Using vImage_Scale with kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange

Question

I am receiving a CMSampleBuffer from the front camera of my iPhone. Currently its size is 1920x1080, and I want to scale it down to 1280x720. I want to use the vImageScale function but I can't get it working correctly. The pixel format from the camera is kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange, so I have tried the following, but it outputs a weird green image which isn't correct:

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,
                                                  nil, nil,
                                                  attributes as CFDictionary?, &newBuffer)

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

  // Do something with the buffer to output it
}

What's going wrong here? Looking at this answer here, it looks like I need to scale the "Y" and the "UV" planes separately. How can I do that in swift and then combine them back into one CVPixelBuffer?

Answer 1

The imageBuffer that's returned from CMSampleBufferGetImageBuffer actually contains two discrete planes - a luminance plane and a chrominance plane (note that for 420, the chrominance plane is half the size of the luminance plane). This is discussed in this sample code project.

This gets you almost there. I don't have experience with the Core Video CVPixelBufferCreateWithBytes, but this code will create you the scaled Yp and CbCr buffers, and convert them to an interleaved ARGB buffer:

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, nil, 0)

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

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

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