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看过<<东成西就>>的小伙伴,都知道周伯通为了给师兄报仇,用了三花聚顶,不慎走火入魔,让时间倒流.于是,神奇的一幕发生了,欧阳锋想杀洪七公就是杀不死,段王爷的小便却总是解不完......看到这一幕,我就忍不住想搞一个视频逆转的方法.

其实,视频逆向播放的原理很简单.用通俗的话说,视频本质上是一张张图片连续播放出来的.所以我们只需要把组成视频的图片逆向排列一下,再播放就OK了.

但是,视频因为是由很多图片组成的,所以如果一下子把所有图片加载到内存中,然后再重排,这样1分钟的视频,就能占到500M,显然不是理想的.

<b>于是想到一种解决内存的方法:</b>

1,我们把视频进行分割成很多1秒的小视频,把这么多视频暂时放在沙盒里.

2,然后逐一的把1秒的视频进行逆向排列,然后得到很多1秒的逆向播放的视频.

3,最后,我们再把1秒的逆向的视频组合起来,完毕.

所以,只要会了分割视频,逆向视频,合并视频,就可以解决问题了.亲测,这样不管视频的长短内存可以稳定在80M左右.但是长视频处理的时间肯定很长.

<b>1,递归分割视频</b>
<pre>

• (void)trimWithAssetPath:(NSString*)assetPath startPoint:(CMTime)startPoint
  complete:(CompletePaths)complete{
  //从路径里获取视频资源
  AVAsset *asset = [AVAsset assetWithURL:[NSURL fileURLWithPath:assetPath]];

  AVAssetTrack *assetVideoTrack = nil;
  AVAssetTrack *assetAudioTrack = nil;
  //获取视频资源里的视频轨道
  if ([[asset tracksWithMediaType:AVMediaTypeVideo] count] != 0) {
  assetVideoTrack = [asset tracksWithMediaType:AVMediaTypeVideo][0];
  }
  //获取视频资源里的音频轨道
  if ([[asset tracksWithMediaType:AVMediaTypeAudio] count] != 0) {
  assetAudioTrack = [asset tracksWithMediaType:AVMediaTypeAudio][0];
  }

  NSError *error = nil;
  CMTime assetTime = [asset duration];
  CMTime sub1Time = CMTimeSubtract(startPoint,assetTime);
  if (CMTimeGetSeconds(sub1Time) == 0) {//此时已经完成了分割,退出递归.
  if(complete){
  complete(self.paths);
  }
  return;
  }
  //设置每段视频的长度为1秒,如果最后不足1秒了, intervalTime 就设置为余下的时间.
  CMTime intervalTime = CMTimeMake(assetTime.timescale, assetTime.timescale);

  CMTime endTime = CMTimeAdd(startPoint, intervalTime);

  CMTime subTime = CMTimeSubtract(endTime,assetTime);

  if (CMTimeGetSeconds(subTime) > 0){
  intervalTime = CMTimeSubtract(intervalTime,subTime);
  endTime = CMTimeAdd(startPoint, intervalTime);
  }
  //相当于创建一个空的视频
  AVMutableComposition *mutableComposition = [AVMutableComposition composition];
  AVMutableCompositionTrack *compositionVideoTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
  //开始往空的视频资源里插入我们想要时间段的视频
  [compositionVideoTrack insertTimeRange:CMTimeRangeMake(startPoint, intervalTime) ofTrack:assetVideoTrack atTime:kCMTimeZero error:&error];

  AVMutableCompositionTrack *compositionAudioTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
  //开始往空的视频资源里插入我们想要时间段的音频
  [compositionAudioTrack insertTimeRange:CMTimeRangeMake(startPoint, intervalTime) ofTrack:assetAudioTrack atTime:kCMTimeZero error:&error];
  //设置分割完之后视频放置的路径
  NSString *outPath = [kVideoPath stringByAppendingPathComponent: [NSString stringWithFormat:@"%d.mp4", self.videoNum]];
  self.videoNum++;

  NSURL *mergeFileURL = [NSURL fileURLWithPath:outPath];
  //利用AVAssetExportSession把新的小视频,输送到沙盒里
  AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mutableComposition presetName:AVAssetExportPresetHighestQuality];
  exporter.outputURL = mergeFileURL;
  exporter.outputFileType = AVFileTypeQuickTimeMovie;
  // exporter.videoComposition = mixVideoComposition;
  exporter.shouldOptimizeForNetworkUse = YES;
  [exporter exportAsynchronouslyWithCompletionHandler:^{
  //把生成的小视频路径用一个数组保存起来
  [self.paths addObject:outPath];
  //递归调用
  [self trimWithAssetPath:assetPath startPoint:endTime complete:complete];
  }];

}
</pre>
<b>2.0,递归逆向处理视频</b>
<pre>

• (void)reversePathsComplete:(CompletePaths)complete{
  if (self.videoNum == self.paths.count) {//逆向完成,退出递归.
  complete(self.paths);
  return;
  }
  NSString *path = self.paths[self.videoNum];
  AVAsset *asset = [AVAsset assetWithURL:[NSURL fileURLWithPath:path]];
  NSString *reversePath = [NSString stringWithFormat:@"reverse%d.mp4", self.videoNum];
  NSString *pathStr = [kVideoPath stringByAppendingPathComponent:reversePath];
  NSURL *outputUrl = [NSURL fileURLWithPath:pathStr];
  //这个方法就是具体怎么逆向操作视频
  [self assetByReversingAsset:asset outputURL:outputUrl complete:^(AVAsset *asset) {
  NSError *error = nil;
  //把视频逆转之后,正序的视频删除就可以了.
  [self.filemanager removeItemAtPath:self.paths[self.videoNum] error:&error];
  //把数组里正序视频的路径替换成逆序视频的路径
  [self.paths replaceObjectAtIndex:self.videoNum withObject:pathStr];
  self.videoNum++;
  [self reversePathsComplete:complete];
  }];
  }
  </pre>

