Question

我才刚刚开始学习AVFoundation框架。导出视频时，我会在导出视频之前将视频合成渲染尺寸缩小到屏幕尺寸（UIScreen.main.bounds），因为它的自然尺寸（我认为是不必要的）会很大。（至少，缩放比例因子也在Ray Wenderlich教程中完成-https://www.raywenderlich.com/5135-how-to-play-record-and-merge-videos-in-ios-and-swift，请参见下面的代码）。但是，我发现这种缩放实际上会降低导出视频的质量（与渲染大小=自然视频大小相比）。为什么会这样呢？我知道将视频扩大到更大的位置会导致质量变差，但是如果我们只是将视频缩小到较小的尺寸（屏幕尺寸），为什么质量会变差？

是否有一种方法可以保持清晰的质量，而不必将渲染尺寸保持在其自然尺寸？

static func videoCompositionInstruction(_ track: AVCompositionTrack, asset: AVAsset) 
  -> AVMutableVideoCompositionLayerInstruction {
  let instruction = AVMutableVideoCompositionLayerInstruction(assetTrack: track)
  let assetTrack = asset.tracks(withMediaType: .video)[0]

  let transform = assetTrack.preferredTransform
  let assetInfo = orientationFromTransform(transform)

  var scaleToFitRatio = UIScreen.main.bounds.width / assetTrack.naturalSize.width
  if assetInfo.isPortrait {
    scaleToFitRatio = UIScreen.main.bounds.width / assetTrack.naturalSize.height
    let scaleFactor = CGAffineTransform(scaleX: scaleToFitRatio, y: scaleToFitRatio)
    instruction.setTransform(assetTrack.preferredTransform.concatenating(scaleFactor), at: kCMTimeZero)
  } else {
    let scaleFactor = CGAffineTransform(scaleX: scaleToFitRatio, y: scaleToFitRatio)
    var concat = assetTrack.preferredTransform.concatenating(scaleFactor)
      .concatenating(CGAffineTransform(translationX: 0, y: UIScreen.main.bounds.width / 2))
    if assetInfo.orientation == .down {
      let fixUpsideDown = CGAffineTransform(rotationAngle: CGFloat(Double.pi))
      let windowBounds = UIScreen.main.bounds
      let yFix = assetTrack.naturalSize.height + windowBounds.height
      let centerFix = CGAffineTransform(translationX: assetTrack.naturalSize.width, y: yFix)
      concat = fixUpsideDown.concatenating(centerFix).concatenating(scaleFactor)
    }
    instruction.setTransform(concat, at: kCMTimeZero)
  }

  return instruction
}

为什么VideoCompositionInstruction Render Size等于屏幕尺寸破坏质量？

0 个答案: