59fc3..768c792 100644
--- a/package.json
+++ b/package.json
@@ -4,7 +4,7 @@
   "description": "Map projections traced as rays from a point of light, through a sphere, onto a plane",
   "type": "module",
   "scripts": {
-    "test": "node test/check-pure.mjs && node test/check-scene.mjs && node test/check-interfaces.mjs && node test/check-proj.mjs && node test/check-inverse.mjs && node test/check-conic.mjs && node test/check-azimuthal.mjs && node test/check-spilhaus.mjs && node test/check-globals.mjs && node test/check-mishaps.mjs && node test/check-landmesh.mjs && node test/check-secant.mjs && node test/check-s2.mjs && node test/check-icosa.mjs && node test/check-oblique.mjs && node test/check-l3bin.mjs && node test/check-grids.mjs && node test/check-mgrs.mjs && node test/check-aatgrid.mjs && node test/check-homage.mjs && node test/check-tissot.mjs && node test/check-distortion.mjs && node test/check-structure.mjs && node test/check-roms.mjs && node test/check-regular.mjs && node test/check-rasterize.mjs && node test/check-cellexport.mjs && node test/check-nuyina.mjs && node test/check-rhumb.mjs && node test/check-aspects.mjs && node test/check-ellipsoid.mjs && node test/check-geoid.mjs && node test/check-vicgrid.mjs && node test/check-texmap.mjs && node test/check-ortho.mjs && node test/check-affine.mjs && node test/check-sphull.mjs",
+    "test": "node test/check-pure.mjs && node test/check-scene.mjs && node test/check-interfaces.mjs && node test/check-proj.mjs && node test/check-inverse.mjs && node test/check-conic.mjs && node test/check-azimuthal.mjs && node test/check-spilhaus.mjs && node test/check-globals.mjs && node test/check-mishaps.mjs && node test/check-landmesh.mjs && node test/check-secant.mjs && node test/check-s2.mjs && node test/check-icosa.mjs && node test/check-oblique.mjs && node test/check-l3bin.mjs && node test/check-grids.mjs && node test/check-mgrs.mjs && node test/check-aatgrid.mjs && node test/check-homage.mjs && node test/check-tissot.mjs && node test/check-distortion.mjs && node test/check-structure.mjs && node test/check-chunkgrid.mjs && node test/check-roms.mjs && node test/check-regular.mjs && node test/check-rasterize.mjs && node test/check-cellexport.mjs && node test/check-nuyina.mjs && node test/check-rhumb.mjs && node test/check-aspects.mjs && node test/check-ellipsoid.mjs && node test/check-geoid.mjs && node test/check-vicgrid.mjs && node test/check-texmap.mjs && node test/check-ortho.mjs && node test/check-affine.mjs && node test/check-sphull.mjs",
     "serve": "python3 -m http.server 8000"
   },
   "devDependencies": {
diff --git a/rasterize.html b/rasterize.html
index 46f7706..7492b3e 100644
--- a/rasterize.html
+++ b/rasterize.html
@@ -37,7 +37,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html" aria-current="page">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html" aria-current="page">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section class="embed-keep">
diff --git a/rectilinear.html b/rectilinear.html
new file mode 100644
index 0000000..5aaff1a
--- /dev/null
+++ b/rectilinear.html
@@ -0,0 +1,232 @@
+<!doctype html>
+<html lang="en">
+<head>
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<title>playproj: rectilinear chunks</title>
+<meta name="description" content="Regular and rectilinear chunk grids in Zarr: the last chunk of a regular grid, days in months and years, uneven, staggered and overlapping tiles in a mosaic, and which chunks a read touches.">
+<link rel="stylesheet" href="css/app.css">
+<script type="importmap">
+{
+  "imports": {
+    "three": "https://cdn.jsdelivr.net/npm/three@0.160.0/build/three.module.js",
+    "three/addons/": "https://cdn.jsdelivr.net/npm/three@0.160.0/examples/jsm/"
+  }
+}
+</script>
+<script type="module" src="js/rectilinear.js"></script>
+</head>
+<body>
+<div id="app">
+  <main id="stage" class="split">
+    <div class="half">
+      <div id="three"></div>
+      <span class="half-label"><span id="case-label"></span> <span id="note3d"></span></span>
+    </div>
+    <div class="half">
+      <canvas id="axis" class="axis" aria-label="The chunk boundaries along the first axis, or across a mosaic"></canvas>
+      <span class="half-label" id="axis-label"></span>
+    </div>
+    <a id="open-full" class="open-full" href="./rectilinear.html" target="_blank" rel="noopener" hidden>Open in playproj</a>
+  </main>
+
+  <aside id="panel">
+    <header>
+      <h1>playproj</h1>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html" aria-current="page">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+    </header>
+
+    <section>
+      <div id="presets" class="presets" role="group" aria-label="Examples"></div>
+    </section>
+
+    <section class="embed-keep">
+      <div class="buttons">
+        <button id="grid-reg" type="button">Regular</button>
+        <button id="grid-rect" type="button">Rectilinear</button>
+      </div>
+      <div id="time-ctl">
+        <label class="row" for="c"><span id="c-label">Regular chunk</span><output id="c-out"></output></label>
+        <input id="c" type="range" min="1" max="400" step="1" aria-label="Regular chunk size along the first axis">
+        <label class="row" for="a"><span>Read from</span><output id="a-out"></output></label>
+        <input id="a" type="range" min="0" max="100" step="1" aria-label="Start of the read">
