Video frame callbacks
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Overlay draws
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Display refresh
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Draw phase
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Prediction error
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Augmentation
– (test pattern)

This is the situation with DRM: the video is an ordinary video element the page cannot read, and the overlay is a separate canvas on top of it. PLAYER and the strip along the top edge are the overlay; SOURCE and the strip along the bottom edge are in the video. They match only when both reached the screen in the same refresh.

The page cannot measure this itself. Run npm run stress:overlay, which reads both strips from what the compositor produced. Depth and phase apply to the two media clock methods, lead to the other two.

Draw phase is where in the refresh cycle the overlay's draws finished, from one refresh (left) to the next (right). A single peak means the draws land at the same point every time; a flat line means it is chance which refresh picks a draw up. Prediction error is how far the media clock, knowing only the previous video frame callback, was from the frame the next callback then reported.