使用AVAssetExportSession旋转AVAsset

Question

我尝试使用AVAssetExportSession将视频旋转到正确的方向，但总是收到以下错误：

Error Domain=AVFoundationErrorDomain Code=-11841 "The operation couldn’t be completed. (AVFoundationErrorDomain error -11841.)"

翻译过来就是AVErrorInvalidVideoComposition，但我看不出我的视频构图有什么问题。代码如下：

AVAssetTrack *sourceVideo = [[avAsset tracksWithMediaType:AVMediaTypeVideo] lastObject];
AVAssetTrack *sourceAudio = [[avAsset tracksWithMediaType:AVMediaTypeAudio] lastObject];
CGAffineTransform preferredTransform = [sourceVideo preferredTransform];

AVMutableComposition *composition = [[AVMutableComposition alloc] init];

AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                            preferredTrackID:kCMPersistentTrackID_Invalid];

AVAssetExportSession *exporter = [[[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetMediumQuality] autorelease];

[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, avAsset.duration)
                               ofTrack:sourceVideo
                                atTime:kCMTimeZero
                                 error:nil];

if( !CGAffineTransformIsIdentity(preferredTransform) ) {

    AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition];
    videoComposition.renderSize = CGSizeMake([avAsset naturalSize].height, [avAsset naturalSize].width);
    videoComposition.frameDuration = CMTimeMake(1, compositionVideoTrack.naturalTimeScale);

    AVMutableVideoCompositionLayerInstruction *instruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:sourceVideo];
    [instruction setTransform:preferredTransform atTime:kCMTimeZero];

    AVMutableVideoCompositionInstruction *videoTrackInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    videoTrackInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, avAsset.duration);
    videoTrackInstruction.layerInstructions = [NSArray arrayWithObject:instruction];

    [videoComposition setInstructions:[NSArray arrayWithObject:videoTrackInstruction]];

    exporter.videoComposition = videoComposition;
}

AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                            preferredTrackID:kCMPersistentTrackID_Invalid];

[compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, avAsset.duration)
                               ofTrack:sourceAudio
                                atTime:kCMTimeZero
                                 error:nil];

exporter.outputURL = tempPathUrl;
exporter.outputFileType = AVFileTypeQuickTimeMovie;
[exporter exportAsynchronouslyWithCompletionHandler:^{ }];

构图可能有什么问题？我已经看过文档了，到目前为止还看不出有什么问题。

Answer 1

这可能与您的帧持续时间有关。您正在使用CMTimeMake(1, naturalTimeScale)

您应该检查naturalTimeScale，因为它并不总是等于fps。有关更多信息，请参阅AVFoundation Programming Guide的“时间表示法”。

Answer 2

我认为这是因为宽度和高度参数的顺序不正确。您必须：

    videoComposition.renderSize = CGSizeMake([avAsset naturalSize].height, [avAsset naturalSize].width);

难道不应该是

    videoComposition.renderSize = CGSizeMake([avAsset naturalSize].width, [avAsset naturalSize].height);

而不是？

Answer 3

该错误与您设置instruction的方式有关。

... videoCompositionLayerInstructionWithAssetTrack:sourceVideo]

sourceVideo不是该组合的成员。在您的示例中，它应该是compositionVideoTrack。