+        <label class="row" for="w"><span>Read how many</span><output id="w-out"></output></label>
+        <input id="w" type="range" min="1" max="100" step="1" aria-label="Length of the read">
+        <label class="row" for="where"><span>Across the map</span></label>
+        <select id="where" class="wide" aria-label="What part of the map to read">
+          <option value="map">All of it</option>
+          <option value="pixel">One pixel</option>
+        </select>
+      </div>
+      <label class="row" for="ex"><span>Pull the chunks apart</span><output id="ex-out"></output></label>
+      <input id="ex" type="range" min="0" max="1" step="0.01" aria-label="Gap between chunks">
+      <div class="buttons">
+        <button id="reset-view" type="button">Reset view</button>
+      </div>
+    </section>
+
+    <section>
+      <table id="out" class="readout"></table>
+      <p id="pick-info" class="badge"></p>
+      <pre id="json" class="projdef" aria-label="The chunk_grid in zarr.json"></pre>
+    </section>
+
+    <section>
+      <details>
+        <summary>Your own chunk grid</summary>
+        <p class="hint">Paste a zarr.json (or a chunk_grid with a shape), or give the URL of one array in a store.</p>
+        <textarea id="paste" rows="5" spellcheck="false">{"shape": [365, 180, 360], "chunk_grid": {"name": "rectilinear", "configuration": {"kind": "inline", "chunk_shapes": [[31, 28, 31, 30, 31, 30, [31, 2], 30, 31, 30, 31], 90, 90]}}}</textarea>
+        <div class="buttons"><button id="draw-json" type="button" class="small">Draw it</button></div>
+        <input id="url" class="token" type="text" spellcheck="false" placeholder="https://.../store.zarr/var" aria-label="URL of a Zarr array">
+        <div class="buttons"><button id="read-url" type="button" class="small">Read zarr.json</button></div>
+        <p id="custom-note" class="hint"></p>
+      </details>
+    </section>
+
+    <section class="about">
+      <details open>
+        <summary>How it works</summary>
+        <p>A Zarr array is stored in <b>chunks</b>, and its <code>chunk_grid</code> says where the cuts are. A
+          <b>regular</b> grid has one chunk length per axis. A <b>rectilinear</b> grid has a list of lengths per
+          axis, so the cuts along an axis can fall anywhere, but every cut still runs the whole way across the
+          array: the grid is the cross product of one list per axis, the same tuple of tuples dask has always
+          used for its own chunks. Finding the chunk that holds an index is a division on a regular axis and a
+          search of the running sums on a rectilinear one (point at the bands on the right to see both).</p>
+        <p>In zarr.json the rectilinear grid is <code>{"name": "rectilinear", "configuration": {"kind":
+          "inline", "chunk_shapes": [...]}}</code>, one entry per axis: an integer (a regular axis), a list of
+          lengths, or <code>[length, count]</code> runs, which can be mixed. The panel shows the
+          <code>chunk_grid</code> of what is drawn. Left, the array in 3D with the chunks pulled apart, the ones a
+          read touches in yellow and the part it wants in orange; right, the cuts along the first axis for
+          both grids, or a mosaic's tiles and the grid their edges make.</p>
+      </details>
+      <details id="about-edge">
+        <summary>The last chunk</summary>
+        <p>When the chunk length does not divide the array's length, a regular grid <b>overhangs</b> the end:
+          there are ceil(n / c) chunks and the last one reaches past the array. Zarr v2 says the contents of
+          that overhang are undefined; Zarr v3 says border chunks always have the full chunk size, and
+          recommends writing the fill value there. A reader decodes the whole chunk, fill and all.</p>
+        <p>This is the general case of the rectilinear grid: a regular grid is exactly a rectilinear grid with
+          an integer for every axis. The rectilinear grid can also say what is left, <code>[[30, 3], 10]</code>
+          for 100 steps in 30s, and then the last chunk is stored at 10, not padded to 30. Its lengths must add
+          up to at least the array's length, so the last chunk may still overhang (and even whole chunks past
+          the end are allowed, which is what a shrunken array keeps). Only the last chunk on an axis can
+          overhang: every chunk before it holds exactly its listed length.</p>
+      </details>
+      <details id="about-cal">
+        <summary>Days in months, days in years</summary>
+        <p>Months have 28, 29, 30 or 31 days and years have 365 or 366, so daily data chunked one month or
+          one year at a time cannot be a regular grid. With a regular 30-day chunk, February 2024 takes two
+          chunks and half the values read are not wanted; with monthly chunks it is one. A leap year is
+          <code>[31, 29, 31, 30, 31, 30, [31, 2], 30, 31, 30, 31]</code>, and years of daily data from 2000 are
+          <code>[366, [365, 3], 366, [365, 3], ...]</code>. This was the leading example in ZEP0003.</p>
+        <p>The same grid is what you get by appending: each day (or batch) added becomes a new chunk of its
+          own length, where a regular grid would have to rewrite its part-filled last chunk. A time series at
