I have some old hand-written course notes scanned that I wrote on green engineering grid paper. I find the greenness of the scan distracting and excessively low-contrast when viewing on a computer, and I am concerned about the sizes of the scans, so I would like to “filter out” the paper print itself, particularly the background itself and the grid, leaving a grayscale image with higher contrast and less noise than would be possible with an off-the-shelf grayscale filter.
In The GIMP, I can specify a color mix with negative coefficients, resulting in more or less precisely the result I want. Here, I used Mono Mixer with RGB=(-1.0, 2.0, -0.213), followed by Levels=(0.2,0.8). This obviously doesn’t look spectacular, but given the source material and the scanner in use, it is probably about as good as it’s going to get.
I don’t believe this process is possible to achieve in Acorn, because the Grayscale filter only permits red/green/blue settings in the 0%-100% range, and I can’t locate any other filters which can achieve the same result. (In particular, Remove Color Cast does nothing to remove the gridlines.)
AFAIK, if Acorn permitted real-valued inputs — or at least inputs between -100% and 200% — it could accomplish what I want to do.