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

Question

我试图用AVFoundation框架正确地翻转视频。出于某种原因，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Photo LIbrary中可见和可播放），但由于未应用层指令，因此不会翻转。所以我认为这个问题出现在层指令被传递到构图中，但它被正确设置。我错过了什么，或没有看到？

下面是代码：

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

呈现大小，你都给人以outputVideoComposition是不正确的。它应该是根据iphone屏幕

Answer 2

我有同样的问题，它旋转视频输出帧。旋转锚点位于左上角。

要修复，应用另一个转变，是这样的：

CGAffineTransformMakeTranslation（exportingVideoWidth，exportingVideoHeight）;

希望这可以帮助，让我知道如果您有任何问题。