AVFoundation - 反转AVAsset并输出视频文件

Question

我已经看过几次这个问题了,但是他们似乎没有任何合适的答案.

要求是反转并输出视频文件(不仅仅是反向播放),保持与源视频相同的压缩,格式和帧速率.

理想情况下,解决方案能够在内存或缓冲区中完成所有操作,并避免将帧生成到图像文件中(例如:使用AVAssetImageGenerator),然后重新编译它(资源密集,不可靠的计时结果,帧/图像质量从原始,等等.).

-

我的贡献:这仍然没有用,但到目前为止我尝试过的最好:

• 将示例帧读入一个CMSampleBufferRef[]使用的数组AVAssetReader.
• 使用相反的顺序将其写回AVAssetWriter.
• 问题:似乎每个帧的时间都保存在CMSampleBufferRef所以即使向后追加它们也行不通.
• 接下来,我尝试用反向/镜像帧交换每帧的定时信息.
• 问题:这会导致未知错误AVAssetWriter.

• 下一步:我要调查一下 AVAssetWriterInputPixelBufferAdaptor

  - (AVAsset *)assetByReversingAsset:(AVAsset *)asset {
      NSURL *tmpFileURL = [NSURL URLWithString:@"/tmp/test.mp4"];    
      NSError *error;
  
      // initialize the AVAssetReader that will read the input asset track
      AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
      AVAssetTrack *videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] lastObject];
  
      AVAssetReaderTrackOutput* readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:videoTrack outputSettings:nil];
      [reader addOutput:readerOutput];
      [reader startReading];
  
      // Read in the samples into an array
      NSMutableArray *samples = [[NSMutableArray alloc] init];
  
      while(1) {
          CMSampleBufferRef sample = [readerOutput copyNextSampleBuffer];
  
          if (sample == NULL) {
              break;
          }
  
          [samples addObject:(__bridge id)sample];
          CFRelease(sample);
      }
  
      // initialize the the writer that will save to our temporary file.
      CMFormatDescriptionRef formatDescription = CFBridgingRetain([videoTrack.formatDescriptions lastObject]);
      AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:nil sourceFormatHint:formatDescription];
      CFRelease(formatDescription);
  
      AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:tmpFileURL
                                                        fileType:AVFileTypeMPEG4
                                                           error:&error];
      [writerInput setExpectsMediaDataInRealTime:NO];
      [writer addInput:writerInput];
      [writer startSessionAtSourceTime:CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)samples[0])];
      [writer startWriting];
  
  
      // Traverse the sample frames in reverse order
      for(NSInteger i = samples.count-1; i >= 0; i--) {
          CMSampleBufferRef sample = (__bridge CMSampleBufferRef)samples[i];
  
          // Since the timing information is built into the CMSampleBufferRef 
          // We will need to make a copy of it with new timing info. Will copy
          // the timing data from the mirror frame at samples[samples.count - i -1]
  
          CMItemCount numSampleTimingEntries;
          CMSampleBufferGetSampleTimingInfoArray((__bridge CMSampleBufferRef)samples[samples.count - i -1], 0, nil, &numSampleTimingEntries);
          CMSampleTimingInfo *timingInfo = malloc(sizeof(CMSampleTimingInfo) * numSampleTimingEntries);
          CMSampleBufferGetSampleTimingInfoArray((__bridge CMSampleBufferRef)sample, numSampleTimingEntries, timingInfo, &numSampleTimingEntries);
  
          CMSampleBufferRef sampleWithCorrectTiming;
          CMSampleBufferCreateCopyWithNewTiming(
                                                kCFAllocatorDefault,
                                                sample,
                                                numSampleTimingEntries,
                                                timingInfo,
                                                &sampleWithCorrectTiming);
  
          if (writerInput.readyForMoreMediaData)  {
              [writerInput appendSampleBuffer:sampleWithCorrectTiming];
          }
  
          CFRelease(sampleWithCorrectTiming);
          free(timingInfo);
      }
  
      [writer finishWriting];
  
      return [AVAsset assetWithURL:tmpFileURL];
  }
  

Answer 1

在过去的几天里完成了这项工作并且能够使其正常运行.

源代码在这里:http://www.andyhin.com/post/5/reverse-video-avfoundation

用于AVAssetReader读出样本/帧,提取图像/像素缓冲区,然后将其与镜像帧的显示时间一起附加.