[digikam] [Bug 376721] New: Poor quality colour management in Preview vs Editor

[Tim]

https://bugs.kde.org/show_bug.cgi?id=376721

            Bug ID: 376721
           Summary: Poor quality colour management in Preview vs Editor
           Product: digikam
           Version: 5.4.0
          Platform: Other
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: ColorManagement
          Assignee: digikam-devel at kde.org
          Reporter: kde at stirfried.vegetable.org.uk
  Target Milestone: ---

Background:
digikam 5.4.0-2 from Arch Linux packages.
My colour-management settings:
enabled
working = BestRGB
Monitor profile = specified an ICC file generated from displayCal
Use colour-managed view in editor: enabled
Use colour-managed view for previews and thumbnails: enabled
Advanced: Use black-point; perceptual

Problem:
I am working with the image http://misc.sty.nu/IMGP0431_1-large_v1.tiff (21MB)
which uses the ProPhotoRGB-linear profile.

In the preview it looks utterly ghastly, with massively quantized shadow areas.
See http://misc.sty.nu/digikam-screenshot-preview.png 

However, in the editor it looks fine:
http://misc.sty.nu/digikam-screenshot-editor.png  (I used the "keep" option on
opening, so there remains a ProPhotoRGB->monitor-space transformation in place
to display this.)

This inaccuracy in the preview makes it difficult to compare/rate images that
rely on shadow zones in ProPhotoRGB-linear space. 

I have tried varying the colour-management Advanced settings and no combination
of blackpoint compensation nor rendering intent makes it go away - and some
look even worse.

The problem does go away when I convert the image to sRGB, but changing now
would be premature in the workflow - there are lots of blends and edits pending
in multiple utilities yet.

I would understand if it was my monitor calibration profile being low quality,
but how come the editor can get it right, so why can't the preview show the
same thing?

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