我正在使用 AVFoundation 将一个视频与另一个具有透明度的视频一起添加水印。我已经设法使用以下代码将一个视频叠加在另一个视频之上，但是一旦我开始使用其中具有透明度的 Assets ，导出就会失败，并且没有任何有用的错误。

This link谈到 PreRes 4444 是 AVFoundation 在 alpha channel 方面支持的唯一编解码器，但我找不到任何官方文档。我目前尝试添加为叠加层的文件是 H.264 编码的 mp4，阅读后似乎是最佳选择 Learning AVFoundation , 它指出

The ProRes codecs are available only on OS X. If you're developing only for iOS, H264 is the only game in town.

我总是可以回退到添加动画层而不是视频叠加层，但如果没有解决方案，我会感到惊讶。

- (void)addWatermarkToAsset:(NSURL *)assetURL completionHandler:(void (^)(NSURL *videoURL))handler
{
    AVURLAsset *videoAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];

    // This asset contains an alpha channel, and has a shorter duration than videoAsset
    NSURL *animationUrl = [[NSBundle mainBundle] URLForResource:@"InstagramAnimation" withExtension:@"mp4"];
    AVURLAsset *animationAsset = [AVURLAsset URLAssetWithURL:animationUrl options:nil];

    AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];
    AVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];
    [videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                        ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
                         atTime:kCMTimeZero error:nil];

    AVMutableCompositionTrack *animationTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                            preferredTrackID:kCMPersistentTrackID_Invalid];

    [animationTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, animationAsset.duration)
                            ofTrack:[[animationAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
                             atTime:kCMTimeZero error:nil];

    AVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, videoAsset.duration);

    AVMutableVideoCompositionLayerInstruction *videoLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
    CGAffineTransform scale = CGAffineTransformMakeScale(0.7f,0.7f);
    CGAffineTransform move = CGAffineTransformMakeTranslation(230,230);
    [videoLayerInstruction setTransform:CGAffineTransformConcat(scale, move) atTime:kCMTimeZero];

    AVMutableVideoCompositionLayerInstruction *animationLayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:animationTrack];
    CGAffineTransform secondScale = CGAffineTransformMakeScale(1.2f,1.5f);
    CGAffineTransform secondMove = CGAffineTransformMakeTranslation(0,0);

    [animationLayerInstruction setTransform:CGAffineTransformConcat(secondScale, secondMove) atTime:kCMTimeZero];

    mainInstruction.layerInstructions = [NSArray arrayWithObjects:videoLayerInstruction, animationLayerInstruction, nil];

    AVMutableVideoComposition *mainCompositionInst = [AVMutableVideoComposition videoComposition];
    mainCompositionInst.instructions = [NSArray arrayWithObject:mainInstruction];
    mainCompositionInst.frameDuration = CMTimeMake(1, 30);
    mainCompositionInst.renderSize = videoTrack.naturalSize;

    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:
                             [NSString stringWithFormat:@"FinalVideo-%d.mov",arc4random() % 1000]];
    NSURL *url = [NSURL fileURLWithPath:myPathDocs];

    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition
                                                                      presetName:AVAssetExportPresetHighestQuality];
    exporter.outputURL = url;
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    exporter.shouldOptimizeForNetworkUse = YES;
    exporter.videoComposition = mainCompositionInst;
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            [self exportDidFinish:exporter];
        });
    }];
} 

最佳答案

我通过做一个
1. H264自定义透明格式，以及
2.自定义透明格式的自定义(perforce)合成器

自定义格式是一个“高”视频，其中每个彩色帧都直接位于其黑白 mask 上方。

合成器从水印和待加水印的文件中提取帧，合成它们，并将结果写入第三个文件。 每一帧的每个像素都被解释为一个 YUV 值，一个用作颜色，另一个用作 mask ，并与背景帧结合。

为了加快速度，合成器是 OpenGL 像素着色器 + TextureCaches，尽管今天您会使用 Metal和我想的 CVMetalTextureCache。

关于iOS - 带有透明视频的水印视频，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/39211966/