在Android上使用MediaCodec录制慢动作视频

Question

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?

Answer 1

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.)

Answer 2

for slow recording.

mMediaRecorder.setVideoFrameRate(QUALITY_HIGH_SPEED_LOW);

for high speed recording.

mMediaRecorder.setVideoFrameRate(QUALITY_HIGH_SPEED_HIGH);