Emulate PhotoShop's "Color Range" Algorithm

Question

I'm trying to replace a manual process done in PhotoShop with an automated process done on the server. Currently in PhotoShop the "Color Range" tool is used to select a range of colors using the "Fuzziness" factor and starting with either Black or White depending on the part of the process.

My initial approaches included both using thresholds for Luminescence in the L*a*b color space as well is DE94 between the candidate color and Black/White. In both cases I selected colors that shouldn't be selected and/or didn't select colors that should.

My hunch is that I should be using cones instead of spheres for my selection.

Can anyone give any insight into what PhotoShop is doing and if I'm heading the right direction? Also, if there is a library out there to do this that would be awesome I'm currently writing this in C.

Answer 1

From what I have seen in Photoshop, the algorithm could probably be similar to the following:

1. Define a function that calculates the closeness of two colors: for example, use a Euclidian distance in the colorspace - that is, calculate the distance between the colors of the two pixels in the RGB space using the Euclidean distance formula.
2. Next, adjust the intensity of each pixel by using a fallof function, such as the Gaussian function. You will probably need to tweak some parameters. To clarify: you calculate the distance of two pixels in RGB space (not the distance in 2D pixel coordinates), and then feed that into the falloff function which will provide a result between 0.0 and 1.0. Multiply all color components of the current pixel with the result of the falloff function for it. Do this for each pixel of the image.
3. If you want to add the range parameter of the effect, simply use the same falloff function for each pixel again, but this time feed it the Euclidean distance between the selected pixel and the current pixel in the 2D space of pixels (the distance between pixel coordinates on the image).

If you only want to select certain pixels, then instead of applying the effect directly at the pixels in the image you could store the falloff values in a matrix of doubles in range from 0.0 to 1.0. Then, choose a threshold value above which you will select the given pixel.

For example, if the step 2. for pixel at the coordinate (x, y) yielded 0.8 and the step 3. yielded 0.5, then the value of the matrix element with coordinates x and y should be 0.8*0.5=0.4. If you picked a selection threshold below 0.4, you would select pixel (x, y), otherwise, you would not.