+          one pixel reads every chunk along time either way: what matters there is how the chunks are cut
+          across the map.</p>
+      </details>
+      <details id="about-native">
+        <summary>Files, and the chunks inside them</summary>
+        <p>The strongest reason for rectilinear grids is <b>virtual references</b>: kerchunk, VirtualiZarr and
+          Icechunk describe a pile of netCDF, HDF5 or TIFF files as one Zarr array whose chunks are byte ranges
+          in the original files. The files' own blocks cannot be changed, so the grid has to fit them. With a
+          regular grid, a file whose length is not a multiple of the chunk leaves a short chunk in the middle
+          of the array; kerchunk refuses it ("irregular chunking at its boundary. This is only allowed for the
+          final array") and VirtualiZarr allows it only on the last file. A rectilinear grid can list each
+          file's chunks, short ones included (orange in the bands).</p>
+        <p>The catch is padding. HDF5 (so netCDF-4) and tiled TIFF store a file's last chunk at the full chunk
+          size, padded, and a rectilinear chunk is stored at its listed length, so a padded 65-day remainder
+          stored as 100 cannot be referenced as a chunk of 65. Proposals to let a chunk's stored size differ
+          from its length were raised in the rectilinear extension's review and the ZEP meetings, but are not
+          in the spec. Sources that do not pad (TIFF strips, netCDF classic records, files whose length is a
+          multiple of their chunk, or one chunk per file) fit cleanly.</p>
+      </details>
+      <details id="about-mosaic">
+        <summary>Mosaics: uneven, staggered and overlapping tiles</summary>
+        <p><b>Uneven tiles.</b> Because there is one list of lengths per axis, every chunk in a column has the
+          same width and every chunk in a row the same height. Tiles of different sizes fit as one chunk each
+          only if they line up in rows and columns. A tile can be several chunks (its own internal blocks, as
+          runs of lengths), so long as those blocks are exact.</p>
+        <p><b>Staggered tiles.</b> When rows of tiles are offset, like bricks, every edge cuts the whole
+          array, so the grid that holds them is cut at every tile's edge and most tiles become two or more
+          chunks. For virtual references that only works where those cuts fall on the files' own block
+          boundaries; otherwise the bytes have to be rewritten.</p>
+        <p><b>Overlap.</b> A chunk grid is a partition: every index is in exactly one chunk, regular or
+          rectilinear. Two scenes that cover the same pixels cannot both be chunks of one array, so overlap
+          (red) has to be settled before Zarr sees it, by a rule such as GDAL VRT's "values will be taken from
+          the last overlapping input", by warping to a new grid, or by cropping each tile so the footprints
+          meet without overlap. Overlapping or haloed chunks have come up in Zarr discussions (they would need
+          an order, such as timestamps), but nothing has been proposed. HDF5 virtual datasets and GDAL VRT,
+          which place sources at any offset, are the nearest things.</p>
+      </details>
+      <details id="about-files">
+        <summary>Inside one TIFF or netCDF file</summary>
+        <p>Within one file, every array is a <b>regular</b> grid with an overhanging last block. Neither format
+          has variable block sizes within one array: they appear across files, or across a COG's overviews,
+          each a regular grid of its own.</p>
+        <table>
+          <tr><th>In one file</th><th>Blocks</th><th>Last block</th></tr>
+          <tr><td>TIFF tiles</td><td>one TileWidth x TileLength per image</td><td>full size, padded</td></tr>
+          <tr><td>TIFF strips</td><td>one RowsPerStrip, full width</td><td>short, not padded</td></tr>
+          <tr><td>COG overviews</td><td>a tiled image per level</td><td>padded, per level</td></tr>
+          <tr><td>netCDF classic</td><td>record variables: 1 along the record dimension</td><td>none</td></tr>
+          <tr><td>netCDF-4, HDF5</td><td>one chunk shape per variable</td><td>full size, padded</td></tr>
+        </table>
+        <p>The exceptions are rare: TIFF-FX's StripRowCounts gives strips of different heights, and an HDF5
+          virtual dataset stitches other datasets of any shape. So both formats have the same style of chunks
+          as a regular Zarr grid, and a rectilinear grid helps only when files are put together.</p>
+      </details>
+      <details id="about-status">
+        <summary>Where it stands, October 2026</summary>
+        <p><b>ZEP0003</b> (Martin Durant and Isaac Virshup, 2022) proposed variable chunking for Zarr and is
+          still a draft. The <b>rectilinear chunk grid</b> extension, by Davis Bennett, building on ZEP0003 and
+          prototyped with Lachlan Deakin at the 2025 Zarr summit, was merged into zarr-extensions in March
+          2026. Support:</p>
+        <table>
+          <tr><th>Library</th><th>Rectilinear</th></tr>
+          <tr><td>zarr-python</td><td>3.2.0 on, behind <code>array.rectilinear_chunks</code></td></tr>
+          <tr><td>zarrs (Rust)</td><td>0.23.8 on</td></tr>
+          <tr><td>Icechunk</td><td>2.0.0 on</td></tr>
+          <tr><td>xarray</td><td>reading, 2026.09.0 on</td></tr>
