Question

Wondering if there is a way to isolate a single color in an image either using masks or perhaps even a custom color space. I’m ultimately looking for a fast way to isolate 14 colors out of an image – figured if there was a masking method it might may be faster than walking through the pixels.

Any help is appreciated!

Answer 1

You could use a custom color space (documentation here) and then substitute it for “CGColorSpaceCreateDeviceGray()” in the following code:

- (UIImage *)convertImageToGrayScale:(UIImage *)image
{
  // Create image rectangle with current image width/height
  CGRect imageRect = CGRectMake(0, 0, image.size.width, image.size.height);

  // Grayscale color space 
  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray(); // <- SUBSTITUTE HERE

  // Create bitmap content with current image size and grayscale colorspace
  CGContextRef context = CGBitmapContextCreate(nil, image.size.width, image.size.height, 8, 0, colorSpace, kCGImageAlphaNone);

  // Draw image into current context, with specified rectangle
  // using previously defined context (with grayscale colorspace)
  CGContextDrawImage(context, imageRect, [image CGImage]);

  // Create bitmap image info from pixel data in current context
  CGImageRef imageRef = CGBitmapContextCreateImage(context);

  // Create a new UIImage object  
  UIImage *newImage = [UIImage imageWithCGImage:imageRef];

  // Release colorspace, context and bitmap information
  CGColorSpaceRelease(colorSpace);
  CGContextRelease(context);
  CFRelease(imageRef);

  // Return the new grayscale image
  return newImage;
}

This code is from this blog which is worth a look at for removing colors from images.