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 frame
if (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 sample
if (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MediaExtractor
mediaMuxer.stop();
mediaMuxer.release();
mediaExtractor.release();
mediaExtractor = null;
return true;
}
}

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。