AVFoundation

Question

我见过这个问题被问过几次，但是 none 似乎有任何可行的答案。

要求反转并输出视频文件（不仅仅是反转播放），保持与源视频相同的压缩、格式和帧速率。

理想情况下，该解决方案能够在内存或缓冲区中完成所有这些操作，并避免将帧生成为图像文件（例如：使用 AVAssetImageGenerator）然后重新编译它（资源密集型、不可靠的计时结果，frame/image 质量与原始质量的变化等）。

--

我的贡献：这仍然无法正常工作，但到目前为止我已经尝试过的最好的方法：

使用 AVAssetReader.

CMSampleBufferRef[]

使用AVAssetWriter倒序写回。
问题：似乎每个帧的时间都保存在 CMSampleBufferRef 中，所以即使向后追加它们也不起作用。
接下来，我尝试将每个帧的时间信息与 reverse/mirror 帧交换。
问题：这会导致 AVAssetWriter 发生未知错误。

下一步：我要调查 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

过去几天一直在处理这个问题，并且能够让它正常工作。

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

使用AVAssetReader读出samples/frames，提取image/pixel缓冲区，然后追加镜像帧的呈现时间。

AVFoundation - 反转 AVAsset 并输出视频文件

AVFoundation - Reverse an AVAsset and output video file

macos

objective-c

ios