Bug 376721

Summary: Poor quality colour management in Preview vs Editor
Product: [Applications] digikam Reporter: Tim <kde>
Component: ColorManagement-ViewsAssignee: Digikam Developers <digikam-bugs-null>
Status: RESOLVED FIXED    
Severity: normal CC: metzpinguin
Priority: NOR    
Version First Reported In: 5.4.0   
Target Milestone: ---   
Platform: Other   
OS: Linux   
Latest Commit: https://commits.kde.org/digikam/0ca4c66af71b2364d69d28cdff4e5364e42db28a Version Fixed/Implemented In: 5.5.0
Sentry Crash Report:

Description Tim 2017-02-20 17:35:09 UTC 
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?
Comment 1 Maik Qualmann 2017-02-20 20:14:46 UTC 
Your image has 14 bits. The preview load for speed reasons images only in 8 bits. The Image Editor can load 16 bits images.

Maik
Comment 2 Maik Qualmann 2017-02-24 20:30:56 UTC 
Git commit 0ca4c66af71b2364d69d28cdff4e5364e42db28a by Maik Qualmann.
Committed on 24/02/2017 at 20:29.
Pushed by mqualmann into branch 'master'.

make it possible to disable converting preview image to 8 bits
FIXED-IN: 5.5.0

M  +2    -1    NEWS
M  +2    -0    libs/settings/applicationsettings.cpp
M  +1    -0    libs/settings/applicationsettings_p.cpp
M  +1    -0    libs/settings/applicationsettings_p.h
M  +6    -4    libs/threadimageio/previewsettings.cpp
M  +2    -2    libs/threadimageio/previewsettings.h
M  +4    -1    libs/threadimageio/previewtask.cpp
M  +20   -12   utilities/setup/album/setupalbumview.cpp

https://commits.kde.org/digikam/0ca4c66af71b2364d69d28cdff4e5364e42db28a
Comment 3 Tim 2017-02-24 21:59:00 UTC 
Many thanks!