+          <tr><td>VirtualiZarr, Zarr.jl</td><td>open pull requests</td></tr>
+          <tr><td>GDAL, tensorstore, zarrita.js, netCDF-C</td><td>regular only</td></tr>
+        </table>
+        <p>Zarr arrays read from a URL here are drawn from their zarr.json alone; no chunk is read. Core in
+          <code>src/core/chunkgrid.js</code>, checked in <code>test/check-chunkgrid.mjs</code> (including the
+          extension's own example).</p>
+        <p>Hash keys: <code>case</code>, <code>grid</code> (reg, rect), <code>c</code> (regular chunk, or the
+          chunk inside each file), <code>a</code> and <code>w</code> (the read), <code>where</code> (map,
+          pixel), <code>ex</code>, <code>cam</code>.</p>
+        <ul class="credits">
+          <li>Durant, M. and Virshup, I. (2022) <a href="https://zarr.dev/zeps/draft/ZEP0003.html">ZEP0003:
+            Variable chunking</a>, draft.</li>
+          <li>Bennett, D. (2026) <a href="https://github.com/zarr-developers/zarr-extensions/tree/main/chunk-grids/rectilinear">Rectilinear
+            chunk grid</a>, zarr-extensions.</li>
+          <li>Zarr v3 core specification, <a href="https://zarr-specs.readthedocs.io/en/latest/v3/chunk-grids/regular-grid/">regular
+            chunk grid</a>; Zarr v2 specification.</li>
+          <li>Hamman, J., Jones, M. and Bennett, D. (2026) <a href="https://www.earthmover.io/blog/zarr-variable-length-chunks">Variable
+            length chunks in Zarr</a>, Earthmover blog.</li>
+          <li>zarr-python <a href="https://github.com/zarr-developers/zarr-python/pull/3802">#3802</a> (Max Jones),
+            zarrs <a href="https://github.com/zarrs/zarrs/pull/284">#284</a> (Lachlan Deakin), Icechunk
+            <a href="https://github.com/earth-mover/icechunk/pull/1806">#1806</a> (Deepak Cherian), VirtualiZarr
+            <a href="https://github.com/zarr-developers/VirtualiZarr/pull/954">#954</a> (Max Jones, Tom Nicholas),
+            kerchunk <a href="https://github.com/fsspec/kerchunk/pull/374">#374</a> (Isaac Virshup).</li>
+          <li>Discussion: <a href="https://github.com/orgs/zarr-developers/discussions/52">ZEP0003 discussion</a>,
+            <a href="https://github.com/zarr-developers/zarr-specs/issues/138">zarr-specs #138</a>,
+            <a href="https://github.com/zarr-developers/zarr-extensions/pull/25">zarr-extensions #25</a>.</li>
+          <li>Adobe (1992) TIFF Revision 6.0; OGC <a href="https://docs.ogc.org/is/21-026/21-026.html">Cloud
+            Optimized GeoTIFF</a> standard; Unidata, <a href="https://docs.unidata.ucar.edu/netcdf-c/current/file_format_specifications.html">netCDF
+            file format specifications</a>; The HDF Group, HDF5 chunking and virtual datasets.</li>
+          <li>GDAL <a href="https://gdal.org/en/stable/programs/gdalbuildvrt.html">gdalbuildvrt</a>.</li>
+        </ul>
+      </details>
+    </section>
+
+    <section class="share">
+      <button id="copy-link" type="button">Copy link</button>
+      <button id="copy-embed" type="button">Copy embed code</button>
+      <a href="https://github.com/hypertidy/playproj">Source and contributing</a>
+    </section>
+  </aside>
+</div>
+</body>
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diff --git a/src/core/chunkgrid.js b/src/core/chunkgrid.js
new file mode 100644
index 0000000..fe558a0
--- /dev/null
+++ b/src/core/chunkgrid.js
@@ -0,0 +1,225 @@
+// Chunk grids: how an array's index space is cut into chunks.
+//
+// A regular grid has one chunk size per axis; a rectilinear grid has a
+// list of sizes per axis, so the cuts along an axis can fall anywhere, but
+// every cut still runs the whole way across the array. Each axis here is a
+// list of chunk edge lengths, and the grid is the cross product of the
+// axes. The edges add up to at least the array's length: the last chunk
+// may overhang the end (always, in a regular grid that does not divide the
+// shape), and a chunk is stored at its edge length, so an overhang is
+// stored as fill. Pure functions, no DOM.
+
+// ---------------------------------------------------------------- axes
+
+// A regular axis of length n cut every c: ceil(n / c) chunks of c, the
+// last overhanging the end when c does not divide n. With trim, the last
+// is cut back to what is left (a rectilinear axis can say that; a regular
+// grid cannot).
+export function regularAxis(n, c, { trim = false } = {}) {
+  if (!(n >= 0) || !(c >= 1)) throw new Error(`bad regular axis ${n} / ${c}`);
+  const out = [];
+  for (let s = 0; s < n; s += c) out.push(trim ? Math.min(c, n - s) : c);
+  return out;
+}
+
+// Run-length form as the rectilinear extension writes it: plain integers
+// and [size, count] pairs, in order. Returns the list of sizes.
+export function expandRuns(list) {
+  if (!Array.isArray(list)) throw new Error("chunk sizes must be a list");
+  const out = [];
+  for (const v of list) {
+    if (Array.isArray(v)) {
+      const [s, k] = v;
+      if (!(Number.isInteger(s) && s >= 1 && Number.isInteger(k) && k >= 1)) throw new Error(`bad run [${v}]`);
+      for (let i = 0; i < k; i++) out.push(s);
+    } else {
+      if (!(Number.isInteger(v) && v >= 1)) throw new Error(`bad chunk size ${v}`);
+      out.push(v);
+    }
+  }
+  return out;
+}
+
+// The shortest run-length form: runs of one stay plain integers.
