[Bug 172196] New: Color managed view is inaccurate

[Milan Knizek]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #19 from Milan Knizek <knizek volny cz>  2009-12-25 09:57:21 ---
It is a question what to do in case of a CMYK image without embedded ICC
profile. There is no straightforward transform to RGB model, hence the ICC
profile of the device (printer) for which the image was created is needed.

Interestingly, only CinePaint and Krita work with the image as with CMYK. GIMP
probably does internal conversion (how?) to RGB and assign sRGB profile then.

CinePaint assigns coated FOGRA CMYK profile (I mistyped in my above comment)
and Krita some internal CMYK profile (Offset printing, ISO/DIS 12647-2:2004,
which is also default CMYK printer profile in the settings).

I wonder whether the Adobe APP14 markers refer to a particular CMYK standard or
not.

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[Marcel Wiesweg]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #20 from Marcel Wiesweg <marcel wiesweg gmx de>  2009-12-25 11:38:50 ---
I just checked with our JPEG loader. The YCCK->CMYK conversion is done by
libjpeg internally. We then convert to RGB using a simplistic routine that we
took from Qt's JPEG loader:
                        int k = ptr[3];
                        ptr2[3] = 0xFF;
                        ptr2[2] = k * ptr[0] / 255;
                        ptr2[1] = k * ptr[1] / 255;
                        ptr2[0] = k * ptr[2] / 255;
Not more.
I dont know if assigning any CMYK ICC profile and later converting to sRGB
would improve the general case of any given CMYK image here.

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["Jens Müller"]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #21 from Jens Mueller <tschenser gmx de>  2009-12-26 09:22:31 ---
The routine is a general way to get the rgb components out of the cmy
components multipilied with the back component. But for real color vision they
have to be adjusted by a cms. The problem with the testfile is that there is no
icc embedded as milan already pointed out (this should not be the case,
especially for offset printing prepared images). 'identify -verbose' say that
the image was edited by photoshop 7. So i don't know if other applications make
some assumptions about adobe markers. And what are they?

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["Jens Müller"]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #22 from Jens Mueller <tschenser gmx de>  2009-12-26 09:50:55 ---
I found a realy good example showing wrong colors in preview widget but not in
thumbnail-view and edit-mode. Look for
http://scummos.sc.funpic.de/upload/farbkreis.jpg from #157200. The words should
match the colors, Red should be on top for not english speakers. But thats only
the case in thumnail-view, thumbnail-bar and when you click on edit image.

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["Jens Müller"]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #23 from Jens Mueller <tschenser gmx de>  2009-12-26 09:54:29 ---
Created an attachment (id=39347)
 --> (http://bugs.kde.org/attachment.cgi?id=39347)
showing wrong colors in digikam preview (red should be on top)

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[Milan Knizek]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #24 from Milan Knizek <knizek volny cz>  2009-12-26 15:44:19 ---
(In reply to comment #23)
> Created an attachment (id=39347)
 --> (http://bugs.kde.org/attachment.cgi?id=39347) [details]
> showing wrong colors in digikam preview (red should be on top)

Jens - this is something different. The image farbkreis.jpg is in RGB
colorspace with a specific ICC profile, which helps the user to see quickly
when Colour Management does not work.
In DigiKam, when CM is off, it is expected to see the RGB image as is, without
any colour transform.
The question is, whether thumbnails should still be generated as if CM is on, I
think they should as it is now.

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["Jens Müller"]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #25 from Jens Mueller <tschenser gmx de>  2009-12-26 16:24:00 ---
Milan you are right, that are two separate problems.
- The second showing a bug with not applied cms in preview (cms is turned on!).
- The first is a special case of cmyk image data with no embedded icc. But both
are problems with inaccurate color management view. But properly it is better
to put them in different bug reports next time.

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[Milan Knizek]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #26 from Milan Knizek <knizek volny cz>  2009-12-26 17:19:40 ---
(In reply to comment #20)
> I dont know if assigning any CMYK ICC profile and later converting to sRGB
> would improve the general case of any given CMYK image here.

I have done more testing with GIMP, CinePaint, EoG, Krita (and digiKam).
Basically, all of them display the CMYK image w/o any ICC profile differently.

CinePaint: somehow selects a default CMYK ICC profile (even that not set in
settings). I guess it is related to Oyranos project, which was implemented for
CinePaint as a show case (even that I have it switched off). Of course, the
user can assign a different profile in the editor.

Krita: automatically assigns a CMYK ICC profile (internal). The user can assign
another profile manually in the editor.

GIMP: automatically converts the image from CMYK to RGB using the default CMYK
profile from the Settings. If none chosen, it probably does something similar
to the current digiKam's implementation. (Yet, the displayed images are
different.)

Out of these, GIMP converts the image to RGB, while CinePaint and Krita
continue to work with the image in CMYK.

I have not found any information on APP14, whether there is an assumed CMYK /
Offset standard or not. I assume not and that the ICC profile should be ideally
embedded.

For digiKam, I would propose similar behaviour like in GIMP:

If the CMYK image has an embedded profile, it should be used (for thumbnail,
preview and editor).

If the CMYK image does not have embedded profile, digiKam should use the
default one from Settings (does not exist yet...).

Further, in case digiKam editor being set to ask when there is a mismatch
between the embedded and working profiles, the user should have a chance to
select CMYK profiles from drop-down list. (Now CMYK profiles are not listed.)

If the user does not set a default CMYK profile or does not use colour
management, then the current method of simple CMY -> (s)RGB conversion could be
used.

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["Jens Müller"]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #27 from Jens Mueller <tschenser gmx de>  2009-12-27 00:33:14 ---
I follow Milans suggestion of digikam behaviour for cymk images in all cases.
From point of implementation i have one notice. What about the following chain:
first convert the cmyk image to internal buffered rgb dimg by standard
approach. Set the contained profile (from the embedded one or a standard one if
no one exists). In a second strep: for all use cases do the cmstransform from
rgb to display/working profile (normally srgb). I dont know if that is
possible, but with the cms on/off selection the second step can simply omitted.
Questions that rise (sorry i dont know much about icc):
-is icc data justified for one special colorspace? In that case the embedded
icc must be adopted for rgb. Is that possible?

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[Milan Knizek]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #28 from Milan Knizek <knizek volny cz>  2009-12-27 08:33:47 ---
(In reply to comment #27)
> first convert the cmyk image to internal buffered rgb dimg by standard
> approach. Set the contained profile (from the embedded one or a standard one if
> no one exists).
I do not think so. ICC profiles are specific to colour model (RGB, CMYK).
Change of colour model from CMYK to RGB is done by littleCMS during the colour
transform from CMYK ICC profile to RGB ICC profile.

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["Jens Müller"]

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--- Comment #29 from Jens Mueller <tschenser gmx de>  2009-12-27 18:04:18 ---
Argh, for the problem with testimage farbkreis.jpg I did'nt see the unset
checkbox 'use color managed view for previews ...' on the second tab in
settings. Sorry about that.

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[Marcel Wiesweg]

https://bugs.kde.org/show_bug.cgi?id=172196


Marcel Wiesweg <[hidden email]> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |220312




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[Marcel Wiesweg]

https://bugs.kde.org/show_bug.cgi?id=172196





--- Comment #30 from Marcel Wiesweg <marcel wiesweg gmx de>  2009-12-27 21:05:47 ---
I created a new entry for the CMYK issues, bug #220312.

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