In GIMP, how do I get the smudge/blur tools to work properly?


Question

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 $. ]

1
9
12/31/2015 11:06:00 PM

Accepted Answer

GIMP 2.10 or higher

Image > Precision > Linear Light

GIMP 2.8 or lower

  1. Download the color profile sRGB-elle-V4-g10.icc from e.g. here.

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


Additional notes

Store the color profile in C:\WINDOWS\System32\spool\drivers\color (Windows) /Library/ColorSync/Profiles (Mac OS X) /usr/share/color/icc (Linux) so it's next to the pre-installed ones. None of the pre-installed profiles seem to do linear light.

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 actually applied!

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

Source: "Radiometrically correct editing"

6
1/5/2020 8:09:00 PM

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