ios - 当视频来自相机胶卷时，AVAssetExportSession 旋转错误

Question

我正在尝试将 .mov 视频转换为 .mp4 并同时更正方向。我在下面使用的代码在使用 UIImagePickerController 录制视频时效果很好，但是如果视频是从相机胶卷中选择的，我会收到此错误，我不明白为什么:

Export failed: Operation Stopped : Error Domain=AVFoundationErrorDomain Code=-11841 "Operation Stopped" UserInfo=0x1815ca50 {NSLocalizedDescription=Operation Stopped, NSLocalizedFailureReason=The video could not be composed.}

我试过先将视频保存到另一个文件，但没有任何区别。

这是我用来转换视频的代码:

- (void)convertVideoToLowQuailtyAndFixRotationWithInputURL:(NSURL*)inputURL handler:(void (^)(NSURL *outURL))handler
{
    if ([[inputURL pathExtension] isEqualToString:@"MOV"])
    {
        NSURL *outputURL = [inputURL URLByDeletingPathExtension];
        outputURL = [outputURL URLByAppendingPathExtension:@"mp4"];

        AVURLAsset *avAsset = [AVURLAsset URLAssetWithURL:inputURL options:nil];

        AVAssetTrack *sourceVideoTrack = [[avAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
        AVAssetTrack *sourceAudioTrack = [[avAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

        AVMutableComposition* composition = [AVMutableComposition composition];

        AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                                    preferredTrackID:kCMPersistentTrackID_Invalid];
        [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, avAsset.duration)
                                       ofTrack:sourceVideoTrack
                                        atTime:kCMTimeZero error:nil];
        [compositionVideoTrack setPreferredTransform:sourceVideoTrack.preferredTransform];

        AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                                    preferredTrackID:kCMPersistentTrackID_Invalid];
        [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, avAsset.duration)
                                       ofTrack:sourceAudioTrack
                                        atTime:kCMTimeZero error:nil];

        AVMutableVideoComposition *videoComposition = [self getVideoComposition:avAsset];

        NSArray *compatiblePresets = [AVAssetExportSession exportPresetsCompatibleWithAsset:avAsset];
        if ([compatiblePresets containsObject:AVAssetExportPresetMediumQuality])
        {
            AVAssetExportSession *exportSession = [[AVAssetExportSession alloc]initWithAsset:composition presetName:AVAssetExportPresetMediumQuality];
            exportSession.outputURL = outputURL;
            exportSession.outputFileType = AVFileTypeMPEG4;
            exportSession.shouldOptimizeForNetworkUse = YES;
            exportSession.videoComposition = videoComposition;
            [exportSession exportAsynchronouslyWithCompletionHandler:^{

                switch ([exportSession status])
                {
                    case AVAssetExportSessionStatusFailed:
                        NSLog(@"Export failed: %@ : %@", [[exportSession error] localizedDescription], [exportSession error]);
                        handler(nil);

                        break;
                    case AVAssetExportSessionStatusCancelled:

                        NSLog(@"Export canceled");
                        handler(nil);

                        break;
                    default:

                        handler(outputURL);

                        break;

                }
            }];
        }

    } else {
        handler(inputURL);
    }
}

- (AVMutableVideoComposition *)getVideoComposition:(AVAsset *)asset
{
    AVAssetTrack *videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    AVMutableComposition *composition = [AVMutableComposition composition];
    AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition];
    CGSize videoSize = videoTrack.naturalSize;
    BOOL isPortrait_ = [self isVideoPortrait:asset];
    if(isPortrait_) {
//        NSLog(@"video is portrait ");
        videoSize = CGSizeMake(videoSize.height, videoSize.width);
    }
    composition.naturalSize     = videoSize;
    videoComposition.renderSize = videoSize;
    videoComposition.frameDuration = CMTimeMakeWithSeconds( 1 / videoTrack.nominalFrameRate, 600);

    AVMutableCompositionTrack *compositionVideoTrack;
    compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack:videoTrack atTime:kCMTimeZero error:nil];
    AVMutableVideoCompositionLayerInstruction *layerInst;
    layerInst = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
    [layerInst setTransform:videoTrack.preferredTransform atTime:kCMTimeZero];
    AVMutableVideoCompositionInstruction *inst = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    inst.timeRange = CMTimeRangeMake(kCMTimeZero, asset.duration);
    inst.layerInstructions = [NSArray arrayWithObject:layerInst];
    videoComposition.instructions = [NSArray arrayWithObject:inst];
    return videoComposition;
}

Answer 1

AVFoundation 错误常量 -11841 表示您的视频合成无效。如果您想了解有关错误常量的更多信息，请参阅此链接: https://developer.apple.com/library/ios/documentation/AVFoundation/Reference/AVFoundation_ErrorConstants/Reference/reference.html

虽然没有立即向我弹出重大错误，但我可以建议以下方法来缩小问题的根源。

首先，在这些调用中，不是为 error 参数传递 nil:

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

创建一个 NSError 对象并将引用传递给它，如下所示:

NSError *error = nil;
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, avAsset.duration)
                                       ofTrack:sourceVideoTrack
                                        atTime:kCMTimeZero error:&error];

检查错误以确保您的视频和音频轨道正确插入合成轨道。如果一切顺利，错误应该是 nil。

if(error)
    NSLog(@"Insertion error: %@", error);

您可能还想检查您的 AVAsset 的 composable 和 exportable，以及 hasProtectedContent 属性。如果这些分别不是"is"、"is"和“否”，则您在创建新视频文件时可能会遇到问题。

我偶尔会看到一个问题，即为音轨创建时间范围与在带有视频轨道的合成中使用时的 600 时间刻度不一样。您可能希望在

中为持续时间 (avAsset.duration) 创建一个新的 CMTime

CMTimeRangeMake(kCMTimeZero, avAsset.duration) 

仅用于插入音轨。在新的 CMTime 中，使用 44100 的时间刻度(或任何音轨的采样率。)同样适用于您的 videoComposition.frameDuration。根据视频轨道的 nominalFrameRate，您的时间可能无法用 600 时间刻度正确表示。

最后，Apple 提供了一个有用的工具来调试视频组合:

https://developer.apple.com/library/mac/samplecode/AVCompositionDebugViewer/Introduction/Intro.html

它提供了您的构图的可视化表示，您可以看到哪些地方看起来不正常。