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

[Elle Stone]

I made some screenshots to show two separate problems in digiKam when 
displaying thumbnails and previews:

https://ninedegreesbelow.com/files/digikam-zoom-and-color-management-problem.jpg

This is for digiKam 5.7.0 on Gentoo Linux.

First problem:

DigiKam is not color-managing the thumbnails, even though in 
Settings/Configure/Color Management I did select to "Use color managed 
view for previews and thumbnails" as well as "Use color managed view in 
editor".

Is anyone else seeing thumbnails that aren't color-managed (not a 
relevant question unless your monitor profile isn't sRGB).

Second problem:

For both thumbnails and previews, and for both high bit depth pngs and 
high bit depth tiffs, for images in linear gamma RGB color spaces, the 
shadow tonality is severel distorted. The smaller the zoom, the greater 
the distortion, but even at 50% and 100% zoom, the shadow tonality is 
distorted. This is true for both pngs and tiffs.

The corresponding images when displayed in GIMP do not show distorted 
shadow tonality even when the zoom is as small as 9%.

Colors do not blend correctly when edited in non-linear RGB color 
spaces. As more and more image editors allow for high bit depth image 
editing, more people are editing in linear gamma RGB color spaces.

Almost all of my own processed image files are in linear gamma color 
spaces. This means digiKam is fairly much useless for allowing me to 
examine and rate my processed image files, as image tonality is a key 
part of judging the differences between images.

As a possibly relevant additional observation, I opened a test image in 
a linear gamma RGB working space from digiKam, using ShowFoto. ShowFoto 
also shows the same distortion in the shadow tonality, though not as 
extreme as in the digiKam thumbnails.

Is digiKam perhaps using LCMS "low quality" conversions for conversions 
from the image color space to the user's monitor profile? In Krita there 
is an option under "Settings/Configure Krita/Color Management" to "Allow 
Little CMS optimizations" with the warning to "uncheck when using linear 
light RGB or XYZ". And indeed having this option in Krita checked 
produces similar shadow tonality distortion as digiKam/showFoto is 
showing, though not as extreme for zooming eg to 8% or 10%.

Is is possible that the zoom algorithm in digiKam is not properly set up 
to handle linear gamma images? For example PhotoShop CS2 (version 
current back around 2004) performed very badly with linear gamma image 
shadow tonality until the zoom factor was at least 67%, at which point 
everything was fine.

Best,
Elle