AVFoundation - 反轉AVAsset並輸出視頻文件

Question

我看過這個問題問過幾次，但沒有一個似乎有任何工作的答案。

要求是扭轉和輸出的視頻文件（不只是反向播放它）保持相同的壓縮，格式，和幀速率作爲源視頻。

理想情況下，解決方案將能夠在內存或緩衝區中完成所有操作，並避免將幀生成爲圖像文件（例如：使用AVAssetImageGenerator），然後重新編譯它（資源密集，不可靠的時序結果，幀/原始圖像質量等）。

-

我的貢獻： 這仍無法正常工作，但最好我試過到目前爲止：

• 讀取樣品架使用AVAssetReader的CMSampleBufferRef[]的數組。
• 使用AVAssetWriter以相反順序將其寫回。
• 問題：好像每個幀定時被保存在CMSampleBufferRef因此，即使追加它們向後將不起作用。
• 接下來，我嘗試用反向/鏡像幀交換每個幀的時間信息。
• 問題：這導致與AVAssetWriter未知的錯誤。

• 下一步：我要去看看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

在過去幾天裏從事過這一點，並能得到它的工作。

源代碼在這裏：http://www.andyhin.com/post/5/reverse-video-avfoundation

用途AVAssetReader讀出樣本/幀，提取圖像/像素緩衝器，然後與反射鏡幀的呈現時間追加它。