Any good source of explanatory documentation on ColorMatrix?

Question

I'd like to try using ColorMatrix, but am only able to find examples that convert an image to grayscale. And even then, they tend to be presented as a chunk of 'magic numbers' code with no explanation.

Does anyone know of a 'tutorial' on how to use ColorMatrix? For example I'd be interested in converting a grayscale image to a color image, where white == transparent, and black = a solid color, with gray pixels somewhere in between. Could ColorMatrix do that?

Answer 1

I don't know about any documentation, but as a ColorMatrix transforms one RGBAW value into another, so you can set the matrix to take the input RGB values and apply them to the output Alpha value. (The W is only there to make the matrix maths work).

OK, I think the following matrix should do what you want:

[1.0  0.0  0.0  0.333  0.0]
[0.0  1.0  0.0  0.333  0.0]
[0.0  0.0  1.0  0.333  0.0]
[0.0  0.0  0.0  0.0    0.0]
[0.0  0.0  0.0  0.0    1.0]

This will leave the current RGB values unchanged and then set the alpha to be R/3 + G/3 + B/3, i.e. the average of the RGB values. Change the 1.0 values along the diagonal to 0.0 to remove the RGB values altogether.

If you know that the image is a greyscale where R == G == B then you could replace any one of the 0.333 (recurring) with 1.0 and set the other two to 0.0