+export function compressRuns(sizes) {
+  const out = [];
+  for (let i = 0; i < sizes.length;) {
+    let j = i;
+    while (j + 1 < sizes.length && sizes[j + 1] === sizes[i]) j++;
+    out.push(j > i ? [sizes[i], j - i + 1] : sizes[i]);
+    i = j + 1;
+  }
+  return out;
+}
+
+// Start of each chunk, and the end of the last: offsets[k] .. offsets[k+1].
+export function offsets(sizes) {
+  const o = [0];
+  for (const s of sizes) o.push(o[o.length - 1] + s);
+  return o;
+}
+
+// The chunk holding index i: a division on a regular axis, a binary search
+// of the offsets on a rectilinear one (the first offset past i, less one).
+export function locate(off, i) {
+  if (i < 0 || i >= off[off.length - 1]) return -1;
+  let lo = 0, hi = off.length - 2;
+  while (lo < hi) {
+    const mid = (lo + hi + 1) >> 1;
+    if (off[mid] <= i) lo = mid; else hi = mid - 1;
+  }
+  return lo;
+}
+export const locateRegular = (c, i) => Math.floor(i / c);
+
+// The chunks a half-open selection [a, b) touches, as [first, last], or null.
+// n is the array's length (defaults to the end of the last chunk).
+export function span(off, a, b, n = off[off.length - 1]) {
+  a = Math.max(0, a); b = Math.min(n, b);
+  if (!(b > a)) return null;
+  return [locate(off, a), locate(off, b - 1)];
+}
+
+// The regular chunk size of an axis of length n, or null: every edge c,
+// and no more chunks than ceil(n / c).
+export function regularSize(sizes, n) {
+  if (!sizes.length) return null;
+  const c = sizes[0];
+  if (sizes.some((s) => s !== c)) return null;
+  return n == null || sizes.length === Math.max(1, Math.ceil(n / c)) ? c : null;
+}
+
+// How much of chunk k of an axis of length n holds data (the rest overhangs).
+export const filled = (off, k, n) => Math.max(0, Math.min(off[k + 1], n) - off[k]);
+
+// ---------------------------------------------------------------- grids
+
+// A grid: { shape, axes: [sizes per axis] }.
+export function grid(shape, axes) {
+  if (shape.length !== axes.length) throw new Error("one list of sizes per axis");
+  axes.forEach((a, d) => {
+    const t = a.reduce((x, y) => x + y, 0);
+    if (t < shape[d]) throw new Error(`axis ${d}: chunk sizes add to ${t}, short of the shape ${shape[d]}`);
+  });
+  return { shape: shape.slice(), axes: axes.map((a) => a.slice()), off: axes.map(offsets) };
+}
+export const regularGrid = (shape, chunks) => grid(shape, shape.map((n, d) => regularAxis(n, chunks[d])));
+export const isRegular = (g) => g.axes.every((a, d) => regularSize(a, g.shape[d]) !== null);
+export const chunkCount = (g) => g.axes.reduce((p, a) => p * a.length, 1);
+
+// The chunk_grid of zarr.json for a grid: regular when every axis is,
+// otherwise rectilinear (zarr-extensions chunk-grids/rectilinear), each
+// axis written as an integer when it is regular and as a list with runs
+// otherwise.
+export function chunkGridJson(g, { force } = {}) {
+  if (isRegular(g) && force !== "rectilinear") {
+    return { name: "regular", configuration: { chunk_shape: g.axes.map((a, d) => regularSize(a, g.shape[d])) } };
+  }
+  return {
+    name: "rectilinear",
+    configuration: {
+      kind: "inline",
+      chunk_shapes: g.axes.map((a, d) => regularSize(a, g.shape[d]) ?? compressRuns(a))
+    }
+  };
+}
+
+// Read a chunk_grid (or a whole zarr.json, or a v2 .zarray) against a shape.
+// An integer inside a rectilinear grid is a regular axis, repeated until it
+// reaches the shape; lists may overflow it by whole chunks.
+export function parseChunkGrid(meta, shape) {
+  if (meta && meta.shape && !shape) shape = meta.shape;
+  if (!shape) throw new Error("no shape: give the zarr.json, or the shape with the chunk_grid");
+  if (meta && meta.chunk_grid) meta = meta.chunk_grid;
+  else if (meta && Array.isArray(meta.chunks)) return { kind: "regular (v2)", grid: regularGrid(shape, meta.chunks) };
+  const name = meta && (meta.name || meta.type);
+  const conf = (meta && meta.configuration) || {};
+  if (name === "regular") return { kind: "regular", grid: regularGrid(shape, conf.chunk_shape) };
+  if (name === "rectilinear") {
+    const cs = conf.chunk_shapes;
+    if (!Array.isArray(cs) || cs.length !== shape.length) throw new Error("chunk_shapes needs one entry per axis");
+    const axes = cs.map((v, d) => (Number.isInteger(v) ? regularAxis(shape[d], v) : expandRuns(v)));
+    return { kind: "rectilinear", grid: grid(shape, axes) };
+  }
+  throw new Error(`unknown chunk grid "${name}"`);
+}
+
+// The key of a chunk under the default v3 encoding ("c/0/1/2") or v2 ("0.1.2").
