[Gimp-developer] Gegl gaussian blur gamma error

[Elle Stone <l elle stone gmail com>]

The general and good advice is to not use Gimp 2.9 for "production"
work. But I've been using Gimp 2.9 for everything I do with Gimp these
days and so far it's been rock solid (albeit very slow on large
images). I quite like the new Gimp and I've finally gotten used to
using "Save" and "Export".

I took a break from working on the lcms2 plugin to write an article on
gamma artifacts from mixing colors (gaussian blur, Normal mode
blending) in a nonlinear RGB color space. I used Gimp 2.9 (last
compiled July 25, 2012, and using my lcms2 plugin) for all of the
image blurring and blending. If anyone is interested, the article is
here:
Linear Gamma vs Higher Gamma RGB Color Spaces: Gaussian Blur and
Normal Blend Mode
http://ninedegreesbelow.com/photography/linear-gamma-blur-normal-blend.html

While working on the article, I noticed that the Gimp Gegl gaussian
blur nicely blurs *regular* sRGB images without the darkening
artifacts that normally accompany gaussian blurring. But when used on
a linear gamma image, the Gegl gaussian blur makes it look like the
image was actually blurred in a gamma=0.45 color space (there are
"lightening" artifacts). So similar to the problem when opening a
16-bit tif, there seems to be a strange gamma error involved.

The regular Gimp 2.9 gaussian blur works exactly as it is supposed to
do on both linear and regular sRGB, that is, there are darkening
artifacts in the regular sRGB color space, but not in the linear sRGB
color space.

I put up some sample images showing the Gimp Gegl gaussian blur vs regular blur:
Gimp 2.9 Gegl and regular gaussian blur
http://ninedegreesbelow.com/temp/gimp29-gegl-blur.html

Also, it seems the Gegl gaussian blur "Size" is different from regular
gaussian blur "pixel": Size 10 seems to be approximately 30 pixels,
which will take some getting used to. I didn't check to see if the
Size/pixel ratio seems constant across different sizes. Does anyone
know if it is?

Elle Stone

-- 
http://ninedegreesbelow.com
Articles and tutorials on open source digital imaging and photography