Reported as "the default video and the green screen video are playing at a hideously slow
rate… I thought the playback timing issues were resolved?!" They were not: two independent
faults, both in the shared clip pipe.
1. QUADRATIC COPY IN THE FRAME READER. `RawVideoFrameReader` re-concatenated the whole pending
buffer on every `Feed`, and the pipe feeds it 64 KB at a time. A 1080p BGRA frame is 8 MB, so
one frame cost ~127 copies of a growing buffer — roughly half a gigabyte of memcpy per frame.
Measured on a 30 fps clip: 6 frames in 3 seconds (2 fps). The reader now fills one preallocated
frame buffer and hands that array to the VideoFrame, so a frame is copied exactly once. Pipe
reads are 256 KB. Same clip: 29 fps.
2. PACING SLEPT ON TOP OF THE WORK. A fixed 1/fps sleep after each frame adds the frame's own
cost, so 20 ms of work on a 30 fps clip yields 1/(33+20) ≈ 19 fps — measured 20.9. Pacing is
now deadline-based (`Helpers/FramePacer.cs`): wait only what is left of the frame's interval,
resync when a frame overruns instead of bursting, never sleep longer than one interval.
The alert clip shares this pipe, so it gained both fixes.
Tests: `FramePacerTests` (subtract work, no drift, late → resync, zero duration, reset);
`ChromaKeyRealClipTests.RealClip_PlaysAtItsNativeRate` (throughput floor of 15 fps on the real
30 fps clip — the old code sat at 2); the two pacing assertions now check "at most one interval"
rather than an exact sleep, which is what deadline pacing means. Full suite: 574 passed,
0 failed, clean build.
Three reports from the creator, plus two bugs the real clip exposed.
1. A video layer was INVISIBLE in the preview. The pane's per-layer Image is Collapsed by
default and switched on by a whitelist of DataTriggers; MediaSource matched none, so the clip
decoded into a bitmap that nothing displayed. Same defect the alert box hit on 2026-09-26 and
the scoreboard after it — now recorded at the top of docs/ai/ui-surface.md with a test that
asserts the bound Image is Visible (`MediaPreviewDisplayTests`).
2. Play is no longer tied to Loop: the button is always enabled (creator ruling — Loop must not
toggle Play). Loop still restarts a finished clip, which is the behaviour that was broken.
3. Geometry is now the LAST section of the properties list for every draggable element, with
opacity at the top; the web source's duplicate geometry block is gone (it now uses the shared
one, which had been sitting above the media panel).
4. The physical mouse-drag test is deleted. It injected real cursor movement, hijacked whatever
machine ran it, and failed whenever the desktop was busy. The arithmetic it covered now lives
in `Helpers/DragReorderMath.cs` — pure, and covered by `DragReorderMathTests` (threshold,
up/down moves, no-op and out-of-range). The two persistence tests were already synthetic.
5. Real-clip check (creator's `test_greenscreen_moving_subject.mp4`, 1920×1080): the detector
REJECTED it — a fully saturated green sits at value 1.0 and the detector's 0.98 brightness
ceiling threw it away. White highlights are excluded by the saturation floor, which is the
property that actually separates them, so the ceiling is gone. With it fixed, the clip is
detected as green (high confidence), and keyed through our own pipe 97.8% of pixels go
transparent. Measured: keying with the canonical #00B140 keys 0.0% of this clip — which is why
the key colour comes from the measured screen.
Tests: 567 passed, 0 failed, clean build 0 warnings. The gate's scope check flags
`Assets/Videos/test_greenscreen_moving_subject.mp4` — the creator's working copy, deliberately not
committed; the fixture lives under ytLive.Tests/Fixtures.
gramps