I use the smudge tool very often in GIMP while painting. Unfortunately, the algorithm tends to generate a much darker version of what the smudged color ought to look like. For instance, a normal smudge on GIMP looks like this:

enter image description here

But I want the smudged color to be a lighter yellow, sort of like this:

enter image description here

In general all smudged colors appear darker and I understand this has something to with GIMP working with sRGB colors as opposed to RGB. But I'd really like to know what setting I need to tweak in GIMP to correct the smudge algorithm (maybe blur too, but I use smudge much more).

[ I assumed sRGB takes the square roots of the original value, so when the smudge tries to average colors between two pixels 'a' and 'b', it takes $ (\sqrt{a} + \sqrt{b})/2 $, which is darker than $ sqrt(a/2 + b/2)^2 $. ]

up vote 7 down vote accepted

GIMP 2.9 or higher

Image > Precision > Linear Light

GIMP 2.8 or lower

Image > Mode > Convert to Color Profile... > Select color profile from disk... > sRGB-elle-V4-g10.icc

You can obtain sRGB-elle-V4-g10.icc e.g. from here.

Source

I summarized what is discussed here as "radiometrically correct editing".

Additional notes

I tested this with brush, smudge and Gaussian blur and they all worked. However, the preview for Gaussian blur showed me the wrong (perceptual gamma) colors, but produced the right (linear light) result when applied.

I also recommend the YouTube video "Computer Color is Broken" explaining the issue.

Linux stores color profiles in /usr/share/color/icc, however none of the preinstalled iccs I found there did the same thing as the linear light icc, so I think you do have to look on the internet for it. That directory might be a good place to put it though. Windows: C:\WINDOWS\System32\spool\drivers\color. Mac OS X: /Library/ColorSync/Profiles

I didn't test the GIMP 2.9+ solution as Arch Linux still ships 2.8 (it usually ships the newest package, so... weird).

  • 1
    The newest stable package is GIMP 2.8 - 2.9.x are development versions. It is easy to build it yourself on any reasonably recent distro, and run it in addition to 2.8, though. – Michael Schumacher Jan 24 '16 at 22:23
  • 1
    Regarding Mac color profiles: ~/Library/ColorSync/Profiles is better if you don't need to install it for all users of the machine, or if you don't have administrator access. – seaturtle Dec 1 '17 at 19:07

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