Thumbnails not color-managed + severely distorted shadow tonality for thumbs, previews

[Marcel Wiesweg]

If my memory is not too wrong, thumbnails are stored in sRGB on disk, and are 
color managed on load, and then cached. This order is mandatory for 
performance reasons. Given that profile loading via X atoms is like a 
technology from the 1980s or 90s and there is no change notification, things 
like you observe can happen.
(multi monitor setups with different profiles another issue)

Regarding the optimization flag, I find Marti Maria's definitive answer to 
this issue raised by you here

https://sourceforge.net/p/lcms/mailman/message/29602585/

asserting that we should not change the flag per default for very convincing 
performance reasons, but a specialized option for 16bit -> 16bit conversions 
appears acceptable

Marcel


> On 11/16/2017 08:00 AM, Elle Stone wrote:
> > Also in the version of digiKam that I installed from git, the thumbnails
> > are color-managed. Of course the shadows are still posterized from being
> > linear gamma 8-bit thumbnails, but that is to be expected.
> > 
> > There is one more variable (setting or not setting the system monitor
> > profile) that I want to double-check, that might affect the display of
> > the thumbnails: When using the version of digiKam from Gentoo Portage, I
> > had set the system monitor profile upon starting X. But this morning,
> > testing my modified "git" version of digiKam, I also unset the system
> > monitor profile and set the digiKam monitor profile from disk, so that
> > the digiKam color management settings and GIMP/Krita color management
> > settings would match as closely as possible. I can't imagine how this
> > would affect display of thumbnails but not the previews, but maybe it
> > somehow does.
> 
> Hmm, experimenting, I can deliberately produce not-color-managed
> thumbnails by setting/unsetting the system monitor profile while digiKam
> is running, and then changing the digiKam color management monitor
> profile setting. Restarting digiKam makes the thumbnails be color-managed.
> 
> So I'm guessing that there was something in how my system monitor
> profile was being set when I was running digikam 5.7.0 from Gentoo
> Portage, that somehow resulted in the thumbnails not being
> color-managed. So my apologies! for saying the thumbnails aren't
> color-managed. They weren't on my system, but I think this was strictly
> a function of when/how the system monitor profile was being set.
> 
> As already noted, the problem with the preview image shadow tonality is
> completely eliminated by changing
> "core/libs/dimg/filters/icc/icctransform.cpp" line 58 from
> "transformFlags = 0;" to "transformFlags = cmsFLAGS_NOOPTIMIZE;" before
> compiling and installing digiKam.
> 
> Best,
> Elle