Is it possible to change a resolution of captured AR Images in CVPixelBuffer?

Question

I'm using pre-trained CoreML model inside ARKit app. And I'm capturing images from ARCamera and placing them into CVPixelBuffer for processing:

let pixelBuffer: CVPixelBuffer? = sceneView.session.currentFrame?.capturedImage

ARKit can capture pixel buffers in a YCbCr format. To correctly render these images on an iPhone's display, you'll need to access the luma and chroma planes of the pixel buffer and convert full-range YCbCr values to an sRGB using float4x4 ycbcrToRGBTransform matrix. So I understand how to handle a color.

But I'd like to know if I can change a resolution of Captured AR Images in CVPixelBuffer?

How to do it? I need a processing to be as low as possible.

Answer 1

Yes, you can do it. Here is how!

/**
 Resizes a CVPixelBuffer to a new width and height.
 */
func resizePixelBuffer(_ pixelBuffer: CVPixelBuffer,
                       width: Int, height: Int) -> CVPixelBuffer? {
    return resizePixelBuffer(pixelBuffer, cropX: 0, cropY: 0,
                             cropWidth: CVPixelBufferGetWidth(pixelBuffer),
                             cropHeight: CVPixelBufferGetHeight(pixelBuffer),
                             scaleWidth: width, scaleHeight: height)
}

func resizePixelBuffer(_ srcPixelBuffer: CVPixelBuffer,
                       cropX: Int,
                       cropY: Int,
                       cropWidth: Int,
                       cropHeight: Int,
                       scaleWidth: Int,
                       scaleHeight: Int) -> CVPixelBuffer? {

    CVPixelBufferLockBaseAddress(srcPixelBuffer, CVPixelBufferLockFlags(rawValue: 0))
    guard let srcData = CVPixelBufferGetBaseAddress(srcPixelBuffer) else {
        print("Error: could not get pixel buffer base address")
        return nil
    }
    let srcBytesPerRow = CVPixelBufferGetBytesPerRow(srcPixelBuffer)
    let offset = cropY*srcBytesPerRow + cropX*4
    var srcBuffer = vImage_Buffer(data: srcData.advanced(by: offset),
                                  height: vImagePixelCount(cropHeight),
                                  width: vImagePixelCount(cropWidth),
                                  rowBytes: srcBytesPerRow)

    let destBytesPerRow = scaleWidth*4
    guard let destData = malloc(scaleHeight*destBytesPerRow) else {
        print("Error: out of memory")
        return nil
    }
    var destBuffer = vImage_Buffer(data: destData,
                                   height: vImagePixelCount(scaleHeight),
                                   width: vImagePixelCount(scaleWidth),
                                   rowBytes: destBytesPerRow)

    let error = vImageScale_ARGB8888(&srcBuffer, &destBuffer, nil, vImage_Flags(0))
    CVPixelBufferUnlockBaseAddress(srcPixelBuffer, CVPixelBufferLockFlags(rawValue: 0))
    if error != kvImageNoError {
        print("Error:", error)
        free(destData)
        return nil
    }

    let releaseCallback: CVPixelBufferReleaseBytesCallback = { _, ptr in
        if let ptr = ptr {
            free(UnsafeMutableRawPointer(mutating: ptr))
        }
    }

    let pixelFormat = CVPixelBufferGetPixelFormatType(srcPixelBuffer)
    var dstPixelBuffer: CVPixelBuffer?
    let status = CVPixelBufferCreateWithBytes(nil, scaleWidth, scaleHeight,
                                              pixelFormat, destData,
                                              destBytesPerRow, releaseCallback,
                                              nil, nil, &dstPixelBuffer)
    if status != kCVReturnSuccess {
        print("Error: could not create new pixel buffer")
        free(destData)
        return nil
    }
    return dstPixelBuffer
}

Usage:

if let pixelBuffer = sceneView.session.currentFrame?.capturedImage, let resizedBuffer = resizePixelBuffer(pixelBuffer, width: 320, height: 480) {
    //Core Model Processing
}

Reference: https://github.com/hollance/CoreMLHelpers/tree/master/CoreMLHelpers