CydandClaude Opus 4.8
66cef329d1
BTDPL.INI's fog values for this mission (testarn.egg = arena1/day/clear,
[ardayclear] leaf: fog=200.0 1250.0 0.32 0.3 0.5, clip=0.25 1300.0) are exact,
verified ground truth. The depth term took two tries: `tz` (texz) looked like
the right candidate (BT411 says fog is linear on "W") but measured live it
DECREASES with distance -- the wrong direction/shape. Re-derived from the
actual firmware source instead of guessing again: AS860/XFPROJ.SS's own
comment on the projection code ("pz=wz*invz, where pz=1 at near clip, 0 at
infinity") establishes the z-buffer value as a normalized hither/distance
quantity, giving distance = hither * 2^20 / raw_zbuf_value.
gpu_raster.py: added a 10th shader output channel carrying the raw z-buffer
float (previously only a filled/empty flag existed), and the fog blend using
this formula + BTDPL.INI's exact color. Verified offline: distance climbs
smoothly (394->1051) across a receding ceiling, landing inside the authored
200-1250 range with no further fudging; visually the horizon shows the
authored blue-purple haze, correctly absent near the camera.
IG-SHADING-MODEL.md: documented the false start (tz) and the source-grounded
fix, with the honest caveats that remain (2^20 exact scale, hither as a fixed
per-mission constant vs. per-view from the wire).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>