iOS如何将两个带有透明度的视频合成？

Question

我有两个视频，其中一个是带透明背景的叠加视频（爆炸视频，我正在尝试将其添加到视频中）。当我尝试使用AVMutableComposition将它们组合时，我发现叠加视频的alpha通道被忽略了。基本上我只看到第二个视频（黑色背景而不是透明背景）。

为了测试，我向叠加视频添加了0.9的不透明度，只是为了确保它们正确合并，并且结果显示主视频在叠加视频下方（这当然不是我想要的，但证明了合成起作用）。有什么办法使第二个视频的alpha通道正常工作吗？

    NSError* error = nil;

    AVMutableComposition *comp = [AVMutableComposition composition];
    AVMutableCompositionTrack* videoCompTrack = [comp addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:0]; 
    AVMutableCompositionTrack* videoCompTrack2 = [comp addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:1]; 
    AVMutableCompositionTrack* audioCompTrack = [comp addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];

    // main video
    AVURLAsset* videoAssetMain = [AVURLAsset URLAssetWithURL:url1 options:nil];
    NSArray* tracks = [videoAssetMain tracksWithMediaType:AVMediaTypeVideo];
    AVAssetTrack* videoTrackMain = [tracks firstObject];
    tracks = [videoAssetMain tracksWithMediaType:AVMediaTypeAudio];
    AVAssetTrack* audioTrackMain = [tracks firstObject];
    CMTimeRange timeRange = CMTimeRangeMake(kCMTimeZero, videoTrackMain.timeRange.duration);

    // overlay video with alpha channel
    AVURLAsset* videoAssetOver = [AVURLAsset URLAssetWithURL:url2 options:nil];
    tracks = [videoAssetOver tracksWithMediaType:AVMediaTypeVideo];
    AVAssetTrack* videoTrackOver = [tracks firstObject];

    [videoCompTrack insertTimeRange:timeRange ofTrack:videoTrackMain atTime:kCMTimeZero error:&error];
    [videoCompTrack2 insertTimeRange:timeRange ofTrack:videoTrackOver atTime:kCMTimeZero error:&error];

    [audioCompTrack insertTimeRange:timeRange ofTrack:audioTrackMain atTime:kCMTimeZero error:&error];

    AVMutableVideoCompositionLayerInstruction *inst1 = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoCompTrack];
    [inst1 setOpacity:1 atTime:kCMTimeZero];
    AVMutableVideoCompositionLayerInstruction *inst2 = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoCompTrack2];
    // adding translucency to test that bottom video is there
    // [inst2 setOpacity:0.9 atTime:kCMTimeZero];
    // stretch overlay video on top of main video
    CGAffineTransform scale = CGAffineTransformMakeScale(videoTrackMain.naturalSize.width/videoTrackOver.naturalSize.width, videoTrackMain.naturalSize.height/videoTrackOver.naturalSize.height);
    [inst2 setTransform:scale atTime:kCMTimeZero];

    AVMutableVideoCompositionInstruction *trans = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    trans.backgroundColor = [UIColor clearColor].CGColor;
    trans.timeRange = timeRange;
    trans.layerInstructions = [NSArray arrayWithObjects:inst2,inst1, nil];

    AVMutableVideoComposition* videoComp = [AVMutableVideoComposition videoComposition];
    videoComp.instructions = [NSArray arrayWithObjects:trans,nil];
    videoComp.frameDuration = CMTimeMake(1, 30);
    videoComp.renderSize = comp.naturalSize;

    AVAssetExportSession* expSession = [[AVAssetExportSession alloc] initWithAsset:comp presetName:AVAssetExportPresetHighestQuality];
    NSString* newVideoPath = [[NSString alloc] initWithFormat:@"%@%@", NSTemporaryDirectory(), @"output_final.mov"];
    if ([[NSFileManager defaultManager] fileExistsAtPath:newVideoPath]) {
        [[NSFileManager defaultManager] removeItemAtPath:newVideoPath error:&error];
    }
    expSession.outputURL = [NSURL fileURLWithPath:newVideoPath];
    expSession.outputFileType = AVFileTypeQuickTimeMovie;
    expSession.videoComposition = videoComp;

    [expSession exportAsynchronouslyWithCompletionHandler:^{
        if (delegate) {
            [delegate videoProcessor:self didFinish:expSession.outputURL];
        }
    }];

- Andrei V

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一个可工作的示例可以在此答案的评论中找到：http://stackoverflow.com/a/26965602/763355 - MoDJ

解决方案有什么进展了吗？ - vatti

你能解决这个问题吗？ - pramod

你能解决这个问题吗？ - Allan Macatingrao

Answer 1

AVVideoCompositionInstruction


/* Indicates the background color of the composition. Solid BGRA colors only are supported; patterns and other color refs that are not supported will be ignored.
   If the background color is not specified the video compositor will use a default backgroundColor of opaque black.
   If the rendered pixel buffer does not have alpha, the alpha value of the backgroundColor will be ignored. */
@property (nonatomic, retain) __attribute__((NSObject)) CGColorRef backgroundColor;

我也遇到了同样的问题，但是找不到任何方法来提供BGRA颜色作为backgroundColor。

- DimaC

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没找到解决方案，所以我使用逐帧手动合并的方式，使用了我在 https://dev59.com/9YLba4cB1Zd3GeqPZRBm 中的方法。 - Andrei V