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[Docs] (tools/trace_replay): localise the improved-transparency ANGLE divergence to the OIT composite

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swung0x48
2026-08-11 21:55:40 -04:00
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@@ -324,3 +324,33 @@ draw-buffer slot restriction is already handled by
means bisecting inside the frame to find which GLES construct ANGLE mistranslates;
the accommodation, once known, belongs with the existing
`--avoid-angle-llvmpipe-*` flags rather than in shared backend code.
#### Where in the frame it goes wrong
Snapshotting the same intra-frame call points on both stacks (`--target-call`,
ten replays in parallel, logs to `/dev/null`) localises it. SSIM against the
golden at each point:
| target call | what runs there | Mesa | ANGLE | gap |
| --- | --- | --- | --- | --- |
| 2667445 | last opaque draw, into FBO 29 | 0.146477 | 0.000195 | - |
| 2667488 | **OIT composite**: fullscreen triangle, program 50, into FBO 3 | 0.976448 | 0.945409 | 0.031 |
| 2667543 | post draw, program 56 | 0.985380 | 0.954480 | 0.031 |
| 2667595 | post draw, program 64 | 0.985685 | 0.954788 | 0.031 |
| 2667619 | golden point | 1.000000 | 0.970127 | 0.030 |
The gap opens at the composite and is then constant to four decimal places -
every pass after 2667488 contributes the same increment on both drivers. So
nothing downstream of the composite is implicated, and the GUI/post chain is
fine. The two stacks already disagree at 2667445, before the composite runs,
which points at the translucent accumulation targets the composite samples
rather than at the composite draw itself.
That is as far as this went. What is still unknown is **which** GLES construct
in the accumulation chain ANGLE translates differently - naming it needs the
intermediate FBO attachments dumped either side of 2667488 on both stacks,
which the replay CLI cannot currently do (it snapshots one framebuffer, not all
attachments). Until it is named there is nothing to put behind an
`--avoid-angle-llvmpipe-*` flag and nothing precise enough to file upstream, so
the fixture stays red on the Android DirectGLES lane rather than being papered
over.