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

[Maik Qualmann]

The thumbnails are color managed. But the color depth of the thumbnails is 
only 8 bit, that's probably the cause. The image preview can be activated in 
the settings to display image with 16 bit, but the thumbnails are not.

Maik

Am Mittwoch, 15. November 2017, 16:16:35 CET schrieb 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