# 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.