Can ImageMagick do channel difference and swap?

Question

In emulating Kodak Infrared Ektachrome (EIR) film (a.k.a. Aerochrome) with images from a full-spectrum-modified digital camera, I want to subtract the blue channel from the red and green channel, then re-map the blue channel to red, red to green, and green to blue. (This is called "IRG to RBG transform".)

Digital infrared enthusiasts typically do this via the Photoshop "channel mixer," but that is tedious and slow in a GUI. I'd like to batch-process it.

Is this the sort of thing ImageMagick can do?

If so, can you provide a lead as to how it would be accomplished? It is not obvious from the documentation — I can see how to swap colour channels around, but not how to subtract one channel from another.

If not, is there some other open-source command-line program that could do this?

Thank you for any leads offered!

Answer 1

Check out the -color-matrix operator to see if it meets your needs.

From your description, I think the following RGB matrix transform will work:

 0  0  1   (R =  B)
 1  0 -1   (G =  R - B)
 0  1 -1   (B =  G - B)

But from the graphic in your post, it looks like the following RGB matrix transform is what you're after:

0    0     1      (R = B)
0.8  0    -0.8    (G = 0.8*R - 0.8*B = 0.8 * (R - B))
0    0.96 -0.96   (B = 0.96*G - 0.96*B = 0.96 * (G - B))

So your command would look like (using the 2nd matrix, for example):

magick <input_file> -color-matrix ` 0    0     1
                                    0.8 -0.8   0
                                    0    0.96 -0.96 ` <output_file>

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Note that color channels are non-negative. Because you are subtracting channels, you might need to make sure your resulting values are non-negative as well, by adding a constant offset for the channels that had subtraction. The color matrix can accommodate 3 additional columns: 'Black', 'Alpha', and Constant, in that order. I assume your Black and Alpha channels are zero. In order to specify Constant, you have to specify '0' for Black and Alpha. So the final matrix might look like:

0    0     1     0  0  0     (R = B)
0.8  0    -0.8   0  0  0.8   (G = 0.8R - 0.8B + 0.8)
0    0.96 -0.96  0  0  0.96  (B = 0.96G - 0.96B + 0.96)

You'll have to experiment to see if you need to specify the constant value.