Finding flicker by diffing frames

Flicker is a pixel that changes when nothing should change it, or that changes by more than the motion explains. Judged by eye it is easy to miss and impossible to compare before and after; diffed, it is a number and a red map.

The hooks

window.__poc (a build with NEXT_PUBLIC_POC_DEBUG=1, poc-debug.ts):

• hold = true stops the loop; step = n lets it draw exactly n more frames (counted down by the frames drawn, tickPocFrame).
• frameMs = 1000 / 60 puts the scene clock on a fixed step per drawn frame (pocFrameTime), so the wind, the water and the rain advance the same between any two frames however slow the renderer is.
• window.__poc is replaced on every patch: read it fresh, never keep it.

What still moves on a held clock, legitimately: the clouds (SkyMesh runs on the renderer's real time), Sin City's rain, the near crowns' sway in the styles that keep it. Mask or name them, never "fix" them.

The procedure

1. NEXT_PUBLIC_POC_DEBUG=1 bunx next build && bunx next start -p 3000 (after bun run build's data preparation; headless SwiftShader is enough — this measures change, not light).

2. Render, per style, a still pair and three moved frames:

   node .claude/skills/flicker-probe/probe.mjs out/p1 pastel,comic,line
   POSE='{"mode":"walk","pos":{"x":-944.2,"y":116,"z":-549.5},"headingDeg":205,"pitchDeg":0,"fov":55}' \
     STEP_M=0.001 node .claude/skills/flicker-probe/probe.mjs out/p2 comic
   

   POSE is a camera state (getCameraState()); without it the site's initial view. In walk mode the eye sinks onto the ground for a while after a teleport: the probe steps until the camera holds still before it takes the still pair, or every "still" diff is motion.

3. Diff:

   • python3 .claude/skills/flicker-probe/diff.py out/p1 comic — the still pair (a:b): anything over 24 that is not on the list above is flicker. b:m1 with STEP_M=0.001 is the sub-pixel test: a millimetre moves the far city by a hundredth of a pixel and the near ground by a fraction, so an antialiased picture barely changes and every binary decision that flips (a stipple cell, a depth-stencil line, a dot under-sampled across a grazing plane) shows in red.
   • python3 .claude/skills/flicker-probe/flicker.py out/p1 comic — at walking pace (2.5 cm a frame), pixels whose change flips sign from one frame to the next. Near the camera a fine line moving a pixel a frame toggles too: read it far off, or use the sub-pixel test.

4. Read the red maps (*.diff.png, *.flicker.png) beside the frame, name the feature, fix it, and render the same pose again. Each run of five styles takes ~15 min in the container (boot ~8 min).

What it found (2026-10)

• A sway or a stipple below a pixel shimmers: fade the sway with distance, draw a crown whole once its stipple cells are smaller than a pixel.
• A world-anchored screen on a swaying surface slides: print the surface flat (Comic's crowns) or hold it still (Strich's crowns, sway).
• A screen across a grazing plane: give it its mean tone once the pixel's long axis spans half a cell, and antialias over the footprint across the edge (fwidth of the distance), not the mean pixel.