AVFoundation variable frame rate output

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Created Dec ’20

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Does AVFoundation exports only have fixed frame rate output in presets other than passthrough? In other words, do AVAssetExportSession & AVAssetWriter/Reader output/input ever output variable frame rate video? If input video was 29.43 fps, I see output video is always 30 fps when I render using the above two interfaces. Looks like AVF applies a default video composition while rendering.

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Answer 1

Media Engineer OP

Apple

Dec ’20

The answer likely varies on a case-by-case basis. For example, with AVAssetReader or AVAssetWriter are used with nil output settings, the timing (and format, and just about everything else) of the source frames is preserved.

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Answer 2

Devgeek OP

Dec ’20

Thanks for the response. So just to give a background of the issue, I am implementing an editing timeline to mix videos & photos which can be zoomed in/out. At the highest zoom level you can scrub through individual frames of AVComposition. So I was thinking of mapping individual points in timeline to time in composition, where time is measured in number of frames, not seconds or milliseconds. I am not sure if this model of representation of time is reliable enough with variable frame rate videos, even if I set AVVideoComposition frameDuration property to CMTime(1,30) or CMTime(1,25).

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