AVMutableComposition y轴反转

Question

我在视频上添加了水印。我设置了原始水印图层（10; 10），但导出后，水印位于左下角。如果我增加y，则水印向上移动。好像y轴从下往上增加。

有人可以提出建议吗？

NSString *path = [[NSBundle mainBundle] pathForResource:@"video" ofType:@"mov"];
NSURL *file = [NSURL fileURLWithPath:path];
AVURLAsset *videoAsset = [[AVURLAsset alloc]initWithURL:file options:nil];
self.asset = videoAsset;
AVMutableComposition *mixComposition = [AVMutableComposition composition];
AVMutableCompositionTrack *compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo  preferredTrackID:kCMPersistentTrackID_Invalid];
AVMutableCompositionTrack *compositionAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio  preferredTrackID:kCMPersistentTrackID_Invalid];
AVAssetTrack *clipVideoTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] firstObject];
AVAssetTrack *clipAudioTrack = [[videoAsset tracksWithMediaType:AVMediaTypeAudio] firstObject];
if (clipAudioTrack)
{
    [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                                   ofTrack:clipAudioTrack
                                    atTime:kCMTimeZero
                                     error:nil];
}
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                               ofTrack:clipVideoTrack
                                atTime:kCMTimeZero
                                 error:nil];
[compositionVideoTrack setPreferredTransform:clipVideoTrack.preferredTransform];

CGSize videoSize = [clipVideoTrack naturalSize];
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);

UIImage *myImage = [UIImage imageNamed:@"watermark"];
CALayer *aLayer = [CALayer layer];
CGRect frame = CGRectMake(10, 10, CGImageGetWidth(myImage.CGImage), CGImageGetHeight(myImage.CGImage));
aLayer.frame = frame;
aLayer.contents = (id)myImage.CGImage;

[parentLayer addSublayer:videoLayer];
[parentLayer addSublayer:aLayer];

AVMutableVideoComposition *videoComposition=[AVMutableVideoComposition videoComposition] ;
videoComposition.frameDuration=CMTimeMake(1, 30);
videoComposition.renderSize = videoSize;
videoComposition.animationTool=[AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];

AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, [mixComposition duration]);
AVAssetTrack *videoTrack = [[mixComposition tracksWithMediaType:AVMediaTypeVideo] firstObject];
AVMutableVideoCompositionLayerInstruction* layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
instruction.layerInstructions = @[layerInstruction];
videoComposition.instructions = @[instruction];

NSString *documentsDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) firstObject];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd_HH-mm-ss"];
NSString *destinationPath = [documentsDirectory stringByAppendingFormat:@"/utput_%@.mov", [dateFormatter stringFromDate:[NSDate date]]];

AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetMediumQuality];
exportSession.videoComposition = videoComposition;

exportSession.outputURL = [NSURL fileURLWithPath:destinationPath];
exportSession.outputFileType = AVFileTypeMPEG4;
[exportSession exportAsynchronouslyWithCompletionHandler:^{
    switch (exportSession.status)
    {
        case AVAssetExportSessionStatusCompleted:
            NSLog(@"Export OK");
            if (UIVideoAtPathIsCompatibleWithSavedPhotosAlbum(destinationPath)) {
                UISaveVideoAtPathToSavedPhotosAlbum(destinationPath, self, nil, nil);
            }
            break;
        case AVAssetExportSessionStatusFailed:
            NSLog (@"AVAssetExportSessionStatusFailed: %@", exportSession.error);
            break;
        case AVAssetExportSessionStatusCancelled:
            NSLog(@"Export Cancelled");
            break;
        default:
            break;
    }
}];