retroDE_ps2/docs/ch403_authentic_vertex_modulation_closeout.md

Ch403 — authentic vertex-color modulation

Objective

Make the 224139 static-frame fixture use the captured GS lighting state instead of drawing every texture at raw DECAL brightness. Geometry, draw order, runtime CLUT handling, persistent Z, and the Ch402 40-epoch staging partition remain unchanged.

Finding

The selected draws carry TEX0.TFX=MODULATE and non-unity per-vertex RGB. Ch402 deliberately forced TFX=DECAL and emitted zero placeholder RGB, which made the composite much brighter than the PCSX2 frame.

The first Ch403 fixture proved a second, narrower gap: the existing MODULATE implementation was only connected to the combined affine texture path. These SH3 triangles use the five-cycle perspective path, whose emit was explicitly hardwired to the raw texel. Consequently the live board completed every gate but produced a framebuffer byte-identical to Ch402.

Implementation

• tools/gs_make_sh3_scheduler_fixture.py
  • adds opt-in --auth-color-tfx;
  • preserves authentic TEX0.TFX and per-vertex RGB;
  • interpolates RGB when clipping triangles;
  • applies the same GS texel * vertex / 128 saturation rule in the independent reference.
• rtl/gif_gs/gs_stub.sv
  • aligns the already-computed S2 Gouraud color with the perspective texel by four registers;
  • applies MODULATE at the perspective S1+5 emit;
  • shares the existing three-channel modulation datapath between affine and perspective paths, avoiding a duplicate multiplier set;
  • leaves DECAL output byte-identical.
• sim/tb/top/tb_top_psmct32_sh3_zint640_shared.sv
  • verifies the full 64-bit staged header and committed TEX0 state;
  • fixes late-epoch diagnostic fill accounting.

Fixture

zsrt139a9 retains all 68 Ch402 draws in 40 staging-safe epochs. The largest list is 1844 of 2048 words (204 words headroom).

The independent modulated reference is:

captures/gs/silenthill3/extracted/recon/sh3_zsrt139a9_ref.png

Against the PCSX2 224139 image resized to the same 640x480 comparison grid, the fixture-only reference improves RGB MAE from 31.34 to 23.31 and RMSE from 45.94 to 33.61. This does not claim full fidelity: missing draws/fog and the perspective/XY precision deficit remain visible.

Verification

• Generator preflight: 40 epochs, max 1844 words, no staging overflow.
• Focused epoch 39 MODULATE proof:
  • old DECAL hash: 37401414dd6ee3b2888d4ddb4aff98903900475e6cf2a47d973959ee05d05fb8
  • MODULATE hash: c85edea3d42e983a31f630b7fffc1457c26233492c332860b7913c20ad7613c0
  • average nonblack RGB sum: 258.49 -> 113.23; geometry remains present.
• Focused Ch402 DECAL regression remains byte-identical at the old hash.
• Full 40-epoch RTL:
  • 649,189 fragments fed; 332,984 passed persistent Z;
  • 40 markers, 40 rises / 39 falls, zero drops/overflows/BRESP errors;
  • Z mismatch 0/307200; color mismatch 0/131618;
  • framebuffer SHA-256 d286e9d5839fc7ab63ae6bc1aa3c26fa867397171ad3d1a5e5edc3b6c5b14964;
  • PASS.
• The final shared-multiplier implementation reproduces the focused MODULATE hash exactly and its late-epoch diagnostic passes with zero errors.
• Owner Quartus 26.1 GUI fit:
  • setup worst slack +0.080 ns, TNS 0;
  • hold passes, worst reported slack 0.000 ns, TNS 0;
  • 37,353 / 46,800 ALMs, 53,238 registers, 284 / 358 RAM blocks, 4,177,064 memory bits, and 153 / 376 DSP blocks;
  • core RBF SHA-256 28659ebfccb19f03478fcd105904cc1b03195e8cc0907fad3903f56d7821975b.
• Live-board acceptance:
  • all 40 epochs completed with every texture CRC, palette checksum, staged word/record count, fresh-drain, fragment-drop, and bus-response gate clean;
  • host runner returned 0;
  • the 307,200-word board framebuffer compares byte-for-byte with RTL;
  • board and RTL framebuffer SHA-256 d286e9d5839fc7ab63ae6bc1aa3c26fa867397171ad3d1a5e5edc3b6c5b14964.

Status

Complete and accepted on silicon. The current HDMI image is the authentic vertex-color-modulated 40-epoch composite; the next fidelity work is geometry precision and remaining GS effects/draw coverage, not Ch403 correctness.