无法为视频添加水印，其轨道的preferredTransform不是使用AVFoundation的身份矩阵？

Question

我想在下载的视频的左下角添加水印图片，我发现下面的代码在视频的视频轨道的preferredTransform属性为CGAffineTransformIdentity时有效（这意味着没有真正的转换），而对于那些视频进行转换，代码失败了。如何解决？

示例视频（已转换）网址为here

BTW，AVFoundations报告的错误代码是-11841

+(void) addWatermarkWithInputFile:(NSString *)inputFile outputFile:(NSString *)outputFile completion:(void (^)(BOOL))completion{
    [[NSFileManager defaultManager] removeItemAtPath:outputFile error:nil];

    AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:[NSURL fileURLWithPath:inputFile] options:nil];
    AVMutableComposition* composition = [AVMutableComposition composition];
    CMTimeRange timeRange = CMTimeRangeMake(kCMTimeZero, videoAsset.duration);
    AVAssetTrack *track = [videoAsset tracksWithMediaType:AVMediaTypeVideo].firstObject;

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

        composedTrack.preferredTransform = track.preferredTransform;

        [composedTrack insertTimeRange:timeRange
                               ofTrack:[videoAsset tracksWithMediaType:AVMediaTypeVideo].firstObject
                                atTime:kCMTimeZero
                                 error:nil];
    }

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

        composedTrack.preferredTransform = track.preferredTransform;

        [composedTrack insertTimeRange:timeRange
                               ofTrack:[videoAsset tracksWithMediaType:AVMediaTypeAudio].firstObject
                                atTime:kCMTimeZero
                                 error:nil];
    }

    AVAssetTrack *clipVideoTrack = [videoAsset tracksWithMediaType:AVMediaTypeVideo].firstObject;
    CGSize videoSize = clipVideoTrack.naturalSize;

    AVMutableVideoComposition* videoComp = [AVMutableVideoComposition videoComposition];
    videoComp.renderSize = videoSize;
    videoComp.frameDuration = CMTimeMake(1, 30);

    {
        UIImage *myImage = [UIImage imageNamed:@"video-watermark"];
        CALayer *aLayer = [CALayer layer];
        aLayer.contents = (id)myImage.CGImage;
        aLayer.frame = CGRectMake(10, 10, myImage.size.width * myImage.scale, myImage.size.height * myImage.scale);

        CALayer *parentLayer = [CALayer layer];
        CALayer *videoLayer = [CALayer layer];
        parentLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height);
        videoLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height);
        [parentLayer addSublayer:videoLayer];
        [parentLayer addSublayer:aLayer];

        videoComp.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];
    }

    /// instruction
    AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    instruction.timeRange = timeRange;
    AVMutableVideoCompositionLayerInstruction* layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:clipVideoTrack];
    instruction.layerInstructions = @[layerInstruction];
    videoComp.instructions = @[instruction];

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

    exporter.outputURL = [NSURL fileURLWithPath:outputFile];
    exporter.outputFileType = AVFileTypeMPEG4;
    exporter.shouldOptimizeForNetworkUse = YES;
    exporter.videoComposition = videoComp;
    exporter.timeRange = timeRange;

    [exporter exportAsynchronouslyWithCompletionHandler:^{
        completion(exporter.status == AVAssetExportSessionStatusCompleted);
    }];
}