Six rounds to light the tunnel walls: Fable 5.1 + Claude Code on a fractal sponge

Six render rounds, and the first five trace one curve:

1. Badly overexposed — the dust haze and the glow term were both too strong.
2. – 3. Exposure corrected, but now the dust was invisible and the lighting flat.
3. – 5. Depth and fog strengthened so the dust became “a few bright, most dim”.
4. Final: switched to a point light ahead of the camera plus half-Lambert shading, which fixed tunnel walls going dark at grazing angles.

Round 6 is worth calling out on its own: “the walls are too dark at grazing angles” is a problem with the lighting model, not with a badly tuned parameter. Five rounds went into adjusting exposure and fog values; only the sixth changed the algorithm. That kind of judgement — this cannot be tuned, it has to be replaced — comes only from looking at the rendered frame repeatedly.

Final numbers: __DEMO_READY=true, __DEMO_ERROR=null, no page, console or request errors; two matching hashes at ?t=7.5&det=1; first frame in 1.8–3.1 seconds and about 1.1 FPS under SwiftShader; the raymarch runs at 50% resolution by default (640×360) at roughly 21 steps per ray; and at t=7.5, t=30 and in live mode mean luminance sits at 0.27–0.35 — neither black nor blown out, with the far end of the tunnel darkening correctly and the dust sparks visible and properly occluded by the fractal.

Implementation: the fractal is a KIFS variant of the Menger sponge (rotate, fold, sort, scale) tiled on a 4-unit period, with a tunnel carved along a curved path driven by simTime; scale, fold offset and rotation all “breathe” slowly over time. The raymarch renders into a HalfFloat target (rgb for colour, a for hit distance); a composite pass upscales it and converts the distance into gl_FragDepthEXT, which is what the million dust points depth-test against — the same approach as the Astra run, at a different density and palette.

A few URL parameters are left exposed for tinkering: rm (raymarch resolution ratio), steps, iter and dust.

Controls: mouse for parallax, scroll to zoom, space to pause, 1/2/3 for palettes (Abyssal Neon, Ember Core, Verdant Circuit), ?t= to freeze the clock.