Re: [Gimp-user] [Gimp-developer] Test images and test suite (was Re: GIMP should fork babl and GEGL)

[Elle Stone <ellestone ninedegreesbelow com>]

On 11/11/2014 11:45 AM, Gary Aitken wrote:
> On 11/11/14 08:15, Elle Stone wrote:
>
> > So the important question is:
> >
> > *What copyrighted or otherwise test images GIMP users might have
> > downloaded from the internet and found useful, and *what kinds of
> > test images GIMP users might have already put together, *for what
> > particular testing purposes.
>
> The things I find most useful are the following:
>
> 1. Peoples' faces.  Important here is a wide selection of ethnicities,
>     such as white, asian, african, latino.  It would be useful to have
>     individual images; and combination images with the extremes (e.g.
>     white, african, and wedding dress in the same image).  In the
>     african group, a wide range from very black to brown.

Despite the plethora of CC- and public domain images, finding suitable 
test images is harder than one might think. It's obvious that a lot of 
time and thought went into Nicolas Robidoux's collection 
(http://www.imagemagick.org/download/image-bank/16bit840x840images).

In particular, it's not easy to find faces that were properly 
white-balanced and haven't been "in-camera jpeg processed" to have 
saturated colors and dark shadows.

Maybe Pat David (and other GIMP users) could suggest some good face 
images that were shot raw, preferably in full spectrum lighting 
(daylight, direct sunlight, high quality studio lighting, etc), and 
properly white balanced, that could be release under a suitable license?

> 2. Highly saturated from the natural world -- birds and flowers, for
>     example.  I may have a few birds of use.  Others may well have
>     better images.

Raw files of such images from a variety of cameras would be very good to 
have.

> 3. Color gradients and ICC targets.
>     In this category I would find useful a chart specifically designed
>     to show the boundaries of different well-known colorspaces, such
>     as sRGB, AdobeRGB, and ProPhoto.

Hmm, ArgyllCMS can be used to relate actual image colors to the 
boundaries of any given RGB working color space, and also to compare 
different RGB working space color gamuts. Are the images on this page 
sort of what you were thinking of?

http://ninedegreesbelow.com/photography/icc-profile-conversion-clipped-colors-examples.html#flower-color-gamut

http://ninedegreesbelow.com/photography/icc-profile-conversion-clipped-colors-examples.html#srgb-prophoto-cielab

>   If the chart had an outline of
>     the colorspaces as separate .xcf layers it would be great.  It
>     might be useful to have layers for some printers as well.

The thing is, color gamuts are 3-dimensional, so 2D charts only tell 
part of the story. 
http://brucelindbloom.com/index.html?WorkingSpaceInfo.html has great 
interactive tools for comparing color spaces. Click on the 3D Gamut 
Viewer link.

Also there is the related question of how much any give RGB working 
space overlaps with real-world colors as seen by the mythical average 
human. Lindbloom's chart indicates that ProPhotoRGB, for example, 
contains a lot of imaginary colors and also holds almost all real 
colors, whereas sRGB has no imaginary colors but only holds about 35% of 
all real colors.

Also see this Lindbloom page for a comparison between ProPhotoRGB and 
real world LAB colors as normally encoded by imaging software:
http://brucelindbloom.com/index.html?LabGamutDisplayHelp.html

I think what you are asking about might fall into the category of better 
soft-proofing tools. Right now GIMP's softproofing is pretty much only 
what's available from LCMS, which sadly doesn't deal with "real vs. 
imaginary" and also does a terrible job with linear gamma working spaces 
(not relevant to 8-bit GIMP, but very important for high bit depth GIMP).

A test image image that shows more or less finely granulated colors 
spanning the full range of real world colors would be *very* nice to 
have, but I don't know how to make such an image. Lindbloom does supply 
Lab Gamut test images, but the license is restrictive (see 
http://brucelindbloom.com/index.html?ProfileEvaluation.html).

Elle