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I'm implementing video recording with Android camera. I use MediaCodec to encode frames from onPreviewFrame() callback.

Now I want to record slow motion video. How can I set the camera capture rate or frame rate? Where can I set these parameters, for example, if I want to record at 120fps, and play at 30fps?

The following is what I have researched:

I recorded a slow-motion video with MOTO X (having built-in slow-motion video recording function) and used FFmpeg to check its fps. I found it records at 110 fps(near 120fps) and playback at 30fps. This proves that it provides hardware support for recording at high fps.

However, when I use getSupportedPreviewFpsRange to check its supported fps range, there are only 4 sets supported fps range with no one greater than 30fps:

min_fps: 15.0 , max_fps15.0

min_fps: 15.0 , max_fps20.0

min_fps: 15.0 , max_fps30.0

min_fps: 24.0 , max_fps30.0

I used setPreviewFpsRange to set the fps higher, but it remains no more than 30fps. I also tried set KEY_FRAME_RATE and KEY_CAPTURE_RATE of MediaFormat. But it still does not work.

I have also searched the solution by setting CamcorderProfile or setVideoFrameRate with MediaRecorder. But I'm doing with MediaCodec.

Can anyone help me with this problem?

Community
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Zebin Xu
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3 Answers3

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Capturing video frames at a higher rate doesn't address the problem. If you capture at 120fps, and play back at 120fps, you have high-speed video playing at a normal rate.

What you want to do is modify the timestamps. MediaCodec supports VFR (variable frame-rate) video. Rather than spending 1 second recording 60 frames that will be played back back at 30fps, you record 30 frames per second that will play back at 15fps.

The H.264 stream generated by MediaCodec's AVC encoder does not include timestamps at all. You specify the timestamp for each frame when you feed the output to MediaMuxer, generating the .mp4 file. Most sample code will simply take the timestamp obtained from Camera and pass it through MediaCodec to MediaMuxer unmodified, but you are allowed to tweak it. The only thing you can't do is allow the timestamp to go backward.

If you look at the way the "eight rects" movie is generated in Grafika, you can see that it plays with the output timestamp to make the movie play slower or faster.

Another easy way to accomplish the same thing without varying the frame rate is to double the frames. For the slow-motion portion, simply hand the same frame to the encoder multiple times. Again, you need to tweak the presentation timestamps for each frame, but your video will have a consistent 30fps. (It would look better if you interpolated frames, but that's a lot harder.)

fadden
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  • Thanks for your answer! But I think the video played back at 15fps may be laggy. If the video captured at 60fps or more, and playback at 30fps will seem to be a better choice. I really wonder if there is any way to truly record video at higher frame rate? – Zebin Xu Nov 30 '15 at 06:09
  • @ZebinXu: That is determined by the video source (i.e. Camera), not MediaCodec. To get 30fps slow-motion, you'd need to tell the camera to double the frame capture rate while simultaneously modifying the timestamps fed to MediaMuxer. The only interest MediaCodec has in the frame timings is when attempting to limit the stream bit rate to a specific value... if you say you want 8Mbps, it needs to be able to tell how many frames appear in a second, and it can use the frame timestamps to figure that out. – fadden Nov 30 '15 at 16:34
  • Yes, thank you. But my point is why I cannot capture high rate video by using API or other code from a device with hardware high rate capturing support? – Zebin Xu Dec 01 '15 at 02:31
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for slow recording.

mMediaRecorder.setVideoFrameRate(QUALITY_HIGH_SPEED_LOW);

for high speed recording. 

mMediaRecorder.setVideoFrameRate(QUALITY_HIGH_SPEED_HIGH);
vishal jangid
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I think this question encloses two separate problems.

  1. How can I activate a high-speed capture mode which is faster than in 60FPS?
  2. How can I slow down the FPS generated by the MediaCodec encoder?

The second part is simple. As described in this answer, you can either insert interpolating/dummy frames, or modify time stamps. The first part is peculiar in that most sample codes deal with normal mode of capturing rather than high-speed captures required in this use case (usually higher than 60FPS, and generally speaking, we are dealing with 120~240FPS.)

About the first part of the question, I would like you to take a look at the description about Camera2 constrained high-speed capture APIs, like this. Unfortunately, there seem to be only niche demand on such an API and I was not able to gather much information upon my own findings, and I am currently dealing with mentally taxing projects myself to create a sample code by myself. However, judging from its similarity with ordinary movie recording and burst shots, I don't think deriving such a use case would be that much complex.

Arle Camille
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