Question

I have an image whose pixel colors I want to change to match a particular color (though not completely).

As an example, I want to tint the image of a red car so that it appears blue. I can do this with the GIMP and with ImageMagick, but I would like to know which algorithm they are using to do this so I can implement it in my own program.

I have tried to do this with simple addition of the difference between the colors but it doesn’t work very well.

Answer 1

As just a shot in the dark, untested suggestion from someone who’s getting into image processing fairly recently… maybe you could just scale the channels?

For example:

RGB_Pixel.r = RGB_Pixel.r * 0.75;
RGB_Pixel.g = RGB_Pixel.g * 0.75;
RGB_Pixel.b = RGB_Pixel.b * 1.25;

If you loop through your image pixel-by-pixel with those three changes, I’d expect you to see the image shift towards blue, and the numbers of course can be trial-and-error’d.

EDIT:
Now if you want to ONLY change the color of pixels that are a certain color to begin with, say, you want to turn a blue car red without doing anything to the rest of the picture, you’ll need to run a check on each pixel to see what color it looks like. One way to do this is to use a Euclidean distance:

int* R = RGB_Pixel.r;
int* G = RGB_Pixel.g;
int* B = RGB_Pixel.b;

// You are looking for Blue, which is [0 0 255];
// this variable D is the distance of your current pixel from the desired color.
float D = sqrt( (R-0)*(R-0) + (G-0)*(G-0) + (B-255)*(B-255) );

if(D < threshold)
{
    R = R * 0.75;
    G = G * 0.75;
    B = B * 1.25;
}

The threshold variable is a number between 1 and 255 that represents the maximum distance a color can be from the color you’re looking for and still be considered “close enough”. This is because you don’t want to only look for [0 0 255], very rarely will you find perfect blue (or perfect anything) in an image.

You want to use the lowest threshold you can get away with so that you don’t end up coloring other things that aren’t part of the object you’re looking for, but you want to make sure your threshold is high enough that it covers your entire image. One way to do this is to set up multiple D variables, each with a different target color, so you can capture a few separate types of “blue” without using a really high threshold. For instance, to the human eye, [102 102 200] looks like blue, but might require a pretty high threshold to catch if [0 0 255] is your target color.

I suggest playing with this calculator to get a feel for which colors you want to search for specifically.