UIImage scale using vImage

gistfile1.m

-(UIImage*)mmg_imageScaledToFitSize:(CGSize)fitSize
{
	// Create a vImage_Buffer from the CGImage
	CGImageRef sourceRef = self.CGImage;
	vImage_Buffer srcBuffer;
	vImage_CGImageFormat format = {
		.bitsPerComponent = 8,
		.bitsPerPixel = 32,
		.colorSpace = NULL,
		.bitmapInfo = (CGBitmapInfo)kCGImageAlphaFirst,
		.version = 0,
		.decode = NULL,
		.renderingIntent = kCGRenderingIntentDefault,
	};
	vImage_Error ret = vImageBuffer_InitWithCGImage(&srcBuffer, &format, NULL, sourceRef, kvImageNoFlags);
	if (ret != kvImageNoError)
	{
		free(srcBuffer.data);
		return nil;
	}

	// Create dest buffer
	const NSUInteger scale = (NSUInteger)[[UIScreen mainScreen] scale];
	const NSUInteger dstWidth = (NSUInteger)fitSize.width * scale;
	const NSUInteger dstHeight = (NSUInteger)fitSize.height * scale;
	const NSUInteger bytesPerPixel = 4;
	const NSUInteger dstBytesPerRow = bytesPerPixel * dstWidth;
	uint8_t* dstData = (uint8_t*)calloc(dstHeight * dstWidth * bytesPerPixel, sizeof(uint8_t));
	vImage_Buffer dstBuffer = {
		.data = dstData,
		.height = dstHeight,
		.width = dstWidth,
		.rowBytes = dstBytesPerRow
	};

	// Scale
	ret = vImageScale_ARGB8888(&srcBuffer, &dstBuffer, NULL, kvImageHighQualityResampling);
	free(srcBuffer.data);
	if (ret != kvImageNoError)
	{
		free(dstData);
		return nil;
	}

	// Create CGImage from vImage_Buffer
	ret = kvImageNoError;
	CGImageRef destRef = vImageCreateCGImageFromBuffer(&dstBuffer, &format, NULL, NULL, kvImageNoFlags, &ret);
	free(dstData);

	// Create UIImage
	UIImage* destImage = [[UIImage alloc] initWithCGImage:destRef scale:0.0 orientation:self.imageOrientation];
	// Free up the remaining memory
	CGImageRelease(destRef);

	return destImage;
}

Swift version:

func scaled(toFit fitSize: CGSize) -> UIImage? {
    // Create a vImage_Buffer from the CGImage
    guard let sourceRef = cgImage else { return nil }
    
    var srcBuffer = vImage_Buffer()
    var format = vImage_CGImageFormat(bitsPerComponent: 8,
                                      bitsPerPixel: 32,
                                      colorSpace: nil,
                                      bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.first.rawValue),
                                      version: 0,
                                      decode: nil,
                                      renderingIntent: .defaultIntent)
    var ret = vImageBuffer_InitWithCGImage(&srcBuffer, &format, nil, sourceRef, vImage_Flags(kvImageNoFlags))
    
    guard ret == kvImageNoError else {
      free(srcBuffer.data)
      return nil
    }
    
    // Create dest buffer
    let scale = UIScreen.main.scale
    let dstWidth = Int(fitSize.width * scale)
    let dstHeight = Int(fitSize.height * scale)
    let bytesPerPixel: Int = 4
    let dstBytesPerRow: Int = bytesPerPixel * dstWidth
    let dstData = UnsafeMutablePointer<UInt8>.allocate(capacity: dstHeight * dstWidth * bytesPerPixel)
    
    var dstBuffer = vImage_Buffer(data: dstData,
                                  height: vImagePixelCount(dstHeight),
                                  width: vImagePixelCount(dstWidth),
                                  rowBytes: dstBytesPerRow)

    // Scale
    ret = vImageScale_ARGB8888(&srcBuffer, &dstBuffer, nil, vImage_Flags(kvImageHighQualityResampling))
    free(srcBuffer.data)
    
    guard ret == kvImageNoError else { return nil }
    
    // Create CGImage from vImage_Buffer
    ret = kvImageNoError
    guard let destRef = vImageCreateCGImageFromBuffer(&dstBuffer, &format, nil, nil, vImage_Flags(kvImageNoAllocate), &ret)?.takeRetainedValue() else {
      return nil
    }
    
    guard ret == kvImageNoError else { return nil }
    
    // Create UIImage
    let destImage = UIImage(cgImage: destRef, scale: 0.0, orientation: imageOrientation)
    return destImage
  }