iOS composing 2 videos with transparency

Question

I have 2 videos where one of them is overlay video with transparent background (explosion video that Im trying to add to the video). When I try to combine them using AVMutableComposition I see that overlay's video alpha channel is ignored. Basically I see 2nd video only (with black background instead of transparent).

As test I added opacity of 0.9 to overlay video just to make sure that they merged correctly and as result I see main video underneath overlay video (not what I want of course but proves that composition works). Any idea as to how to make alpha channel work for 2nd 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];
        }
    }];

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;

have same problem, but did'nt find any way to provide BGRA color for backgroundColor