使用AVFoundation叠加两个视频

我正在尝试叠加两个视频，前景视频有些透明。我一直在关注 Apple Docs 以及本教程。

I am trying to overlay two videos, with the foreground video being somewhat alpha transparent. I have been following the Apple Docs as well as This tutorial.

每当我尝试通过我的代码它没有崩溃;但是，当我尝试提供两个不同的视频时，我收到此错误：

Whenever I try putting two of the same video through my code it doesn't crash; however, when I try feeding it two different videos I receive this error:

VideoMaskingUtils.exportVideo Error: Optional(Error Domain=AVFoundationErrorDomain Code=-11841 "Operation Stopped" UserInfo={NSLocalizedDescription=Operation Stopped, NSLocalizedFailureReason=The video could not be composed.})
VideoMaskingUtils.exportVideo Description: <AVAssetExportSession: 0x1556be30, asset = <AVMutableComposition: 0x15567f10 tracks = (
"<AVMutableCompositionTrack: 0x15658030 trackID = 1, mediaType = vide, editCount = 1>",
"<AVMutableCompositionTrack: 0x1556e250 trackID = 2, mediaType = vide, editCount = 1>"
)>, presetName = AVAssetExportPresetHighestQuality, outputFileType = public.mpeg-4
Error Domain=AVFoundationErrorDomain Code=-11841 "Operation Stopped" UserInfo={NSLocalizedDescription=Operation Stopped, NSLocalizedFailureReason=The video could not be composed.}

我明白您无法在iOS上保存带有Alpha通道的视频 - - 我想将两个视频压缩成一个不透明的视频。

I understand that you can't save a video with an alpha channel on iOS -- I want to flatten the two videos into one opaque video.

当尝试重叠两个视频并使用CATransforms应用PiP样式时，它会崩溃;简单地重叠它们（没有alpha或任何其他效果应用工作）
任何帮助表示赞赏。

When trying to overlap the two videos and apply a PiP style using CATransforms, it crashes; simply overlapping them (w/o alpha or any other effects applied work) Any help is appreciated.

这是我的代码（包含两种方法）：

Here's my code (with both approaches in it):

class func overlay(video firstAsset: AVURLAsset, withSecondVideo secondAsset: AVURLAsset, andAlpha alpha: Float) {

    let mixComposition = AVMutableComposition()

    let firstTrack = mixComposition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)
    let secondTrack = mixComposition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)


    guard let firstMediaTrack = firstAsset.tracksWithMediaType(AVMediaTypeVideo).first else { return }
    guard let secondMediaTrack = secondAsset.tracksWithMediaType(AVMediaTypeVideo).first else { return }
    do {
        try firstTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, firstAsset.duration), ofTrack: firstMediaTrack, atTime: kCMTimeZero)
        try secondTrack.insertTimeRange(CMTimeRangeMake(kCMTimeZero, secondAsset.duration), ofTrack: secondMediaTrack, atTime: kCMTimeZero)
    } catch (let error) {
        print(error)
    }

    let width = max(firstMediaTrack.naturalSize.width, secondMediaTrack.naturalSize.width)
    let height = max(firstMediaTrack.naturalSize.height, secondMediaTrack.naturalSize.height)

    let videoComposition = AVMutableVideoComposition()
    videoComposition.renderSize = CGSizeMake(width, height)
    videoComposition.frameDuration = firstMediaTrack.minFrameDuration


    let firstApproach = false
    if firstApproach {
        let mainInstruction = AVMutableVideoCompositionInstruction()
        mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, firstAsset.duration)
        mainInstruction.backgroundColor = UIColor.redColor().CGColor

        let firstlayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: firstTrack)
        firstlayerInstruction.setTransform(firstAsset.preferredTransform, atTime: kCMTimeZero)

        let secondInstruction = AVMutableVideoCompositionInstruction()
        secondInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, secondAsset.duration)
        let backgroundColor = UIColor(colorLiteralRed: 1.0, green: 1.0, blue: 1.0, alpha: alpha)
        secondInstruction.backgroundColor = backgroundColor.CGColor

        let secondlayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: secondTrack)
        secondlayerInstruction.setTransform(secondAsset.preferredTransform, atTime: kCMTimeZero)

        secondInstruction.layerInstructions = [secondlayerInstruction]

        mainInstruction.layerInstructions = [firstlayerInstruction]//, secondlayerInstruction]

        videoComposition.instructions = [mainInstruction, secondInstruction]

    } else {
        let firstLayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: firstMediaTrack)
        firstLayerInstruction.setTransform(firstMediaTrack.preferredTransform, atTime: kCMTimeZero)
        firstLayerInstruction.setOpacity(1.0, atTime: kCMTimeZero)

        let secondlayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: secondMediaTrack)
        secondlayerInstruction.setTransform(secondMediaTrack.preferredTransform, atTime: kCMTimeZero)
        secondlayerInstruction.setOpacity(alpha, atTime: kCMTimeZero)


        let instruction = AVMutableVideoCompositionInstruction()
        instruction.timeRange = CMTimeRangeMake(kCMTimeZero, min(firstAsset.duration, secondAsset.duration))
        instruction.layerInstructions = [firstLayerInstruction, secondlayerInstruction]

        videoComposition.instructions = [instruction]
    }



    let outputUrl = VideoMaskingUtils.getPathForTempFileNamed("output.mov")

    VideoMaskingUtils.exportCompositedVideo(mixComposition, toURL: outputUrl, withVideoComposition: videoComposition)

    VideoMaskingUtils.removeTempFileAtPath(outputUrl.absoluteString)
}

这是我的 exportCompositedVideo 函数。

private class func exportCompositedVideo(compiledVideo: AVMutableComposition, toURL outputUrl: NSURL, withVideoComposition videoComposition: AVMutableVideoComposition) {
    guard let exporter = AVAssetExportSession(asset: compiledVideo, presetName: AVAssetExportPresetHighestQuality) else { return }
    exporter.outputURL = outputUrl
    exporter.videoComposition = videoComposition
    exporter.outputFileType = AVFileTypeQuickTimeMovie
    exporter.shouldOptimizeForNetworkUse = true
    exporter.exportAsynchronouslyWithCompletionHandler({
        switch exporter.status {
        case .Completed:
            // we can be confident that there is a URL because
            // we got this far. Otherwise it would've failed.
            UISaveVideoAtPathToSavedPhotosAlbum(exporter.outputURL!.path!, nil, nil, nil)
            print("VideoMaskingUtils.exportVideo SUCCESS!")
            if exporter.error != nil {
                print("VideoMaskingUtils.exportVideo Error: \(exporter.error)")
                print("VideoMaskingUtils.exportVideo Description: \(exporter.description)")
            }

            NSNotificationCenter.defaultCenter().postNotificationName("videoExportDone", object: exporter.error)
            break

        case .Exporting:
            let progress = exporter.progress
            print("VideoMaskingUtils.exportVideo \(progress)")

            NSNotificationCenter.defaultCenter().postNotificationName("videoExportProgress", object: progress)
            break

        case .Failed:
            print("VideoMaskingUtils.exportVideo Error: \(exporter.error)")
            print("VideoMaskingUtils.exportVideo Description: \(exporter.description)")

            NSNotificationCenter.defaultCenter().postNotificationName("videoExportDone", object: exporter.error)
            break

        default: break
        }
    })
}