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you'd have to "rebase" all the other frames to be derived from those

You wouldn't have to any more than you need 0 through N frames in memory to calculate frame N+1. Whatever your decoding state completes at frame N can be considered a key frame.

decoding state can be bulkier than a key frame and opaque to the CPU if hardware acceleration is used

I wonder if caching semi-compressed frames would be more efficient in either case (CPU or GPU)

It doesn’t have to be every frame though. Pre calculate to 25%/50%/75%, then as I’m playing, save key frames for more incremental points, and if I start scrubbing, calculate more around that region.

Edit: this doesn’t have to happen synchronously either, it can occur in a background thread or passively.

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