Per Miguel's review: the previous fixture had no body / root background, so it
passed against both the buggy and fixed code. The fix this PR makes (the
initTransparentBackground stylesheet injection in initializeSession) only
matters when a composition paints over the CDP default-background-color
override — exactly what we tell users not to do, but exactly what a
regression test must do.
Reproduced locally:
- base SHA (2935be6): pixel (10,10) decodes as rgba [16,16,16,255]
(opaque #111 body bg leaks through the pre-navigation override that
Chrome resets on goto)
- this head: pixel (10,10) decodes as rgba [0,0,0,0]
(initTransparentBackground injects [data-composition-id]{background:transparent !important}
AFTER navigation, force-overriding the body bg)
The pixel-level assertions in transparency-test.ts are unchanged — they
already require alpha=0 at (10,10). With the body bg painted, that
assertion now fails on any code path that doesn't actually preserve alpha
end-to-end.
Extends RenderConfig.format with "png-sequence" and patches two correctness
gaps so the existing "webm" / "mov" values actually preserve the alpha
channel end-to-end.
Engine fixes:
- screenshotService.pageScreenshotCapture: drop optimizeForSpeed for PNG
captures. The fast path uses an alpha-unaware codec that crushes real
alpha values; kept for opaque jpeg captures where it is harmless.
- frameCapture: replace the inline setDefaultBackgroundColorOverride
block (which fired pre-navigation and was reset by page.goto) with a
proper initTransparentBackground() call inside initializeSession,
after the window.__hf readiness poll. This also injects the
html/body/[data-composition-id]{background:transparent !important}
stylesheet so compositions with custom body / #root backgrounds do not
defeat the override. Wired into both screenshot-mode and beginframe-mode
branches.
Producer:
- RenderConfig.format extended to "mp4" | "webm" | "mov" | "png-sequence"
with full JSDoc.
- Streaming encode is bypassed for png-sequence (frames go straight to
disk). FORMAT_EXT extended.
- New Stage-5 png-sequence branch: mkdir outputPath, copy captured PNGs as
frame_NNNNNN.png, copy audio.aac sidecar when audio is present.
- Stage-6 mux/faststart and the debug copy are wrapped in !isPngSequence.
- README.md: new "Transparent Video Output" section.
Tests:
- New fixture tests/transparency-regression/ tagged "transparency".
- New tsx script src/transparency-test.ts asserts pixel-level alpha for
webm + png-sequence outputs. Wired as "test:transparency".
- Default "test" / "test:update" scripts pass --exclude-tags transparency
so the golden-MP4 harness ignores the new fixture.
Verified locally on macOS arm64: typecheck clean across engine + producer,
producer renderOrchestrator vitest 10/10, transparency-test passes for
both webm and png-sequence with end-to-end pixel assertions.
Youssef Toufik