AVFoundation AVAssetExportSession在尝试用图层指令翻转视频时产生空白视频

Question

我正试图使用AVFoundation框架以正确的方式翻转视频。由于某些原因，下面的代码每次都会生成一个空白视频。

我注意到，当我不应用AVAssetExportSession的videoComposition设置时，它确实通过视频(它在照片库中可见和可播放)，但它没有被翻转，因为层指令没有应用。所以我假设问题是在图层指令被传递到合成中，但它是正确设置的。我错过了什么，或者没有看到什么?

代码如下:

    NSURL *assetURL = [NSURL fileURLWithPath:path];
    AVAsset *movieAsset = [AVAsset assetWithURL:assetURL];
    NSLog(@"Asset: %0.1f",CMTimeGetSeconds(movieAsset.duration));
    NSLog(@"Asset Preferred Transform %@", NSStringFromCGAffineTransform(movieAsset.preferredTransform));

    //Output Composition
    AVMutableComposition *outputComposition = [[AVMutableComposition alloc] init];
    AVMutableCompositionTrack *videoTrack = [outputComposition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                           preferredTrackID:kCMPersistentTrackID_Invalid];
    [videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, movieAsset.duration)
                        ofTrack:[[movieAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
                         atTime:kCMTimeZero
                          error:nil];
    [videoTrack setPreferredTransform:CGAffineTransformMakeScale(1, -1)];
    AVMutableVideoCompositionLayerInstruction *videoLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
    [videoLayerInstruction setTransform:CGAffineTransformMakeScale(1, -1) atTime:kCMTimeZero];
    AVMutableVideoCompositionInstruction *videoInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    videoInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, movieAsset.duration);
    videoInstruction.layerInstructions = [NSArray arrayWithObjects:videoLayerInstruction, nil];

    AVMutableVideoComposition *outputVideoComposition = [AVMutableVideoComposition videoComposition];
    outputVideoComposition.instructions = [NSArray arrayWithObjects:videoInstruction, nil];
    outputVideoComposition.frameDuration = CMTimeMake(1, 30);
    outputVideoComposition.renderSize = CGSizeMake(480, 640);

    //Export
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:outputComposition presetName:AVAssetExportPresetHighestQuality] ;
    exporter.videoComposition = outputVideoComposition;
    NSString *newPath = [documentsDirectory stringByAppendingPathComponent:[self.filePath stringByAppendingString:@"-fb.mov"]];
    [[NSFileManager defaultManager] removeItemAtPath:newPath error:nil];
    exporter.outputURL = [NSURL fileURLWithPath:newPath];
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    NSLog(@"Starting export");

    [exporter exportAsynchronouslyWithCompletionHandler:^(void){
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"Video exported");
            NSLog(@"%@", newPath);
            NSURL *newAssetURL = [NSURL fileURLWithPath:newPath];
            AVAsset *newMovieAsset = [AVAsset assetWithURL:newAssetURL];
            NSLog(@"New Asset %@",newMovieAsset);
            NSLog(@"New Asset Duration: %0.1f",CMTimeGetSeconds(newMovieAsset.duration));
            NSLog(@"New Asset Preferred Transform %@", NSStringFromCGAffineTransform(newMovieAsset.preferredTransform));

            UISaveVideoAtPathToSavedPhotosAlbum(newPath, nil, nil, nil);}}];

Answer 1

你给outputVideoComposition的渲染大小是不正确的。应该是根据iPhone屏幕

Answer 2

我有同样的问题，它是旋转的视频导出帧。旋转锚点位于左上角的东西。

要修复，应用另一个转换，像这样:

CGAffineTransformMakeTranslation (exportingVideoWidth exportingVideoHeight);

希望这对你有帮助，如果你有任何问题请告诉我。