利用MediaExtractor和MediaMuxer实现视频剪切

客户要在Android手机上做个能视频剪切的app，由于视频源只是MP4，所以就想到了用MediaExtractor和MediaMuxer来实现功能，直接上代码。

public class VideoDecoder {private final static String TAG = "VideoDecoder";private MediaCodec mediaDecoder;private MediaExtractor mediaExtractor;private MediaFormat mediaFormat;private MediaMuxer mediaMuxer;private String mime = null;public boolean decodeVideo(String url, long clipPoint, long clipDuration) {int videoTrackIndex = -1;int audioTrackIndex = -1;int videoMaxInputSize = 0;int audioMaxInputSize = 0;int sourceVTrack = 0;int sourceATrack = 0;long videoDuration, audioDuration;//创建分离器mediaExtractor = new MediaExtractor();try {//设置文件路径mediaExtractor.setDataSource(url);//创建合成器mediaMuxer = new MediaMuxer(url.substring(0, url.lastIndexOf(".")) + "_output.mp4", OutputFormat.MUXER_OUTPUT_MPEG_4);} catch (Exception e) {Log.e(TAG, "error path" + e.getMessage());}//获取每个轨道的信息for (int i = 0; i < mediaExtractor.getTrackCount(); i++) {try {mediaFormat = mediaExtractor.getTrackFormat(i);mime = mediaFormat.getString(MediaFormat.KEY_MIME);if (mime.startsWith("video/")) {sourceVTrack = i;int width = mediaFormat.getInteger(MediaFormat.KEY_WIDTH);int height = mediaFormat.getInteger(MediaFormat.KEY_HEIGHT);videoMaxInputSize = mediaFormat.getInteger(MediaFormat.KEY_MAX_INPUT_SIZE);videoDuration = mediaFormat.getLong(MediaFormat.KEY_DURATION);//检测剪辑点和剪辑时长是否正确 if (clipPoint >= videoDuration) {Log.e(TAG, "clip point is error!");return false;}if ((clipDuration != 0) && ((clipDuration + clipPoint) >= videoDuration)) {Log.e(TAG, "clip duration is error!");return false;}Log.d(TAG, "width and height is " + width + " " + height+ ";maxInputSize is " + videoMaxInputSize+ ";duration is " + videoDuration);//向合成器添加视频轨videoTrackIndex = mediaMuxer.addTrack(mediaFormat);}else if (mime.startsWith("audio/")) {sourceATrack = i;int sampleRate = mediaFormat.getInteger(MediaFormat.KEY_SAMPLE_RATE);int channelCount = mediaFormat.getInteger(MediaFormat.KEY_CHANNEL_COUNT);audioMaxInputSize = mediaFormat.getInteger(MediaFormat.KEY_MAX_INPUT_SIZE);audioDuration = mediaFormat.getLong(MediaFormat.KEY_DURATION);Log.d(TAG, "sampleRate is " + sampleRate+ ";channelCount is " + channelCount+ ";audioMaxInputSize is " + audioMaxInputSize+ ";audioDuration is " + audioDuration);//添加音轨audioTrackIndex = mediaMuxer.addTrack(mediaFormat);}Log.d(TAG, "file mime is " + mime);} catch (Exception e) {Log.e(TAG, " read error " + e.getMessage());}}//分配缓冲ByteBuffer inputBuffer = ByteBuffer.allocate(videoMaxInputSize);//根据官方文档的解释MediaMuxer的start一定要在addTrack之后mediaMuxer.start();//视频处理部分mediaExtractor.selectTrack(sourceVTrack);MediaCodec.BufferInfo videoInfo = new MediaCodec.BufferInfo();videoInfo.presentationTimeUs = 0;long videoSampleTime;//获取源视频相邻帧之间的时间间隔。