如何同时使用 AVAssetReader 和 AVAssetWriter 处理多个轨道（音频和视频）？

Question

我知道如何使用 AVAssetReader 和 AVAssetWriter，并成功使用它们从一部电影中获取视频轨道并将其转码为另一部电影。不过，我也想用音频来做到这一点。完成初始转码后是否必须创建 AVAssetExportSession，或者是否有某种方法可以在写入会话期间在轨道之间切换？我不想处理 AVAssetExportSession 的开销。

我问这个问题是因为，使用拉式方法 - while ([assetWriterInput isReadyForMoreMediaData]) {...} - 假设仅一个轨道。它如何用于多个轨道，即音频轨道和视频轨道？

Answer 1

AVAssetWriter 将自动交错关联的 AVAssetWriterInput 上的请求，以便将不同的轨道集成到输出文件中。只需为您拥有的每个轨道添加一个 AVAssetWriterInput ，然后在每个 AVAssetWriterInput 上调用 requestMediaDataWhenReadyOnQueue:usingBlock: 即可。

这是我调用 requestMediaDataWhenReadyOnQueue:usingBlock: 的方法。我从对我拥有的输出/输入对的数量的循环中调用此方法。 （单独的方法不仅有利于代码的可读性，而且还因为与循环不同，每个调用都会为该块设置一个单独的堆栈帧。）

您只需要一个 dispatch_queue_t 并且可以将其重用于所有轨道。请注意，您绝对不应该从块中调用 dispatch_async，因为 requestMediaDataWhenReadyOnQueue:usingBlock: 期望块阻塞，直到它填满AVAssetWriterInput 将获取尽可能多的数据。在那之前你不想回来。

- (void)requestMediaDataForTrack:(int)i {
  AVAssetReaderOutput *output = [[_reader outputs] objectAtIndex:i];
  AVAssetWriterInput *input = [[_writer inputs] objectAtIndex:i];

  [input requestMediaDataWhenReadyOnQueue:_processingQueue usingBlock:
    ^{
      [self retain];
      while ([input isReadyForMoreMediaData]) {
        CMSampleBufferRef sampleBuffer;
        if ([_reader status] == AVAssetReaderStatusReading &&
            (sampleBuffer = [output copyNextSampleBuffer])) {

          BOOL result = [input appendSampleBuffer:sampleBuffer];
          CFRelease(sampleBuffer);

          if (!result) {
            [_reader cancelReading];
            break;
          }
        } else {
          [input markAsFinished];

          switch ([_reader status]) {
            case AVAssetReaderStatusReading:
              // the reader has more for other tracks, even if this one is done
              break;

            case AVAssetReaderStatusCompleted:
              // your method for when the conversion is done
              // should call finishWriting on the writer
              [self readingCompleted];
              break;

            case AVAssetReaderStatusCancelled:
              [_writer cancelWriting];
              [_delegate converterDidCancel:self];
              break;

            case AVAssetReaderStatusFailed:
              [_writer cancelWriting];
              break;
          }

          break;
        }
      }
    }
  ];
}

Answer 2

您是否尝试过使用两个 AVAssetWriterInputs 并将样本推送到工作队列？这是一个粗略的草图。

processing_queue = dispatch_queue_create("com.mydomain.gcdqueue.mediaprocessor", NULL);

[videoAVAssetWriterInput requestMediaDataWhenReadyOnQueue:myInputSerialQueue usingBlock:^{
    dispatch_asyc(processing_queue, ^{process video});
}];

[audioAVAssetWriterInput requestMediaDataWhenReadyOnQueue:myInputSerialQueue usingBlock:^{
    dispatch_asyc(processing_queue, ^{process audio});
}];

Answer 3

您可以使用调度组！

查看 MacOSX 的 AVReaderWriter 示例...

我直接引用示例 RWDocument.m：

- (BOOL)startReadingAndWritingReturningError:(NSError **)outError
{
    BOOL success = YES;
    NSError *localError = nil;

    // Instruct the asset reader and asset writer to get ready to do work
    success = [assetReader startReading];
    if (!success)
        localError = [assetReader error];
    if (success)
    {
        success = [assetWriter startWriting];
        if (!success)
            localError = [assetWriter error];
    }

    if (success)
    {
        dispatch_group_t dispatchGroup = dispatch_group_create();

        // Start a sample-writing session
        [assetWriter startSessionAtSourceTime:[self timeRange].start];

        // Start reading and writing samples
        if (audioSampleBufferChannel)
        {
            // Only set audio delegate for audio-only assets, else let the video channel drive progress
            id <RWSampleBufferChannelDelegate> delegate = nil;
            if (!videoSampleBufferChannel)
                delegate = self;

            dispatch_group_enter(dispatchGroup);
            [audioSampleBufferChannel startWithDelegate:delegate completionHandler:^{
                dispatch_group_leave(dispatchGroup);
            }];
        }
        if (videoSampleBufferChannel)
        {
            dispatch_group_enter(dispatchGroup);
            [videoSampleBufferChannel startWithDelegate:self completionHandler:^{
                dispatch_group_leave(dispatchGroup);
            }];
        }

        // Set up a callback for when the sample writing is finished
        dispatch_group_notify(dispatchGroup, serializationQueue, ^{
            BOOL finalSuccess = YES;
            NSError *finalError = nil;

            if (cancelled)
            {
                [assetReader cancelReading];
                [assetWriter cancelWriting];
            }
            else
            {
                if ([assetReader status] == AVAssetReaderStatusFailed)
                {
                    finalSuccess = NO;
                    finalError = [assetReader error];
                }

                if (finalSuccess)
                {
                    finalSuccess = [assetWriter finishWriting];
                    if (!finalSuccess)
                        finalError = [assetWriter error];
                }
            }

            [self readingAndWritingDidFinishSuccessfully:finalSuccess withError:finalError];
        });

        dispatch_release(dispatchGroup);
    }

    if (outError)
        *outError = localError;

    return success;
}