+export function chunkKey(idx, { version = 3, separator } = {}) {
+  if (version === 2) return idx.join(separator || ".");
+  return ["c", ...idx].join(separator || "/");
+}
+
+// What a selection costs: per axis a half-open [a, b); returns the chunks
+// touched, and the elements decoded (whole chunks at their edge lengths,
+// overhang and all) and wanted.
+export function readCost(g, sel) {
+  let chunks = 1, read = 1, want = 1;
+  const spans = [];
+  for (let d = 0; d < g.axes.length; d++) {
+    const [a, b] = sel[d] || [0, g.shape[d]];
+    const s = span(g.off[d], a, b, g.shape[d]);
+    if (!s) return { chunks: 0, read: 0, want: 0, spans: null };
+    spans.push(s);
+    let r = 0;
+    for (let k = s[0]; k <= s[1]; k++) r += g.axes[d][k];
+    chunks *= s[1] - s[0] + 1; read *= r; want *= Math.min(b, g.shape[d]) - Math.max(a, 0);
+  }
+  return { chunks, read, want, spans };
+}
+
+// ---------------------------------------------------------------- calendars
+
+export const isLeap = (y) => (y % 4 === 0 && y % 100 !== 0) || y % 400 === 0;
+const MONTH = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
+export const daysInMonth = (y, m) => (m === 1 && isLeap(y) ? 29 : MONTH[m]);
+export const MONTH_NAMES = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
+
+// Days of each month from January y0 to December y1, with labels.
+export function months(y0, y1) {
+  const sizes = [], labels = [];
+  for (let y = y0; y <= y1; y++) for (let m = 0; m < 12; m++) { sizes.push(daysInMonth(y, m)); labels.push(`${MONTH_NAMES[m]} ${y}`); }
+  return { sizes, labels };
+}
+export function years(y0, y1) {
+  const sizes = [], labels = [];
+  for (let y = y0; y <= y1; y++) { sizes.push(isLeap(y) ? 366 : 365); labels.push(String(y)); }
+  return { sizes, labels };
+}
+// Day index (from 1 January y0) of the first day of month m of year y.
+export function dayOf(y0, y, m = 0, d = 1) {
+  let n = 0;
+  for (let yy = y0; yy < y; yy++) n += isLeap(yy) ? 366 : 365;
+  for (let mm = 0; mm < m; mm++) n += daysInMonth(y, mm);
+  return n + d - 1;
+}
+
+// ---------------------------------------------------------------- mosaics
+
+// Rectangles in index space: { x0, x1, y0, y1 } half-open, from files or
+// scenes of a mosaic. The rectilinear grid that can hold them is cut at
+// every edge any of them has; a tile is a single chunk of it only when no
+// other tile's edge runs through it.
+export function mosaicGrid(rects) {
+  const xs = [...new Set(rects.flatMap((r) => [r.x0, r.x1]))].sort((a, b) => a - b);
+  const ys = [...new Set(rects.flatMap((r) => [r.y0, r.y1]))].sort((a, b) => a - b);
+  const diff = (e) => e.slice(1).map((v, i) => v - e[i]);
+  return { x0: xs[0], y0: ys[0], cols: diff(xs), rows: diff(ys), xs, ys };
+}
+
+// How each rectangle sits on the grid of all their edges: the cells it
+// covers, and whether two rectangles claim the same cell (overlap).
+export function mosaicReport(rects) {
+  const m = mosaicGrid(rects);
+  const nx = m.cols.length, ny = m.rows.length;
+  const owners = Array.from({ length: nx * ny }, () => []);
+  const per = rects.map((r, t) => {
+    const i0 = m.xs.indexOf(r.x0), i1 = m.xs.indexOf(r.x1), j0 = m.ys.indexOf(r.y0), j1 = m.ys.indexOf(r.y1);
+    for (let j = j0; j < j1; j++) for (let i = i0; i < i1; i++) owners[j * nx + i].push(t);
+    return { cells: (i1 - i0) * (j1 - j0), i0, i1, j0, j1 };
+  });
+  const overlapCells = owners.filter((o) => o.length > 1).length;
+  const holes = owners.filter((o) => o.length === 0).length;
+  const oneChunkEach = per.every((p) => p.cells === 1) && overlapCells === 0;
+  return { grid: m, per, owners, overlapCells, holes, oneChunkEach };
+}
diff --git a/src/core/index.js b/src/core/index.js
index 2c1ffba..b4f86bc 100644
--- a/src/core/index.js
+++ b/src/core/index.js
@@ -47,6 +47,7 @@ export * as sphull from "./sphull.js";
 export * as affine from "./affine.js";
 
 export * as structure from "./structure.js";
+export * as chunkgrid from "./chunkgrid.js";
 export * as basemap from "./basemap.js";
 export * as geotiff from "./geotiff-write.js";
 export * as cellexport from "./cellexport.js";
diff --git a/structure.html b/structure.html
index 79740d2..3d639a4 100644
--- a/structure.html
+++ b/structure.html
@@ -28,7 +28,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html" aria-current="page">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html" aria-current="page">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
     </header>
 
     <section>
diff --git a/test/check-chunkgrid.mjs b/test/check-chunkgrid.mjs
new file mode 100644
index 0000000..5c928f3
--- /dev/null
+++ b/test/check-chunkgrid.mjs
@@ -0,0 +1,102 @@
+// Chunk grids: regular axes and their edge chunk, run-length sizes, chunk
+// lookup against a brute-force scan, the zarr.json chunk_grid round trip,
+// calendar axes and the mosaic edge grid.
