Wanted: negative color mixing

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.

That’s a nifty idea, and I’m surprised it hasn’t been brought up yet. I’ve just implemented it for the next update of Acorn, which you can grab a preview of from here: https://flyingmeat.com/download/latest/

Try it out and let me know what you think.

From the release notes:

  • New: The Grayscale Filter has extended the range of values for the RGB sliders from 0-100% to -200-200%. In addition to this, it now includes an Intensity slider which controls the blending of the grayscale filter. This allows for more creative channel mixing when making monochromatic images.

Yup! That’s what I was thinking of, thank you.

If there was a way of adding detents to the RGB sliders at 0% and 100% that would probably be advised — I freely admit that the vast-majority use case for this filter probably involves those two values specifically.

Detents are a good idea! I’ll consider it.