# Ch425 — post-process bisection closeout

## Objective

Append the captured post-process families to the accepted Ch424 scene and
admit them only if the production replay stays exact and the PCSX2 fidelity
score improves.

## Candidates and isolation

`zsrt139f20` concatenated the 125-epoch Ch424 scene, the 20-epoch PSMCT32
darken pass, the 212-epoch Ch405 PSMT8 alpha-fan fixture, and the 18-epoch
PSMT4 pass.  The 375-epoch production replay was functionally clean but its
frame was catastrophically white (mapped MAE 165.23 / RMSE 189.17), so it was
not deployed.

Epoch-boundary replays isolated each family:

- Ch424 + CT32 darken: mapped MAE 21.3646 / RMSE 30.1358, worse than the
  Ch424 baseline 20.9469 / 29.7626.
- Ch424 + CT32 + old alpha fans: mapped MAE 161.9749 / RMSE 185.4484; this is
  the white-frame source.
- Ch424 + PSMT4 only (`zsrt139f21`): mapped MAE 23.4472 / RMSE 32.3453 and
  most scene detail was erased.

Every isolation replay had zero fragment drops and exact RTL Z/color
scoreboards.  The problem was therefore captured-state interpretation, not
timing, loss, or board nondeterminism.  Ch425 rejected all three fixtures as
generated and left the live board on Ch424 while the alpha source was audited.

## Status

Closed as a diagnostic chapter.  Ch426 supersedes the alpha result after
repairing the fixture's RGBAQ alpha packing.