<b>2.1,递归逆向处理具体实现</b>

<pre>

• (void)assetByReversingAsset:(AVAsset *)asset outputURL:(NSURL *)outputURL complete:( void (^)(AVAsset *asset))complete{
  NSError *error;

  // AVAssetReader 把视频资源读取出来
  AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
  AVAssetTrack *videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] lastObject];
  NSDictionary readerOutputSettings = [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:kCVPixelFormatType_420YpCbCr8BiPlanarFullRange], kCVPixelBufferPixelFormatTypeKey, nil];
  AVAssetReaderTrackOutput readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:videoTrack
  outputSettings:readerOutputSettings];
  [reader addOutput:readerOutput];
  [reader startReading];

  // read in the samples
  NSMutableArray *samples = [[NSMutableArray alloc] init];

  CMSampleBufferRef sample;
  //读取的每一帧放到数组里.
  while((sample = [readerOutput copyNextSampleBuffer])) {
  [samples addObject:(__bridge id)sample];
  CFRelease(sample);
  }

  // AVAssetWriter 把数组里的每一帧重组,输送到制定的沙盒文件中去
  AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:outputURL
  fileType:AVFileTypeMPEG4
  error:&error];
  NSDictionary *videoCompressionProps = [NSDictionary dictionaryWithObjectsAndKeys:
  @(videoTrack.estimatedDataRate), AVVideoAverageBitRateKey,
  nil];
  NSDictionary *writerOutputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
  AVVideoCodecH264, AVVideoCodecKey,
  [NSNumber numberWithInt:videoTrack.naturalSize.width], AVVideoWidthKey,
  [NSNumber numberWithInt:videoTrack.naturalSize.height], AVVideoHeightKey,
  videoCompressionProps, AVVideoCompressionPropertiesKey,
  nil];
  AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo
  outputSettings:writerOutputSettings
  sourceFormatHint:(__bridge CMFormatDescriptionRef)[videoTrack.formatDescriptions lastObject]];
  [writerInput setExpectsMediaDataInRealTime:NO];

  // Initialize an input adaptor so that we can append PixelBuffer
  AVAssetWriterInputPixelBufferAdaptor *pixelBufferAdaptor = [[AVAssetWriterInputPixelBufferAdaptor alloc] initWithAssetWriterInput:writerInput sourcePixelBufferAttributes:nil];

  [writer addInput:writerInput];

  [writer startWriting];
  [writer startSessionAtSourceTime:CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)samples[0])];

  // Append the frames to the output.
  // Notice we append the frames from the tail end, using the timing of the frames from the front.
  for(NSInteger i = 0; i < samples.count; i++) {
  // Get the presentation time for the frame
  CMTime presentationTime = CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)samples[i]);
  // take the image/pixel buffer from tail end of the array
  CVPixelBufferRef imageBufferRef = CMSampleBufferGetImageBuffer((__bridge CMSampleBufferRef)samples[samples.count - i - 1]);

    while (!writerInput.readyForMoreMediaData) {
        [NSThread sleepForTimeInterval:0.01];
    }
    
    [pixelBufferAdaptor appendPixelBuffer:imageBufferRef withPresentationTime:presentationTime];
  

  }

  [writer finishWritingWithCompletionHandler:^{
  complete([AVAsset assetWithURL:outputURL]);
  }];

}

</pre>

<b>3,合并视频</b>
<pre>

• (void)mergeVideosWithPaths:(NSArray *)paths outputPath:(NSString )outputPath completed:(void(^)())completed {
  if (!paths.count) return;
  AVMutableComposition mixComposition = [[AVMutableComposition alloc] init];

    AVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    
    videoTrack.preferredTransform = CGAffineTransformIdentity;
    
    for (int i = 0; i < paths.count; i++) {//遍历数组,获取每一个视频资源
        AVURLAsset *asset = [AVURLAsset assetWithURL:[NSURL fileURLWithPath:paths[i]]];
        
        AVAssetTrack *assetVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo]firstObject];
        
        NSError *errorVideo = nil;
        //把视频插入到新建的空视频资源里,每一次插入都是查在头部.就不用把数组逆向排列了.
        BOOL bl = [videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack:assetVideoTrack atTime:kCMTimeZero error:&errorVideo];
        NSLog(@"errorVideo:%@--%d",errorVideo,bl);
  

//每拼完一个小视频就直接删除
[self.filemanager removeItemAtPath:paths[i] error:&error];
}
//合并之后视频的输出路径
NSURL *mergeFileURL = [NSURL fileURLWithPath:outputPath];

    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];
    exporter.outputURL = mergeFileURL;
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    //        exporter.videoComposition = mixVideoComposition;
    exporter.shouldOptimizeForNetworkUse = YES;
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            completed();
        });
    }];

}
</pre>

<a href="https://github.com/YY415263/VideoHandler">Demo github地址</a>.如果您有更好的方法,也请不吝赐教.