[krita] [Bug 373267] New: mergedimage.png has problems of rendering and color-profile in *.kra RGB 16bit files

[David REVOY]

https://bugs.kde.org/show_bug.cgi?id=373267

            Bug ID: 373267
           Summary: mergedimage.png has problems of rendering and
                    color-profile in *.kra RGB 16bit files
           Product: krita
           Version: 3.1 Beta
          Platform: Appimage
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: Color models
          Assignee: krita-bugs-n...@kde.org
          Reporter: i...@davidrevoy.com
  Target Milestone: ---

Created attachment 102627
  --> https://bugs.kde.org/attachment.cgi?id=102627&action=edit
[*.kra file, 16bit, to reproduce the bug]

Hi, 

I test digital-painting with RGB 16bit ClayRGB-elle-V4-srgbtrc *.kra document,
and I like it! Smooth gradients, less artefact for mixing colors, precision in
blending modes... But!

## The bug:
The mergedimage.png (inside the *.kra file) has problems of rendering and
color-profile. It's not the same colors than my artwork.
If I export my *.kra artwork manually to *.png via Krita GUI; I obtain a 16bit
ClayRGB-elle-V4-srgbtrc *.png picture. Here it's fine, color and profile is
perfect, even in other apps, imagemagick, etc... But ; the extracted
mergedimage.png is not the same: it's a 16bit sRGB-elle-V2-srgbtrc *.png
picture with bad color rendering. This bug prevents me to use 16bit for my
comic pages on Pepper&Carrot; because my renderfarm script works with the
mergedimage.png from my *.kra pages. 

Screenshot: perceptual comparison of "manually exported PNG" VS
"mergedimage.png" :
Eg.
http://www.peppercarrot.com/extras/forum/2016-12-04_bug-report_color-profile-mergedimage.jpg
 
(source *.kra file to reproduce the bug in attachment)

## Expected:
At the save time, convert mergedimage.png to 8bit sRGB-elle-V2-srgbtrc
perceptual. If Krita wants to keep a 16bit mergedimage.png, then it has keep
the correct colorprofile or be converted (perceptual) to sRGB-elle-V2-srgbtrc.

## Note:
For a CMYK 16Bit ISO Coated v2 300%(ECI), the mergedimage.png is correct: it's
a RGB 8Bit sRGB-elle-V2-srgbtrc.icc with perceptual rendering. So both of the
CMYK 16bit and the mergedimage looks the same. It means probably that the bug
happens only with Krita file with this settings: RGB+16bit profiles other than
sRGB-elle-V2-srgbtrc.icc.

Tested on 3.1RC appimage, ManjaroXFCE.

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