On Tue, 2017-06-06 at 06:02 +0200, Lancer wrote:
> I am a school teacher. One of the checks I ask students to do in
> order to test the contrast of their graphics work, is to convert the
> images to grayscale and see whether images are still clear.
As you discovered, there are different ways to convert to greys.
However, people's colour perception varies, with more than one in 10
having some form of "colour blindness" (depending on how you measure)
or eye difficulty - and more than that percentage unable to read small
text, of course.
I don't know of any accessibility checkers for GIMP; there are
PhotoShop plugins. It'd be a good Google Summer of Code project I
suppose, if that's still going. I might even be able to drum up some
funding for work in the area, and/or technical resources.
Because of the differences in people's vision, I don't think it matters
which method is used to convert. The people who have poor colour vision
will be exactly the ones who see the brightnesses differently, e.g. if
their eye doesn't respond well to reds (the most common problem with
human males) then reds will likely appear darker to them. My father
couldn't tell the difference between a traffic light that was all dark
and one with just red showing. So all the methods will be "wrong".
If the goal is just to make sure the image reproduces OK on a black-
and-white printer, have them send the RGB image to the laser printer,
then convert to greyscale in several different ways and compare, and
they can learn a lot, it's a good exercise. The default dot screen on
PostScript printers is actually fairly mediocre and most professional
graphic design software replaces it, or used to.
Numerically, it's about colour spaces and gamma and precision and the
purpose ("intent") of the conversion. The gegl c-to-g filter sometimes
gets much better results than either the mono mixer or desaturating.
Liam
--
Liam R E Quin <l...@holoweb.net>
ankh on irc
Web slave for fromoldbooks.org
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