同时使用AVComposition和AVVideoComposition时转换错误

编程问答 2022-06-09

问题描述

我正在创建一个AVMutableComposition。我需要将资产水平翻转，因此我要像这样设置合成轨道的变换：

compositionTrack.preferredTransform = assetTrack.preferredTransform.scaledBy(x: -1,y: 1)

如果我导出此文件（我使用AVAssetPreset960x640作为预设），则可以正常工作。

但是，我还需要添加一个AVMutableVideoComposition叠加层才能使用此copmosition进行渲染。此叠加层不应水平翻转。我这样指定：

// Create video composition
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: composition.duration
)
let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: compositionTrack)
layerInstruction.setTransform(assetTrack.preferredTransform,at: .zero)
    
instruction.layerInstructions = [layerInstruction]
videoComposition.instructions = [instruction]

当我以此导出视频时，它不会水平翻转。它将停止执行应用于合成的PreferredTransform。大概是这条线导致了这一点：

layerInstruction.setTransform(assetTrack.preferredTransform,at: .zero)

如果将图层变换设置为assetTrack.preferredTransform.scaledBy(x: -1,y: 1)或compositionTrack.preferredTransform，则导出时会出现黑屏。

Apple有these docs解释了AVVideoComposition上的转换。如果我理解正确，他们会说我应该设置图层指令的转换。如果我这样做-对图层指令而不是对合成应用转换-我仍然会看到黑屏。

这是为什么，我该如何解决？

解决方法

编辑：有效的转换是

CGAffineTransform(a: -1.0,b: 0.0,c: 0.0,d: 1.0,tx: videoSize.width,ty: 0.0)

因此，在RayWenderlich tutorial之后，我又遇到了这个问题（您的代码看起来也非常熟悉！）。问题是assetTrack.preferredTransform不可靠，在某些视频上它可以工作，而在另一些视频上则是黑屏。

代替

let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: compositionTrack)
layerInstruction.setTransform(assetTrack.preferredTransform,at: .zero)

尝试一下：

func compositionLayerInstruction(for track: AVCompositionTrack,assetTrack: AVAssetTrack,orientation: UIImage.Orientation) -> AVMutableVideoCompositionLayerInstruction {

    let instruction = AVMutableVideoCompositionLayerInstruction(assetTrack: track)
    
    var transform = CGAffineTransform.identity
    let assetSize = assetTrack.naturalSize
    
    /// you should be able to play with these values to make a horizontal flip (try changing `a` and `d`)
    switch orientation {
    case .up:
        transform = CGAffineTransform(a: 1,b: 0,c: 0,d: 1,tx: 0,ty: 0)
    case .down:
        transform = CGAffineTransform(a: -1,d: -1,tx: assetSize.width,ty: assetSize.height)
    case .left:
        transform = CGAffineTransform(a: 0,b: -1,c: 1,d: 0,ty: assetSize.width)
    case .right:
        transform = CGAffineTransform(a: 0,b: 1,c: -1,tx: assetSize.height,ty: 0)
     default:
        print("Unsupported orientation")
    }
    
    instruction.setTransform(transform,at: .zero)
    
    return instruction
}

let layerInstruction = compositionLayerInstruction(
    for: compositionTrack,assetTrack: assetTrack,orientation: videoInfo.orientation)
/// assetTrack is the original video

我是根据SO的另一个答案，虽然不记得在哪里。

这是一个帮助函数，用于获取orientation 

/// adjust the video orientation is the source has a different orientation
private func orientation(from transform: CGAffineTransform) -> (orientation: UIImage.Orientation,isPortrait: Bool) {
    var assetOrientation = UIImage.Orientation.up
    var isPortrait = false
    if transform.a == 0 && transform.b == 1.0 && transform.c == -1.0 && transform.d == 0 {
        assetOrientation = .right
        isPortrait = true
    } else if transform.a == 0 && transform.b == -1.0 && transform.c == 1.0 && transform.d == 0 {
        assetOrientation = .left
        isPortrait = true
    } else if transform.a == 1.0 && transform.b == 0 && transform.c == 0 && transform.d == 1.0 {
        assetOrientation = .up
    } else if transform.a == -1.0 && transform.b == 0 && transform.c == 0 && transform.d == -1.0 {
        assetOrientation = .down
    }
    return (assetOrientation,isPortrait)
}

完整用法：

/// get adjusted orientation of the video
let videoInfo = orientation(from: assetTrack.preferredTransform)
let videoSize: CGSize

if videoInfo.isPortrait {
    videoSize = CGSize(
    width: assetTrack.naturalSize.height,height: assetTrack.naturalSize.width)
} else {
    videoSize = assetTrack.naturalSize
}


 /// the video
let videoLayer = CALayer()
videoLayer.frame = CGRect(origin: .zero,size: videoSize)
        
/// overlay where you add graphics and other stuff
let overlayLayer = CALayer()
overlayLayer.frame = CGRect(origin: .zero,size: videoSize)

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)

// MARK: - previous `let layerInstruction` code goes here...

instruction.layerInstructions = [layerInstruction]

/// export session code here...
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