swift - AVFoundation:缩放视频改变颜色

Question

我正在通过使用 AVFoundation 和 AVMutableComposition 缩放单个视频来创建视频组合。当我应用 1 以外的任何缩放比例(更大或更小)时，最终结果会出现颜色偏移。 无论将视频用作源还是将预设/格式用作目标(MOV/MP4、ProRes/H24...)，我都会遇到此问题。

这是相关代码，灵感来自另一个 StackOverflow 问题 (Swift Video Resizer AVAsset)

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

一个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]

不知道为什么，但如果您有多个轨道用于合成，也可以工作。