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I have a problem with the iOS SDK. I can't find the API to slowdown a video with continuous values.

I have made an app with a slider and an AVPlayer, and I would like to change the speed of the video, from 50% to 150%, according to the slider value.

As for now, I just succeeded to change the speed of the video, but only with discrete values, and by recompiling the video. (In order to do that, I used AVMutableComposition APIs.

Do you know if it is possible to change continuously the speed, and without recompiling?

Thank you very much!

Jery

darksider
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1 Answers1

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The AVPlayer's rate property allows playback speed changes if the associated AVPlayerItem is capable of it (responds YES to canPlaySlowForward or canPlayFastForward). The rate is 1.0 for normal playback, 0 for stopped, and can be set to other values but will probably round to the nearest discrete value it is capable of, such as 2:1, 3:2, 5:4 for faster speeds, and 1:2, 2:3 and 4:5 for slower speeds.

With the older MPMoviePlayerController, and its similar currentPlaybackRate property, I found that it would take any setting and report it back, but would still round it to one of the discrete values above. For example, set it to 1.05 and you would get normal speed (1:1) even though currentPlaybackRate would say 1.05 if you read it. Set it to 1.2 and it would play at 1.25X (5:4). And it was limited to 2:1 (double speed), beyond which it would hang or jump.

For some reason, the iOS API Reference doesn't mention these discrete speeds. They were found by experimentation. They make some sense. Since the hardware displays video frames at a fixed rate (e.g.- 30 or 60 frames per second), some multiples are easier than others. Half speed can be achieved by showing each frame twice, and double speed by dropping every other frame. Dropping 1 out of every 3 frames gives you 150% (3:2) speed. But to do 105% is harder, dropping 1 out of every 21 frames. Especially if this is done in hardware, you can see why they might have limited it to only certain multiples.

Glen Cunningham
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  • One more thing... The way a video editing program can provide more fine control of the relative speed is by doing interpolation between frames to resample the video at a different rate. That's what was happening when you recompiled the video. But the complex calculations involved take a lot of processing power to do live (if it can even keep up), so the players generally have just discrete speed ratios based on simply dropping or doubling up on some of the video frames. – Glen Cunningham Aug 20 '16 at 11:30