第二次导出 AVAsset 使视频空白答案

【问题标题】：Exporting AVAsset second time makes video blank第二次导出 AVAsset 使视频空白
【发布时间】：2020-12-11 15:38:54
【问题描述】：

我正在使用AVMutableComposition() 将多个视频文件拼接成一个，添加这样的轨道：

let compositionVideoTrack = mainComposition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)
let soundtrackTrack = mainComposition.addMutableTrack(withMediaType: .audio, preferredTrackID: kCMPersistentTrackID_Invalid)

private var insertTime = CMTime.zero
     
for videoAsset in arrayVideos {
       try! compositionVideoTrack?.insertTimeRange(CMTimeRangeMake(start: .zero, duration: videoAsset.duration), of: videoAsset.tracks(withMediaType: .video)[0], at: insertTime)
       try! soundtrackTrack?.insertTimeRange(CMTimeRangeMake(start: .zero, duration: videoAsset.duration), of: videoAsset.tracks(withMediaType: .audio)[0], at: insertTime)
            
       insertTime = CMTimeAdd(insertTime, videoAsset.duration)
}

然后使用AVAssetExportSession(asset: mainComposition, presetName: AVAssetExportPresetMediumQuality) 将它们导出到 .mov 文件中。

这会将拼接的视频保存到 url，我可以使用 AVAsset 访问并显示给用户。之后，我尝试将图像叠加层添加到视频中并再次导出。

在第二种方法中，我从 url AVAsset(url: fileUrl) 实例化 AVAsset。并创建新的AVMutableComposition()。我将视频和音频轨道添加到资产的合成中：

    compositionTrack = composition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)
    compositionTrack.insertTimeRange(timeRange, of: asset.tracks(withMediaType: .video)[], at: .zero)
...

然后我使用图层和AVVideoCompositionCoreAnimationTool() 为视频添加叠加层，如下所示：

 let videoLayer = CALayer()
 videoLayer.frame = CGRect(origin: .zero, size: videoSize)
 let overlayLayer = CALayer()
 overlayLayer.frame = CGRect(origin: .zero, size: videoSize)
        
 overlayLayer.contents = watermark.cgImage
 overlayLayer.contentsGravity = .resizeAspect
        
 let outputLayer = CALayer()
 outputLayer.frame = CGRect(origin: .zero, size: videoSize)
 outputLayer.addSublayer(videoLayer)
 outputLayer.addSublayer(overlayLayer)
        
 let videoComposition = AVMutableVideoComposition()
 videoComposition.renderSize = videoSize
 videoComposition.frameDuration = CMTime(value: 1, timescale: 30)
 videoComposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videoLayer, in: outputLayer)
        
 let instruction = AVMutableVideoCompositionInstruction()
 instruction.timeRange = CMTimeRange(start: .zero, duration: asset.duration)
 videoComposition.instructions = [instruction]
 let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: assetTrack)
 layerInstruction.setTransform(assetTrack.preferredTransform, at: .zero)
 instruction.layerInstructions = [layerInstruction]

然后我以与第一次导出相同的方式导出视频。

问题是当我把它结合起来的时候。 如果我只使用第二种方法导出一些示例视频，则会添加到视频的叠加层，并且一切都符合预期。如果我使用第一种方法拼接视频，那么视频就会完美拼接。但是，当我结合这些方法时，生成的视频是黑屏（音频和覆盖都很好，生成的视频时长也适合）。

【问题讨论】：

标签： ios swift video avfoundation avvideocomposition

【解决方案1】：

这个问题可能与AVVideoCompositionCoreAnimationTool() 有关。我能够通过在第二个函数中使用另一种技术在视频中添加叠加来解决这个问题。我没有使用AVVideoCompositionCoreAnimationTool() 堆叠层，而是使用了CIFilter，如下所示：

let watermarkFilter = CIFilter(name: "CISourceOverCompositing")!
let watermarkImage = CIImage(image: watermark)!
let videoComposition = AVVideoComposition(asset: asset) { (filteringRequest) in
    let source = filteringRequest.sourceImage
    watermarkFilter.setValue(source, forKey: kCIInputBackgroundImageKey)
    let widthScale = source.extent.width/watermarkImage.extent.width
    let heightScale = source.extent.height/watermarkImage.extent.height
    watermarkFilter.setValue(watermarkImage.transformed(by: .init(scaleX: widthScale, y: heightScale)), forKey: kCIInputImageKey)
    filteringRequest.finish(with: watermarkFilter.outputImage!, context: nil)
    }

【讨论】：