+++ This bug was initially created as a clone of Bug #172196 +++ Color management is missing when loading CMYK images. Discussion at the bottom of bug #172196.
Suggestions from Milan Knizek: 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.
My remarks on this: To apply color management according to user settings, we must preserve the original CMYK data in the image loaders (JPEG, TIFF). DImg is always expected to contain RGB data, but when explicitly switched on before loading, and with guaranteed conversion afterwards, we can probably do that. If it proves not to be possible, we need to keep the data in an extra place inside the DImg container. If we want full support, the settings and the color management dialog need to support this as well. Milan, any suggestion for a default CMYK profile that we can ship?
I will try to do some research on the issue of default cmyk profile.
There appears to be a standardisation effort in the industry to introduce recommended ICC profiles for a few selected printing conditions. The printing houses are then promoted to calibrate their printers to match the profiles. In Europe, there is an initiative called ECI, which offers a set of ICC profiles for download for free. Their profiles are based on freely available specifications made by FOGRA. Similar efforts probably exists for other parts of the world (US, Japan, etc.). For the situations, when the printer is unknown, ECI recommends [1] to use ISO Coated v2 300% ICC profile for CMYK images. The profile is made by ECI according to specifications of FOGRA39L [2]. At the moment, it is unclear under which license the ECI ICC profiles are available - I have sent them an email to confirm if free distribution with GPL'ed software is possible. However, since the FOGRA specifications are free, everyone can prepare the ICC profiles on his/her own. Kai-Uwe Behrmann did so (using Argyllcms) and the profiles are available in his project Oyranos [3]. Kai-Uwe confirmed to me: "The Cmyk profiles in Oyranos are licensed like libpng and thus are liberal to use and redistribute. I wish to see the profiles distributed with Oyranos. So I would be glad when digikam avoids placing them directly into /usr/share/color/icc . Digikam might choose a subdirectory or a application specific path. Otherwise I am fine." Unless anyone else has another opinion, I would propose to use coated_FOGRA39L_argl.icc as the standard CMYK profile for digiKam. [1] http://www.eci.org/doku.php?id=en:colorstandards:offset "Which profile should I use?" [2] http://www.fogra.org/products-en/icc1.html [3] http://www.oyranos.org/#about
Created attachment 39440 [details] CMYK ICC profile made by Kai-Uwe Behrmann with Argyllcms according to FOGRA39L spec.
*** Bug 291356 has been marked as a duplicate of this bug. ***
This problem still reproducible with digiKam 5.0.0 ? Gilles Caulier
Please test with 6.2.0 pre-release bundles available here : https://files.kde.org/digikam/ Thanks in advance Gilles Caulier