AVFoundation：缩放视频更改颜色

Question

我正在通过使用AVFoundation和AVMutableComposition缩放单个视频来创建视频合成。当我应用除1之外的任何缩放（更大或更小）时，我得到最终结果的颜色偏移。无论用作源的视频还是用作目标的预设/格式（MOV / MP4，ProRes / H24 ......），我都会遇到此问题。

以下是相关代码，受到另一个StackOverflow问题（Swift Video Resizer AVAsset）的广泛启发

func resizer(inputURL: URL , completion: @escaping (URL?) -> Void ){
    let videoAsset = AVAsset(url: inputURL) as AVAsset
    let clipVideoTrack = videoAsset.tracks(withMediaType: AVMediaTypeVideo).first! as AVAssetTrack

    let composition = AVMutableComposition()
    composition.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID())

    let videoComposition = AVMutableVideoComposition()
    videoComposition.renderSize = clipVideoTrack.naturalSize
    videoComposition.frameDuration = CMTimeMake(1, 30)

    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRange(start: kCMTimeZero, duration: clipVideoTrack.asset!.duration)

    let transformer : AVMutableVideoCompositionLayerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: clipVideoTrack)

    // If anything that 1 on both, colors are shifted!
    let transform = CGAffineTransform(scaleX: 1.001, y: 1.001)
    transformer.setTransform(transform, at: kCMTimeZero)
    instruction.layerInstructions = [transformer]
    videoComposition.instructions = [instruction]

    // Export part, left for facility    
    let exporter = AVAssetExportSession(asset: videoAsset, presetName: AVAssetExportPresetHighestQuality)!
    exporter.videoComposition = videoComposition
    let outputURL = URL(fileURLWithPath: "/path/to/destination/video.mov")
    exporter.outputURL = outputURL
    exporter.outputFileType = AVFileTypeQuickTimeMovie

    exporter.exportAsynchronously(completionHandler: { () -> Void in
        completion(outputURL)
    })
}

Answer 1

解决方案是在视频结束时重置转换：

// Add this line to fix the issue:
transformer.setTransform(CGAffineTransform.identity, at: kCMTimeZero + clipVideoTrack.asset!.duration)

// Before this lines:
instruction.layerInstructions = [transformer]
videoComposition.instructions = [instruction]

不确定原因，但如果您有多个曲目用于合成，也可以使用。