+import * as g from "../src/core/chunkgrid.js";
+
+let fails = 0;
+const check = (ok, msg) => { if (!ok) { fails++; console.log("FAIL " + msg); } };
+const same = (a, b) => JSON.stringify(a) === JSON.stringify(b);
+
+// regular axes: whole chunks, the last overhanging, or trimmed to what is left
+check(same(g.regularAxis(100, 30), [30, 30, 30, 30]), "100 by 30");
+check(same(g.regularAxis(100, 30, { trim: true }), [30, 30, 30, 10]), "100 by 30, trimmed");
+check(same(g.regularAxis(90, 30), [30, 30, 30]), "90 by 30");
+check(same(g.regularAxis(5, 30), [30]) && same(g.regularAxis(5, 30, { trim: true }), [5]), "one short chunk");
+check(g.regularSize([30, 30, 30, 30], 100) === 30 && g.regularSize([30, 30, 30, 10], 100) === null && g.regularSize([30, 31, 30]) === null, "regular sizes");
+check(g.regularSize([30, 30, 30, 30, 30], 100) === null, "an overflow chunk is not regular");
+{
+  const off = g.offsets(g.regularAxis(100, 30));
+  check(g.filled(off, 3, 100) === 10 && g.filled(off, 0, 100) === 30, "filled");
+}
+
+// runs
+check(same(g.expandRuns([[31, 1], 28, [30, 2]]), [31, 28, 30, 30]), "expand runs");
+check(same(g.compressRuns([31, 28, 31, 30, 30, 30, 1]), [31, 28, 31, [30, 3], 1]), "compress runs");
+for (const bad of [[0], [[3, 0]], [1.5], "1,2"]) {
+  let threw = false;
+  try { g.expandRuns(bad); } catch (e) { threw = true; }
+  check(threw, `rejects ${JSON.stringify(bad)}`);
+}
+
+// lookup against a scan, on a calendar axis
+{
+  const { sizes } = g.months(2023, 2025);
+  const off = g.offsets(sizes);
+  check(off[off.length - 1] === 365 + 366 + 365, "three years of days");
+  let ok = true;
+  for (let i = 0; i < off[off.length - 1]; i++) {
+    let k = 0;
+    while (off[k + 1] <= i) k++;
+    if (g.locate(off, i) !== k) ok = false;
+  }
+  check(ok, "locate matches a scan");
+  check(g.locate(off, -1) === -1 && g.locate(off, 2000) === -1, "outside is -1");
+  const feb24 = g.dayOf(2023, 2024, 1);
+  check(feb24 === 365 + 31, "1 Feb 2024");
+  check(same(g.span(off, feb24, feb24 + 29), [13, 13]), "February 2024 is one chunk");
+  check(same(g.span(g.offsets(g.regularAxis(1096, 30)), feb24, feb24 + 29, 1096), [13, 14]), "February 2024 in 30-day chunks is two");
+  check(g.years(2023, 2025).sizes.join() === "365,366,365", "years");
+  check(g.isLeap(2000) && !g.isLeap(1900) && g.isLeap(2024), "leap years");
+}
+
+// chunk_grid round trip
+{
+  const r = g.regularGrid([100, 180, 360], [30, 90, 90]);
+  check(g.isRegular(r) && g.chunkCount(r) === 4 * 2 * 4, "regular grid");
+  check(same(g.chunkGridJson(r), { name: "regular", configuration: { chunk_shape: [30, 90, 90] } }), "regular json");
+  const back = g.parseChunkGrid({ shape: [100, 180, 360], chunk_grid: g.chunkGridJson(r) });
+  check(same(back.grid.axes, r.axes), "regular round trip");
+
+  const days = g.months(2024, 2024).sizes;
+  const rg = g.grid([366, 180, 360], [days, g.regularAxis(180, 90), g.regularAxis(360, 90)]);
+  const j = g.chunkGridJson(rg);
+  check(j.name === "rectilinear" && same(j.configuration.chunk_shapes, [[31, 29, 31, 30, 31, 30, [31, 2], 30, 31, 30, 31], 90, 90]), "rectilinear json");
+  const p = g.parseChunkGrid(j, [366, 180, 360]);
+  check(p.kind === "rectilinear" && same(p.grid.axes, rg.axes), "rectilinear round trip");
+  check(same(g.chunkGridJson(g.grid([100], [g.regularAxis(100, 30, { trim: true })])).configuration.chunk_shapes, [[[30, 3], 10]]), "trimmed edge chunk as runs");
+  check(same(g.chunkGridJson(g.grid([100], [g.regularAxis(100, 30)]), { force: "rectilinear" }).configuration.chunk_shapes, [30]), "overhanging edge chunk as an integer");
+  let threw = false;
+  try { g.grid([100], [[30, 30, 30]]); } catch (e) { threw = true; }
+  check(threw, "sizes must reach the shape");
+  check(g.grid([12], [[4, 4, 4, 4]]).axes[0].length === 4, "overflow chunks are allowed");
+  check(g.parseChunkGrid({ shape: [10, 10], chunks: [4, 5] }).grid.axes[0].join() === "4,4,4", "v2 .zarray");
+  // the extension's own example (zarr-extensions chunk-grids/rectilinear README)
