Calculate color similarity/distance in RGBA color space

distanceToNSColor.m

@implementation NSColor (DDExtensions)

- (float)distanceTo:(NSColor*)color
{
    // Calculate color similarity/distance in RGBA color space
    // http://stackoverflow.com/questions/4754506/color-similarity-distance-in-rgba-color-space
    
    //    max((r₁-r₂)², (r₁-r₂ - a₁+a₂)²) +
    //    max((g₁-g₂)², (g₁-g₂ - a₁+a₂)²) +
    //    max((b₁-b₂)², (b₁-b₂ - a₁+a₂)²)
    return MAX(pow(self.redComponent - color.redComponent, 2), pow(self.redComponent - color.redComponent - self.alphaComponent + color.alphaComponent, 2))
           + MAX(pow(self.greenComponent - color.greenComponent, 2), pow(self.greenComponent - color.greenComponent - self.alphaComponent + color.alphaComponent, 2))
           + MAX(pow(self.blueComponent - color.blueComponent, 2), pow(self.blueComponent - color.blueComponent - self.alphaComponent + color.alphaComponent, 2));
}

@end