Question

For image scanning purposes, I’d like a pixel (which I can get from a UIImage) to match (for a certain percentage) to a pre-set color.

Say pink. When I scan the image for pixels that are pink, I want a function to return a percentage of how much the RGB value in the pixel looks like my pre-set RGB value. This way I’d like all (well, most) pink pixels to become ‘visible’ to me, not just exact matches.

Is anyone familiar with such an approach? How would you do something like this?

Thanks in advance.

UPDATE: thank you all for your answers so far. I accepted the answer from Damien Pollet because it helped me further and I came to the conclusion that calculating the vector difference between two RGB colors does it perfectly for me (at this moment). It might need some tweaking over time but for now I use the following (in objective c):

float difference = pow( pow((red1 - red2), 2) + pow((green1 - green2), 2) + pow((blue1 - blue2), 2), 0.5 );

If this difference is below 85, I accept the color as my target color. Since my algorithm needs no precision, I’m ok with this solution 🙂

UPDATE 2: on my search for more I found the following URL which might be quite (understatement) useful for you if you are looking for something similar.

http://www.sunsetlakesoftware.com/2010/10/22/gpu-accelerated-video-processing-mac-and-ios

Answer 1

I would say just compute the vector difference to your target color, and check that it’s norm is less than some threshold. I suspect some color spaces are better than others at this, maybe HSL or L*ab, since they separate the brightness from the color hue itself, and so might represent a small perceptual difference by a smaller color vector…

Also, see this related question