+  const ex = g.parseChunkGrid({ shape: [6, 6, 6, 6, 6], chunk_grid: { name: "rectilinear", configuration: { kind: "inline", chunk_shapes: [4, [1, 2, 3], [[4, 2]], [[1, 3], 3], [4, 4, 4]] } } });
+  check(same(ex.grid.axes, [[4, 4], [1, 2, 3], [4, 4], [1, 1, 1, 3], [4, 4, 4]]), "the extension's example");
+}
+
+// keys and read cost
+{
+  check(g.chunkKey([1, 0, 3]) === "c/1/0/3" && g.chunkKey([1, 0, 3], { version: 2 }) === "1.0.3", "chunk keys");
+  const r = g.regularGrid([100, 10], [30, 10]);
+  const c = g.readCost(r, [[95, 100], [0, 10]]);
+  check(c.chunks === 1 && c.read === 300 && c.want === 50, `edge chunk read ${JSON.stringify(c)}`);
+  const t = g.grid([100, 10], [g.regularAxis(100, 30, { trim: true }), [10]]);
+  check(g.readCost(t, [[95, 100], [0, 10]]).read === 100, "trimmed edge chunk read");
+}
+
+// mosaics
+{
+  const tidy = [{ x0: 0, x1: 100, y0: 0, y1: 50 }, { x0: 100, x1: 130, y0: 0, y1: 50 }, { x0: 0, x1: 100, y0: 50, y1: 70 }, { x0: 100, x1: 130, y0: 50, y1: 70 }];
+  const a = g.mosaicReport(tidy);
+  check(a.oneChunkEach && same(a.grid.cols, [100, 30]) && same(a.grid.rows, [50, 20]), "uneven tiles in rows and columns");
+  const brick = [{ x0: 0, x1: 100, y0: 0, y1: 50 }, { x0: 100, x1: 200, y0: 0, y1: 50 }, { x0: 50, x1: 150, y0: 50, y1: 100 }, { x0: 0, x1: 50, y0: 50, y1: 100 }, { x0: 150, x1: 200, y0: 50, y1: 100 }];
+  const b = g.mosaicReport(brick);
+  check(!b.oneChunkEach && b.overlapCells === 0 && b.holes === 0 && same(b.grid.cols, [50, 50, 50, 50]), "brick bond splits tiles");
+  const lap = [{ x0: 0, x1: 110, y0: 0, y1: 50 }, { x0: 100, x1: 200, y0: 0, y1: 50 }];
+  const c = g.mosaicReport(lap);
+  check(c.overlapCells === 1 && same(c.grid.cols, [100, 10, 90]), "overlap claims a cell twice");
+}
+
+if (fails) { console.log(`${fails} failed`); process.exit(1); }
+console.log("chunk grids ok");
diff --git a/texture.html b/texture.html
index 00ae24a..c2eae94 100644
--- a/texture.html
+++ b/texture.html
@@ -34,7 +34,7 @@
   <aside id="panel">
     <header>
       <h1>playproj</h1>
-      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html" aria-current="page">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
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diff --git a/tissot.html b/tissot.html
index c1e8a4f..afb901e 100644
--- a/tissot.html
+++ b/tissot.html
@@ -35,7 +35,7 @@
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+      <nav class="pages"><span class="group"><a href="./">Perspective</a><a href="./cones.html">Cones and cylinders</a></span><span class="group"><a href="./spilhaus.html">Spilhaus</a><a href="./globals.html">Globals and interrupteds</a><a href="./tissot.html" aria-current="page">Distortion</a></span><span class="group"><a href="./ellipsoid.html">Ellipsoid</a><a href="./geoid.html">Geoid</a><a href="./ortho.html">Orthorectification</a></span><span class="group"><a href="./cells.html">s2 cells</a><a href="./h3.html">H3 hexagons</a><a href="./bins.html">L3 bins</a><a href="./grids.html">Model grids</a><a href="./indexspace.html">Index space</a><a href="./sentinel.html">MGRS and Sentinel-2</a><a href="./aatgrid.html">AAT grid</a><a href="./structure.html">Tiles and overviews</a><a href="./rectilinear.html">Rectilinear chunks</a><a href="./rasterize.html">Rasterize</a><a href="./affine.html">Affine</a><a href="./regular.html">Regular in what?</a><a href="./hull.html">Convex hull</a></span><span class="group"><a href="./catastrophes.html">Catastrophes</a><a href="./texture.html">Texture mapping</a><a href="./example.html">For example</a></span><span class="group"><a href="./homage.html">Homage</a><a href="./vicgrid.html">VicGrid</a></span><span class="group"><a href="./nuyina.html">Nuyina</a><a href="./rhumb.html">Rhumb lines</a></span></nav>
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diff --git a/vicgrid.html b/vicgrid.html
index 338e68c..63d2135 100644
--- a/vicgrid.html
+++ b/vicgrid.html
@@ -27,7 +27,7 @@
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