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IOS component 2 video with transparency

I have 2 videos in which one of them is an overlay video with a transparent background (the video for the explosion that I am trying to add to the video). When I try to combine them using AVMutableComposition, I see that the alpha channel of the video overlap is ignored. Basically I see only the second video (with a black background instead of a transparent one).

As a test, I added an opacity of 0.9 for the video overlay to make sure that they merged correctly, and as a result I see the main video under the overlay video (not what I want, of course, but proves that the composition works). Any idea on how to make an alpha channel for the second video?

    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];
        }
    }];
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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;

​​ , BGRA backgroundColor

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