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feat(chroma): detect a green or blue screen and build the key filter

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TASK 64 units B and C — the two pure halves, no wiring yet.

- `Services/ChromaKeyDetector.cs`: samples the frame's outer ring (the screen is at the
  edges, the subject in the middle), reports Green/Blue/None plus the colour actually shot
  and a confidence. Confidence is coverage × hue tightness, which is what keeps a lawn or
  a football pitch from being offered as a keyable screen. Letterbox bars are excluded from
  the denominator rather than counted against the screen. Never decides anything: an offer
  is the creator's to accept.
- `Services/VideoFilterChain.cs`: options → `-vf` (scale → colorkey → despill → format=bgra)
  plus a `Signature` so two layers keyed identically can share one decoder. Numbers clamp and
  an unparseable colour falls back to the app default, so ffmpeg is never handed a filter it
  rejects at spawn.
- Keying is done by the bundled ffmpeg, not by us: verified against the shipped binary that
  `colorkey=…,format=bgra` emits straight-alpha BGRA (alpha 0 on keyed pixels), which is what
  the compositor already blends. No per-pixel loop lands in the 16.67 ms render budget.

Test: `ChromaKeyDetectorTests` — green, blue, letterboxed, lawn (must say None), mixed scene,
dark frame, plus the chain strings and signature sharing.

Docs: TASK 64 queued in TASKS.md with its slice table; the decisions.md chroma-key reversal
recorded as a reversal (it was buried for webcam background removal, not for supplied clips).
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gramps
2026-11-05 08:05:18 -08:00
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@@ -42,7 +42,8 @@ blocker; the smallest slice is 32·4 (fragmented MP4 on the record block).
| 53 | ✅ | Output-format contract — six size/FPS modes, vertical YouTube playback, and recordings visually accepted | [`task-53`](TASKS/task-53-output-formats.md) |
| — | ✅ | Output entitlement code: free at 30 fps / paid at 60 fps regardless of resolution; Steam App ID is still pending | [`commercial`](docs/ai/commercial.md) |
| 3·17 | ✅ | **Text layer** — content, font family/size/weight/style, colour, alignment, drag/resize, persisted (done 2026-11-04) | [`task-03`](TASKS/task-03-capture-pipeline.md) |
| 21 | ✅ | Media source: video file added from the (+) menu decodes, composites, loops, and stops with its last reference (done 2026-11-05) | [`task-21`](TASKS/task-21-media-source.md) |
| 21 | ✅ | Media source: video file added from the (+) menu decodes, composites, plays its soundtrack, loops, and stops with its last reference (done 2026-11-05) | [`task-21`](TASKS/task-21-media-source.md) |
| 64 | 🔶 **NEXT** | **Green/Blue Screen** — detect a green/blue backdrop in a clip and key it out through the decode pipe; suggest-only, opt-in auto-apply, alert clips included | [`task-64`](TASKS/task-64-green-blue-screen.md) |
| — | ☐ | Transitions **Move**: per-layer carry-across-scenes flag, per-scene placement, fixed z band | [`ui-surface`](docs/ai/ui-surface.md) |
| 55·2–3 | ☐ | Disk hygiene: delete `%TEMP%` alert clips on close + sweep at startup; low-disk guard before REC (slice 1 shipped) | [`task-55`](TASKS/task-55-disk-hygiene.md) |
| 40·A/C/D | ☐ | App Settings: active-camera picker; persist output resolution + surface transition defaults + confirm-before-End; accent consolidation | [`task-40`](TASKS/task-40-app-settings-round.md) |