双视频图层显示异常：两层均展示同一视频问题排查

问题现象 #

最终输出视频中，视频图层与叠加图层均仅显示来自videoURL的视频，未展示overlayVideoURL的视频。

已尝试操作 #

• 检查并确认CALayer的frame设置正确
• 验证AVVideoCompositionCoreAnimationTool的使用无误
• 确认AVMutableComposition已插入两个视频轨道

疑问 #

1. 视频图层的配置是否存在错误？
2. 如何确保每个图层正确展示对应URL的视频？

补充说明：视频已正确加载并插入至AVMutableComposition，但最终合成视频两层均显示videoURL的视频。

相关代码 #

func makeVideo(videoURL: URL, overlayVideoURL: URL, forName name: String, onComplete: @escaping (URL?) -> Void) {
    let asset = AVURLAsset(url: videoURL)
    let overlayAsset = AVURLAsset(url: overlayVideoURL)
    let composition = AVMutableComposition()
    guard
        let compositionTrack = composition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid),
        let assetTrack = asset.tracks(withMediaType: .video).first,
        let overlayCompositionTrack = composition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid),
        let overlayAssetTrack = overlayAsset.tracks(withMediaType: .video).first
    else {
        print("Something is wrong with the assets.")
        onComplete(nil)
        return
    }
    do {
        let timeRange = CMTimeRange(start: .zero, duration: asset.duration)
        try compositionTrack.insertTimeRange(timeRange, of: assetTrack, at: .zero)
        try overlayCompositionTrack.insertTimeRange(timeRange, of: overlayAssetTrack, at: .zero)
    } catch {
        print(error)
        onComplete(nil)
        return
    } 
    let videoInfo = orientation(from: assetTrack.preferredTransform)
    let videoSize = videoInfo.isPortrait ? CGSize(width: assetTrack.naturalSize.height, height: assetTrack.naturalSize.width) : assetTrack.naturalSize
    let videoComposition = AVMutableVideoComposition()
    videoComposition.renderSize = videoSize
    videoComposition.frameDuration = CMTime(value: 1, timescale: 30) 
    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRange(start: .zero, duration: composition.duration)
    let layerInstruction = compositionLayerInstruction(for: compositionTrack, assetTrack: assetTrack)
    let overlayLayerInstruction = compositionLayerInstruction(for: overlayCompositionTrack, assetTrack: overlayAssetTrack)
    instruction.layerInstructions = [layerInstruction, overlayLayerInstruction]
    videoComposition.instructions = [instruction]
    let videoLayer = CALayer()
    videoLayer.frame = CGRect(origin: .zero, size: videoSize)
    let overlayVideoLayer = CALayer()
    overlayVideoLayer.frame = CGRect(x: videoSize.width / 4, y: videoSize.height / 4, width: videoSize.width / 2, height: videoSize.height / 2)
    let outputLayer = CALayer()
    outputLayer.frame = CGRect(origin: .zero, size: videoSize)
    outputLayer.addSublayer(videoLayer)
    outputLayer.addSublayer(overlayVideoLayer)
    videoComposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayers: [videoLayer, overlayVideoLayer], in: outputLayer)
    guard let export = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetHighestQuality) else {
        print("Cannot create export session.")
        onComplete(nil)
        return
    }
    let videoName = UUID().uuidString
    let exportURL = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(videoName).appendingPathExtension("mov")
    export.videoComposition = videoComposition
    export.outputFileType = .mov
    export.outputURL = exportURL
    export.exportAsynchronously {
        DispatchQueue.main.async {
            switch export.status {
            case .completed:
                onComplete(exportURL)
            default:
                print("Something went wrong during export.")
                print(export.error ?? "unknown error")
                onComplete(nil)
            }
        }
    }
}

问题原因与解决方法 #

核心问题在于错误使用了AVVideoCompositionCoreAnimationTool的初始化方法：你当前用的postProcessingAsVideoLayers:in:是对单个视频轨道做后期动画处理的接口，并非用于多轨道到多图层的映射，导致两个视频轨道的内容都渲染到了第一个图层上。

具体修改步骤 #

1. 替换动画工具初始化方式
   改用videoLayers:usingRenderingMode:方法，将每个视频轨道与对应的CALayer绑定，数组顺序与轨道添加顺序一致（第一个轨道对应第一个图层，第二个轨道对应第二个图层）：

   videoComposition.animationTool = AVVideoCompositionCoreAnimationTool(
       videoLayers: [videoLayer, overlayVideoLayer],
       usingRenderingMode: .passthrough
   )
   

2. 调整叠加层的图层指令
   需要给叠加层轨道添加缩放、平移变换，让视频内容适配overlayVideoLayer的尺寸和位置，修改compositionLayerInstruction方法：

   func compositionLayerInstruction(for compositionTrack: AVCompositionTrack, assetTrack: AVAssetTrack, overlayFrame: CGRect) -> AVMutableVideoCompositionLayerInstruction {
       let instruction = AVMutableVideoCompositionLayerInstruction(assetTrack: compositionTrack)
       let baseTransform = assetTrack.preferredTransform
       let naturalSize = assetTrack.naturalSize
       
       // 处理叠加层轨道
       if compositionTrack == composition.tracks(withMediaType: .video)[1] {
           // 计算缩放比例适配叠加层尺寸
           let scaleX = overlayFrame.width / naturalSize.width
           let scaleY = overlayFrame.height / naturalSize.height
           let scaleTransform = CGAffineTransform(scaleX: scaleX, y: scaleY)
           // 平移到叠加层位置
           let translateTransform = CGAffineTransform(translationX: overlayFrame.origin.x, y: overlayFrame.origin.y)
           // 合并变换：先处理原视频方向，再缩放平移
           let finalTransform = baseTransform.concatenating(scaleTransform).concatenating(translateTransform)
           instruction.setTransform(finalTransform, at: .zero)
       } else {
           // 处理主视频轨道的方向适配
           let videoInfo = orientation(from: baseTransform)
           let renderSize = videoInfo.isPortrait ? CGSize(width: naturalSize.height, height: naturalSize.width) : naturalSize
           let adjustTransform = baseTransform.concatenating(CGAffineTransform(translationX: renderSize.width/2, y: renderSize.height/2))
               .concatenating(CGAffineTransform(rotationAngle: CGFloat(videoInfo.rotationAngle)))
               .concatenating(CGAffineTransform(translationX: -renderSize.width/2, y: -renderSize.height/2))
           instruction.setTransform(adjustTransform, at: .zero)
       }
       return instruction
   }
   

   调用时传入叠加层的frame：

   let overlayLayerInstruction = compositionLayerInstruction(for: overlayCompositionTrack, assetTrack: overlayAssetTrack, overlayFrame: overlayVideoLayer.frame)
   

3. 统一时间范围
   避免叠加视频时长不足出现空白，取两个视频的最小时长作为合成时间范围：

   let minDuration = CMTimeMinimum(asset.duration, overlayAsset.duration)
   let timeRange = CMTimeRange(start: .zero, duration: minDuration)
   try compositionTrack.insertTimeRange(timeRange, of: assetTrack, at: .zero)
   try overlayCompositionTrack.insertTimeRange(timeRange, of: overlayAssetTrack, at: .zero)
   

内容的提问来源于stack exchange，提问作者Игорь Шварев