(1){mediaExtractor.readSampleData(inputBuffer, 0);//skip first I frameif (mediaExtractor.getSampleFlags() == MediaExtractor.SAMPLE_FLAG_SYNC)mediaExtractor.advance();mediaExtractor.readSampleData(inputBuffer, 0);long firstVideoPTS = mediaExtractor.getSampleTime();mediaExtractor.advance();mediaExtractor.readSampleData(inputBuffer, 0);long SecondVideoPTS = mediaExtractor.getSampleTime();videoSampleTime = Math.abs(SecondVideoPTS - firstVideoPTS);Log.d(TAG, "videoSampleTime is " + videoSampleTime);}//选择起点mediaExtractor.seekTo(clipPoint, MediaExtractor.SEEK_TO_PREVIOUS_SYNC);while (true) {int sampleSize = mediaExtractor.readSampleData(inputBuffer, 0);if (sampleSize < 0) {//这里一定要释放选择的轨道，不然另一个轨道就无法选中了mediaExtractor.unselectTrack(sourceVTrack);break;}int trackIndex = mediaExtractor.getSampleTrackIndex();//获取时间戳long presentationTimeUs = mediaExtractor.getSampleTime();//获取帧类型，只能识别是否为I帧int sampleFlag = mediaExtractor.getSampleFlags();Log.d(TAG, "trackIndex is " + trackIndex+ ";presentationTimeUs is " + presentationTimeUs+ ";sampleFlag is " + sampleFlag+ ";sampleSize is " + sampleSize);//剪辑时间到了就跳出if ((clipDuration != 0) && (presentationTimeUs > (clipPoint + clipDuration))) {mediaExtractor.unselectTrack(sourceVTrack);break;}mediaExtractor.advance();videoInfo.offset = 0;videoInfo.size = sampleSize;videoInfo.flags = sampleFlag;mediaMuxer.writeSampleData(videoTrackIndex, inputBuffer, videoInfo);videoInfo.presentationTimeUs += videoSampleTime;//presentationTimeUs;}//音频部分mediaExtractor.selectTrack(sourceATrack);MediaCodec.BufferInfo audioInfo = new MediaCodec.BufferInfo();audioInfo.presentationTimeUs = 0;long audioSampleTime;//获取音频帧时长{mediaExtractor.readSampleData(inputBuffer, 0);//skip first sampleif (mediaExtractor.getSampleTime() == 0)mediaExtractor.advance();mediaExtractor.readSampleData(inputBuffer, 0);long firstAudioPTS = mediaExtractor.getSampleTime();mediaExtractor.advance();mediaExtractor.readSampleData(inputBuffer, 0);long SecondAudioPTS = mediaExtractor.getSampleTime();audioSampleTime = Math.abs(SecondAudioPTS - firstAudioPTS);Log.d(TAG, "AudioSampleTime is " + audioSampleTime);}mediaExtractor.seekTo(clipPoint, MediaExtractor.SEEK_TO_CLOSEST_SYNC);while (true) {int sampleSize = mediaExtractor.readSampleData(inputBuffer, 0);if (sampleSize < 0) {mediaExtractor.unselectTrack(sourceATrack);break;}int trackIndex = mediaExtractor.getSampleTrackIndex();long presentationTimeUs = mediaExtractor.getSampleTime();Log.d(TAG, "trackIndex is " + trackIndex+ ";presentationTimeUs is " + presentationTimeUs);if ((clipDuration != 0) && (presentationTimeUs > (clipPoint + clipDuration))) {mediaExtractor.unselectTrack(sourceATrack);break;}mediaExtractor.advance();audioInfo.offset = 0;audioInfo.size = sampleSize;mediaMuxer.writeSampleData(audioTrackIndex, inputBuffer, audioInfo);audioInfo.presentationTimeUs += audioSampleTime;//presentationTimeUs;}//全部写完后释放MediaMuxer和MediaExtractormediaMuxer.stop();mediaMuxer.release();mediaExtractor.release();mediaExtractor = null;return true;}
}
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这里要对(1)进行一下解释，如果视频中带了B帧，源视频时间戳会按照播放顺序来打，这样通过读每帧获取到的时间戳就不是一直增长的，MediaMuxer的writeSampleData函数在写每帧的时间戳时如果时间戳不是增长顺序就会报错，帧率也无法通过MediaFormat.KEY_FRAME_RATE来获取，也可以通过查找MP4文件的stsz关键字来获取总帧数，进而算出帧率，来按帧率给每一帧打时间戳，但是stsz一般都在文件的后面而且含有音视频的文件会有两个stsz，如果文件很大，查找起来就会很慢，所以这里我用了个偷懒的办法，用I帧后面两帧的时间戳绝对差值来当做每帧的时间，然后+=这个时间来为写入的帧打时间戳，虽然时间戳不是源视频的顺序了，但并不影响解码。 
还有一点就是unselectTrack这个函数，用selectTrack选择轨道后，如果要切换轨道一定要调用unselectTrack函数来释放轨道，不然读的还是之前的轨道。