feat(view): add turntable mode to interactive viewer

[pfmephisto]

Problem

The interactive rux view viewer uses VTK's default trackball camera, which makes it hard to present or record a scene. There is no way to continuously orbit the scene at a steady pace, which is useful for reviewing geometry or capturing footage.

Goal

Add a turntable mode that:

• Rotates the camera continuously around the scene's vertical axis (world Z)
• Keeps the focal point fixed at the scene centre
• Can be toggled on/off and paused with keyboard shortcuts

Implementation context

Current viewer architecture

The viewer is built on pcl::visualization::PCLVisualizer, which wraps VTK's vtkInteractorStyleTrackballCamera. The main loop runs in apps/rux/src/processing_observer.cpp:671 via viewer->spinOnce(100ms). No custom interactor style exists today.

Camera manipulation uses the VTK API directly — see apps/rux/src/view/panorama_handler.cpp:135–146 for a working example:

cam->SetPosition(x, y, z);
cam->SetFocalPoint(cx, cy, cz);
cam->SetViewUp(0, 0, 1);

Orbit math (reference)

libs/reusex/src/visualize/render_view.cpp:612–625 already implements a circular orbit:

const double azimuth = 2.0 * pi * orbit_index / orbit_count;
const double elevation = elevation_deg * pi / 180.0;
cam->SetPosition(
    center[0] + reach * cos(azimuth) * cos(elevation),
    center[1] + reach * sin(azimuth) * cos(elevation),
    center[2] + reach * sin(elevation));
cam->SetViewUp(0.0, 0.0, 1.0);

For turntable mode the same formula is used with elevation fixed (e.g. 20°) and azimuth incrementing each frame.

State pattern

Follow the PanoramaState pattern (apps/rux/include/view/panorama_handler.hpp:24–35) — a small struct held by shared_ptr and shared with callbacks:

struct TurntableState {
    bool active = false;
    bool spinning = true;
    double azimuth = 0.0;            // radians
    double angular_velocity = 0.3;   // rad/s
    double radius = 5.0;
    double elevation_deg = 20.0;
    std::array<double,3> center = {0,0,0};
};

Compute center and radius from the scene bounding box on activation (see render_view.cpp:580–582 for the bbox query pattern).

Integration points

1. State + keyboard callback — add to apps/rux/src/view.cpp after the pano_state block (around line 130). Register keys via viewer->registerKeyboardCallback(...) — see the Escape key handler at lines 220–238 for the pattern.

   Suggested keys: t = toggle on/off, Space = pause/resume spin, [/] = decrease/increase speed.

2. Per-frame update — enqueue a camera update task each tick. The task queue is drained every 100 ms in processing_observer.cpp:671. Use observer.viewer_enqueue_task(...) (same pattern as other renderers).

3. Help text — add turntable controls to the help overlay following the pattern in apps/rux/src/view/component_renderer.cpp:106–145.

4. No interactor changes needed — when turntable is active, manually calling SetPosition/SetFocalPoint each frame overrides VTK's trackball. When it is off, normal mouse interaction resumes automatically.

Acceptance criteria

• t toggles turntable mode; viewer prints a brief status message
• Space pauses/resumes spinning while in turntable mode
• [ / ] decrease/increase angular speed
• Camera stays locked to world-Z up; scene centre auto-detected from loaded geometry
• Normal trackball interaction works when turntable is off

Key files

File	Lines	Purpose
apps/rux/src/processing_observer.cpp	645–676	Viewer init and main spin loop
apps/rux/src/view.cpp	105–319	Callback registration, pano_state pattern to follow
apps/rux/src/view/panorama_handler.cpp	135–146	Camera SetPosition/FocalPoint/ViewUp example
apps/rux/include/view/panorama_handler.hpp	24–35	State struct pattern
libs/reusex/src/visualize/render_view.cpp	612–625	Orbit math to adapt
apps/rux/src/view/component_renderer.cpp	106–145	Help text addition point
apps/rux/include/view/label_renderer.hpp	77–173	Keyboard callback template pattern