[Gimp-developer] Gimp and OpenEXR and Darktable

[Elle Stone <ellestone ninedegreesbelow com>]

Darktable can save OpenEXR files and Gimp can open them. Which 
theoretically means that an interpolated raw file can be processed at 
32-bit floating point all the way from opening the raw file, until the 
final image is exported from Gimp.

However, Gimp acts in an unexpected way.

When Gimp opens an OpenEXR file, it applies an inverse sRGB tone 
response curve correction. If the user hasn't chosen another RGB color 
space, then Gimp also automatically assigns the sRGB built-in color 
space. The inverse sRGB TRC means the image looks correct if, and only 
if, the OpenEXR file is in a necessarily linear gamma version of the 
sRGB color space.

This is wrong behavior for an OpenEXR file.

OpenEXR is supposed to be ***linear***. So applying an inverse sRGB tone 
response curve correct makes no sense at all, unless you assume that the 
only color space any OpenEXR image will ever be in, is the sRGB color 
space. But OpenEXR files can be in any color space, as long as it's a 
linear gamma color space.

So the Gimp code leaves out in the cold anyone who tries to open OpenEXR 
files that aren't also sRGB color space files. Personally, the only 
thing I ever use sRGB for is as an output space for posting images to 
the web; I suspect many people who shoot raw have a similar workflow.

Gimp's automatically applied inverse sRGB tone response curve correction 
means that if the user assigns an actual linear gamma sRGB profile to 
the OpenEXR image), the image will look way too light.

If the OpenEXR image is really supposed to be ProPhoto or ACES or a 
camera input profile or anything other than sRGB, in Gimp the colors 
will look very, very wrong, and especially after the user assigns what 
in reality actually is the right (linear gamma) ICC profile, because the 
inverse sRGB TRC correction means that as soon as a linear gamma ICC 
profile is assigned, the tonality is completely wrong.

Gimp doesn't do this when opening a linear gamma tiff or png. It only 
does it for file formats that can only be linear. So the behavior is 
inconsistent. If I make three files, one tiff, one png, one OpenEXR, all 
in the same linear gamma color space, Gimp will open the png and tiff 
correctly, but not the OpenEXR.

I have two versions of babl/gegl/gimp from git installed on my computer, 
in different prefixes. I keep them both updated and run both of them 
regularly. One version is completely unmodified. The other version (the 
one I use for my own image editing) has two major departures from the 
unmodified version: All the back-and-forth Babl conversions between 
linear and regular srgb tone response curve are completely disabled. And 
I reverted the EXR handling to before the most recent changes so I can 
open and also export EXR files.

In my modified babl/gegl/gimp, OpenEXR files open as true linear gamma 
files, such that applying the correct linear gamma ICC profile makes the 
colors look right. That's because those sRGB-TRC-to-Linear Babl 
conversions are all disabled.

I realize that proper handling of OpenEXR is just one of many 
enhancements many people want for Gimp to have. After looking over 
OpenEXR code from various programs that read and write OpenEXR files, 
and reading through some of the OpenEXR documentation, I also realize 
that coding up proper OpenEXR support is not a simple undertaking. For 
one thing, color space information is carried in a metadata tag that 
Exiv2 probably can't read.

But one simple change in the default Gimp code will make it possible for 
people to open OpenEXR files right now, the right way (that is, open the 
file and assign a linear gamma version of whatever ICC profile matches 
the color space the file is actually in):

Open this file: 
https://git.gnome.org/browse/gimp/tree/plug-ins/file-exr/file-exr.c
and change line 204 to read "image_precision = 
GIMP_PRECISION_FLOAT_GAMMA; /* GIMP_PRECISION_FLOAT_LINEAR; */".

There are several other lines in that file that deal with different 
formats for OpenEXR files, for which similar changes would be needed. 
But the one change will take care of 32-bit floating point RGB OpenEXR 
files, which is the kind of OpenEXR file that Darktable saves when you 
interpolate a raw file and ask for OpenEXR output.

The user still won't be able to export OpenEXR files to disk, but at 
least s/he can open them. It wouldn't be full and proper OpenEXR 
support, but it would make OpenEXR useable right now for more than just 
those people who only use the sRGB color space.

If this change is made, the user must actually have a linear gamma 
version of the color space the OpenEXR file is in. Linear gamma versions 
of sRGB are packaged with many image editing programs.

This page: http://ninedegreesbelow.com/gimpgit/gimp-openexr.html has 
pictures showing the problem. There are also download links for several 
linear gamma ICC profiles and some sample OpenEXR files in different 
color spaces. I could make a complete set of linear gamma working space 
profiles available, if anyone is interested.

Elle

--
Elle Stone
http://ninedegreesbelow.com
Articles on color management and open source photography