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gramps ea24405ba2 feat(chroma): offer to remove a green or blue screen, and key it in the decode

TASK 64 slices D and E: the feature is usable end to end.

- Add a video file → one frame is probed (new `IChromaProbe`/`FfmpegChromaProbe`, one ffmpeg
  pass at 160x90, off the UI thread) and the layer gets an offer: "Looks like a green screen —
  Remove it", with the measured colour as a swatch.
- Accepting it turns the offer into settings (Amount / Softness / Fringe) that reach ffmpeg as
  `colorkey` + `despill` + `format=bgra`, so the key is computed in the pipe and both consumers
  (the shared preview bitmap and the compositor's frame) carry the same alpha.
- Media sessions are now keyed by **path + look** (`MainViewModel.MediaSessionKey`), so two
  layers with the same settings still share one decoder, and changing the look releases the old
  session and starts the new one. Removal, load reconciliation, loop and audio settings all move
  to the same key.
- Settings persist on the `Source` row (six ALTER TABLE guards + SELECT/INSERT), so a keyed layer
  comes back keyed.
- Panel section is plain creator words: Green/Blue Screen, Amount, Softness, Fringe.

Test: `ChromaKeyApplyTests` — the pipe asks ffmpeg to key (and does not when off); a green-screen
clip is offered, accepting it starts a keyed session, the layer's resolver then reads that session
rather than the old one; the setting survives a save.

Docs: TASK 64 D/E ticked with slice F (auto-apply setting + alert clips) left open; frame-pipeline
records the decode-option design; HANDOFF rewritten. verify.sh: 544 passed, 0 failed, 0 warnings.

2026-11-05 08:15:32 -08:00

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TASK 64 — Green/blue screen (chroma) for video clips

Catalog: TASKS.md — status and requirements live here. Queued 2026-11-05 at the creator's direction, straight after TASK 21's audio slice.

Goal: a creator who drops in a clip shot against a green or blue screen gets the background removed — without knowing what "chroma key", "similarity" or "spill" mean. The app notices the screen and offers to remove it; the creator never has to pick a colour.

Why this is not the retired "colour-key" item: docs/ai/decisions.md buried chroma key in the webcam background-removal context (ML model, lighting, NVIDIA gives it free — support hell). This is a different job: the creator supplies the clip, ffmpeg does the arithmetic, there is no model and no lighting to blame. The reversal is recorded there.

0. What already exists (verified 2026-11-05, bundled ffmpeg N-126342)

colorkey / chromakey (colour, similarity, blend) and despill (green/blue, mix) are all present in the shipped binary and slice-threaded.
A probe of the bundled binary showed keyed pixels come out of the pipe with alpha 0 and straight (un-premultiplied) colour — colorkey=…,format=bgra → 63 175 0 0 vs 63 175 0 255 unkeyed. The compositor already blends straight alpha, and media frames are not flagged IsOpaque, so no compositor change is needed.
Consequence: keying costs zero app-side CPU — it is one more term in the decode pipe's -vf, which is also the only place that feeds both consumers (the shared preview bitmap and the compositor's LatestFrame). A C# post-process would have to be wired twice.

1. Decisions (creator, 2026-11-05)

Suggest, never override. Detection offers; the creator clicks. Opt-in setting for auto-apply.
Green and blue. The keyed colour is the detected screen colour (real screens read 0x00B140, not 0x00FF00), not a canonical green.
"Auto Green/Blue Screen" is the setting name. No engineering words in the UI (never "chroma", never "colour key"): the panel section is Green/Blue Screen, the sliders are Amount, Softness, Fringe.
A clip is a clip — alert clips get the same treatment from the same settings block.
High confidence only may auto-apply; medium shows the offer; low says nothing.
Settings are per layer (they belong to the layer in the UI). Sessions are keyed by path + settings signature, so two layers with identical settings still share one decoder.

2. Slices (one Good Dog each)

#	Slice	Test
A	✅ media audio + shared clip pipe (TASK 21)	shipped `2670ac0`
B	✅ Pure `Services/ChromaKeyDetector.cs`: sample a `VideoFrame` → green / blue / none + suggested colour + confidence	synthetic frames: green, blue, gradient, letterboxed, lawn (must say none)
C	✅ Pure `Services/VideoFilterChain.cs`: options → `-vf` string (`scale → colorkey → despill → format=bgra`)	exact string assertions (encoder `FfmpegArgs` style)
D	✅ Detect on add: one `-frames:v 1` probe (mirror `IFrameRateProbe`) → suggestion in the panel; no rendering change	add a green-screen file → suggestion appears with its colour
E	✅ Apply: session key `path	key-settings`, filter chain in the pipe, level/mute untouched
F	☐ Remaining: "Auto Green/Blue Screen" opt-in in App Settings (high confidence only), alert clips wired to the same settings block, spill quality pass	slider reaches the chain and the restart preserves loop state

3. Landmines for whoever builds this

Two consumers. The preview bitmap and LatestFrame fork inside MediaVideoSourceManager; keying must happen upstream (in the pipe), or one path shows green.
Parameter change = decoder restart (-vf is fixed at spawn). Debounce, and carry loop + refcount across the restart instead of tearing the session down.
Session identity. ResolveOutputFrame and the preview-bitmap adoption must derive the session key from the same helper as the panel, or a keyed and an unkeyed layer on one file fight over one decoder.
Detection is a second reader of the file — one frame, async, never on the UI thread; a failed probe means "no suggestion", never a broken layer.
Letterboxing (black bars) breaks naive border sampling — detect and exclude them first.

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