使用 vImage_Scale 和 kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange

如何解决使用 vImage_Scale 和 kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange

我从 iPhone 的前置摄像头收到了 CMSampleBuffer。目前它的大小是 1920x1080，我想把它缩小到 1280x720。我想使用 vImageScale 函数，但无法正常工作。来自相机的像素格式是 kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange，所以我尝试了以下方法，但它输出了一个不正确的奇怪的绿色图像：

private var scaleBuffer: vImage_Buffer = {
  var scaleBuffer: vImage_Buffer = vImage_Buffer()
  let newHeight = 720
  let newWidth = 1280
  scaleBuffer.data = UnsafeMutableRawPointer.allocate(byteCount: Int(newWidth * newHeight * 4),alignment: MemoryLayout<UInt>.size)
  scaleBuffer.width = vImagePixelCount(newWidth)
  scaleBuffer.height = vImagePixelCount(newHeight)
  scaleBuffer.rowBytes = Int(newWidth * 4)
  return scaleBuffer
}()

func captureOutput(_ output: AVCaptureOutput,didOutput sampleBuffer: CMSampleBuffer,from connection: AVCaptureConnection)
    {

  guard let imageBuffer = CMSampleBufferGetimageBuffer(sampleBuffer) else {
    return
  }

  CVPixelBufferLockBaseAddress(imageBuffer,CVPixelBufferLockFlags(rawValue: 0))

  // create vImage_Buffer out of CVImageBuffer
  var inBuff: vImage_Buffer = vImage_Buffer()
  inBuff.width = UInt(CVPixelBufferGetWidth(imageBuffer))
  inBuff.height = UInt(CVPixelBufferGetHeight(imageBuffer))
  inBuff.rowBytes = CVPixelBufferGetBytesPerRow(imageBuffer)
  inBuff.data = CVPixelBufferGetBaseAddress(imageBuffer)

  // perform scale
  var err = vImageScale_CbCr8(&inBuff,&scaleBuffer,nil,0)
  if err != kvImageNoError {
      print("Can't scale a buffer")
      return
  }
  CVPixelBufferUnlockBaseAddress(imageBuffer,CVPixelBufferLockFlags(rawValue: 0))

  var newBuffer: CVPixelBuffer?
  let attributes : [NSObject:AnyObject] = [
    kCVPixelBufferCGImageCompatibilityKey : true as AnyObject,kCVPixelBufferCGBitmapContextCompatibilityKey : true as AnyObject
  ]

  let status = CVPixelBufferCreateWithBytes(kcfAllocatorDefault,Int(scaleBuffer.width),Int(scaleBuffer.height),kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange,scaleBuffer.data,Int(scaleBuffer.width) * 4,attributes as CFDictionary?,&newBuffer)

  guard status == kCVReturnSuccess,let b = newBuffer else {
    return
  }

  // Do something with the buffer to output it
}

这里出了什么问题？看看这个答案 here，看起来我需要分别缩放“Y”和“UV”平面。我怎样才能迅速做到这一点，然后将它们组合回一个 CVPixelBuffer？

解决方法

从 imageBuffer 返回的 CMSampleBufferGetImageBuffer 实际上包含两个离散平面 - 一个亮度平面和一个色度平面（请注意，对于 420，色度平面是亮度平面的一半大小）。这在 this sample code project 中讨论。

这让您几乎完成了。我没有使用 Core Video CVPixelBufferCreateWithBytes 的经验，但此代码将为您创建缩放的 Yp 和 CbCr 缓冲区，并将它们转换为交错的 ARGB 缓冲区：

let lumaBaseAddress = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer,0)
let lumaWidth = CVPixelBufferGetWidthOfPlane(pixelBuffer,0)
let lumaHeight = CVPixelBufferGetHeightOfPlane(pixelBuffer,0)
let lumaRowBytes = CVPixelBufferGetBytesPerRowOfPlane(pixelBuffer,0)

var sourceLumaBuffer = vImage_Buffer(data: lumaBaseAddress,height: vImagePixelCount(lumaHeight),width: vImagePixelCount(lumaWidth),rowBytes: lumaRowBytes)

let chromaBaseAddress = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer,1)
let chromaWidth = CVPixelBufferGetWidthOfPlane(pixelBuffer,1)
let chromaHeight = CVPixelBufferGetHeightOfPlane(pixelBuffer,1)
let chromaRowBytes = CVPixelBufferGetBytesPerRowOfPlane(pixelBuffer,1)

var sourceChromaBuffer = vImage_Buffer(data: chromaBaseAddress,height: vImagePixelCount(chromaHeight),width: vImagePixelCount(chromaWidth),rowBytes: chromaRowBytes)

var destLumaBuffer = try! vImage_Buffer(size: CGSize(width: Int(sourceLumaBuffer.width / 4),height: Int(sourceLumaBuffer.height / 4)),bitsPerPixel: 8)

var destChromaBuffer = try! vImage_Buffer(size: CGSize(width: Int(sourceChromaBuffer.width / 4),height: Int(sourceChromaBuffer.height / 4)),bitsPerPixel: 8 * 2)

vImageScale_CbCr8(&sourceChromaBuffer,&destChromaBuffer,nil,0)
vImageScale_Planar8(&sourceLumaBuffer,&destLumaBuffer,0)

var argbBuffer = try! vImage_Buffer(size: destLumaBuffer.size,bitsPerPixel: 8 * 4)

vImageConvert_420Yp8_CbCr8ToARGB8888(&destLumaBuffer,&argbBuffer,&infoYpCbCrToARGB,255,vImage_Flags(kvImagePrintDiagnosticsToConsole))

destLumaBuffer.free()
destChromaBuffer.free()
argbBuffer.free()