Alpha mask from CGImage

CGImage-AlphaMask.m

CGImageRef createAlphaMask(CGImageRef inputImage) {
  int width = (int)CGImageGetWidth(inputImage);
  int height = (int)CGImageGetHeight(inputImage);
  int bytesPerRow = (int)CGImageGetBytesPerRow(inputImage);
  CGColorSpaceRef colorSpace = CGImageGetColorSpace(inputImage);
  CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(inputImage);

  unsigned char *data = (unsigned char *)calloc(bytesPerRow * height, 1);
  CGContextRef ctx = CGBitmapContextCreate(data,
                                           width,
                                           height,
                                           8,
                                           bytesPerRow,
                                           colorSpace,
                                           bitmapInfo);
  
  if (!ctx) {
    return nil;
  }

  int alphaIndex = bitmapInfo & (kCGImageAlphaFirst | kCGImageAlphaPremultipliedFirst)
    ? 0
    : 3;
  
  if (bitmapInfo & kCGImageByteOrder32Little) {
    alphaIndex = 3 - alphaIndex;
  }

  CGDataProviderRef inputProvider = CGImageGetDataProvider(inputImage);
  NSData *inputData = (__bridge NSData *)CGDataProviderCopyData(inputProvider);
  unsigned char *inputBytes = (unsigned char *)inputData.bytes;
  
  for (int y = 0; y < height; y++) {
    for (int x = 0; x < width; x++) {
      int offset = (x + y * width) * 4;
      int alpha = inputBytes[offset + alphaIndex];
      if (alpha > 0) {
        alpha = 255;
      }
      data[offset + 0] = alphaIndex == 0 ? alpha : 0;
      data[offset + 1] = 0;
      data[offset + 2] = 0;
      data[offset + 3] = alphaIndex == 3 ? alpha : 0;
    }
  }
  
  CGImageRef image = CGBitmapContextCreateImage(ctx);
  CGContextRelease(ctx);
  free(data);
  
  return image;
}