ICC: don't apply gray profiles to color images

Like libpng, which rejects them: a gray profile embedded in a color image
(Pillow keeps the profile when converting gray to RGB) set all channels
to the transformed red one. It's applied only when the pixels are gray.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lfv41PqHEGbU7SLg5onpG1

Author: matmen

CHANGELOG.md

@@ -26,8 +26,8 @@
   (ICCP, also animated), AVIFs (colr prof/rICC, which then describe the RGB values instead of the nclx box) and TIFFs.
   Matrix/TRC profiles (RGB and gray; gamma, table and parametric curves) are applied relative colorimetric, colors
   outside sRGB clipped: Display P3 photos from phones, Adobe RGB and ProPhoto images no longer look washed out. The
-  results are within 1 of LittleCMS's. Profiles with only lookup tables, CMYK profiles and ones that are sRGB (like
-  most cameras embed) leave the pixels as they are; converting a 1920x1080 image takes about 35 ms.
+  results are within 1 of LittleCMS's. Profiles with only lookup tables, CMYK profiles, gray profiles of color images (like libpng,
+  which rejects them) and ones that are sRGB (like most cameras embed) leave the pixels as they are; converting a 1920x1080 image takes about 35 ms.
 - `font-size` resolves `em`, `ex`, percentages and keywords against the parent's size for all elements (lengths in `em`
   used the default 16 pixels when an ancestor's size was relative), and `font-weight` `bolder` and `lighter`.
 - `Image.renderSVG()` blends elements with `mix-blend-mode` (all 16 modes and `plus-lighter`, in CSS) and isolates

scripts/icc-fixtures.py

@@ -3,7 +3,7 @@
 # ImageCms) converts them to, relative colorimetric to sRGB, as PNGs: from the source pixels for the lossless formats,
 # from what Pillow decodes for JPEG and AVIF. The profiles are matrix/TRC ones written here (Display P3 with a
 # parametric curve, Adobe RGB with a gamma, P3 with a table curve, Rec. 2020 with a type 4 parametric curve, gray), and
-# one that is sRGB (its pixels stay as they are). Needs Pillow (with AVIF) and numpy:
+# one that is sRGB (its pixels stay as they are), and a gray one on a color image (not applied). Needs Pillow (with AVIF) and numpy:
 #   python scripts/icc-fixtures.py
 import io, struct
 from pathlib import Path
@@ -127,3 +127,5 @@ def convert(im, icc):
 gray = source.convert('L')
 gray.save(out / 'gray.png', icc_profile=gray_profile)
 convert(gray, gray_profile).save(out / 'gray.ref.png')
+# a gray profile on a color image (invalid, like libpng says: not applied)
+source.save(out / 'gray-on-rgb.png', icc_profile=gray_profile)

src/util/icc.ts

@@ -135,8 +135,10 @@ export function profileTransform(icc: Uint8Array): ((rgba: Uint8Array | Uint8Cla
 
     if (p.gray) {
         // (the gray's luminance in all three channels: the profile's white is sRGB's)
+        // (only for gray images: like libpng, a gray profile of a color image isn't applied)
         const k = linear[0], lut = Uint8Array.from({ length: 256 }, (_, i) => out(k[i]));
         return rgba => {
+            for (let i = 0; i < rgba.length; i += 4) if (rgba[i] !== rgba[i + 1] || rgba[i] !== rgba[i + 2]) return;
             for (let i = 0; i < rgba.length; i += 4) rgba[i] = rgba[i + 1] = rgba[i + 2] = lut[rgba[i]];
         };
     }

tests/icc.js

@@ -32,6 +32,9 @@ const difference = (a, b) => {
         const raw = await Image.decode(data, {applyColorProfile: false});
         if (file !== 'srgb.png' && difference(raw, ref) <= tolerance) panic(`${file}: applyColorProfile: false should keep the stored colors`);
     }
+    // (a gray profile of a color image isn't applied)
+    const color = await Image.decode(await fs.readFile('./tests/icc/gray-on-rgb.png'));
+    if (difference(color, await Image.decode(await fs.readFile('./tests/icc/gray-on-rgb.png'), {applyColorProfile: false})) !== 0) panic('a gray profile of a color image should be ignored');
     // (an sRGB profile leaves the pixels as they are)
     const srgb = await fs.readFile('./tests/icc/srgb.png');
     if (difference(await Image.decode(srgb), await Image.decode(srgb, {applyColorProfile: false})) !== 0) panic('an sRGB profile should leave the pixels as they are');