[Digikam-users] underlying / basic concepts of 0.9 colour management?

[Daniel Bauer]

Hello everybody

I am still trying to become an expert in color management while testing 
0.9.0 ;-)

While reading (again) thru my years-old photoshop books and spending hours in 
google and on colour websites some questions are coming up and I'd like to 
start a discussion here with pointing out some of them. Developpers: please 
do *not* take this as criticism on your great work, which I truly estimate 
and admire!

My goal is not only to get a better understanding myself, but also - maybe - 
to help clarify for the users what the basic concepts of color managements in 
digiKam are and eventually identify what and how things must be explained and 
named in the digiKam dialogs and hand book.

1) ICC profile settings

The basics of what a color management system (CMS) does is explained very 
understandable on the digikam.org website (Color Management) by Paco. He 
points out, that there are 3 types of icc color profiles (input, display, 
output). 

But digiKam 0.9.0 asks for 4 profiles, and they are named differently:
- Workspace
- Monitor
- Input
- Soft proof

While it is quite clear what "Input" and "Monitor" stand for, the other two 
names are confusing to me... 

As much as I understand (please correct me, if I'm wrong) the "Workspace" 
represents the internal colour gamut or space of the CMM (colour management 
module or colour engine). 

Said very simplified: the CMM "takes in" the camera picture, places it 
according to the input profile in it's own colour space, then does the 
calculations according to the display or output device profile and finally 
gives out the transformed picture.

So that this is possible in every case, from any input to any output, the 
colour of the CMM space must be "big" enough to accept any given range of 
colours, because otherwise it could happen, that a specific input "has no 
place" in CMM's own space, therefore leading to errors. In my eyes then the 
CMM's workspace can only be CIELAB, that can take every possible colour. 

So why does digiKam ask for a workspace profile? digiKam sais, this is the 
"editor"-space, but I edit on my monitor, which would be the display 
space..., now what am I getting wrong here?

And what exactly is meant by "Soft proof"? 

I'd rather prefer another setting, for printer-output (besides of what Gilles 
already indicated in another mail: that a user can have different input, 
devices, cameras, files; different printers; even two monitors attached, but 
only one ICC setting..., but that's another story, for later, I guess)

2) IO files settings

This might be my personal inability, but I just don't get what the following 
really means/does:

- ICC profile correction during decoding
To get acceptable results I have to "disable" this. Then when opening a 
picture in the editor I am asked if I want it to be processed. As it then 
processes according to the settings I gave on the ICC page, I don't 
understand why there is a difference to selecting "using digiKam ICC 
settings". Maybe this is just a question of clear wording or my lack of 
english, but I just didn't find out, what the explaning text tried to tell 
me :-)

- enable RAW decoding quality
I tried with different settings from 0 to 3 but couldn't see a difference. 
Well, I don't have a super monitor nor did I use a special colour chart, but 
in a simple photo I can't find any significant changes.
Without further explenation it is also not clear for what purpose one should 
check "enable" and the set to 0, as zero in my eyes is the same as 
"nothing" (unchecked). I just guess, that there *is* a difference between 
leaving the box unchecked or check it and set to 0. To me it is not clear 
what the purpose of this setting means.

3) converting dialog

Such as I have set it now, when opening a raw image I am asked the following:

"this picture has not assigned any color profile.
Do you want to convert it to your workspace color profile?"

Now, this again is very confusing to me: 

First, why does it want to convert to "workspace" and not to "monitor"? 
For having a correct display it need's to use my monitors profile, doesn't it?

Then, *from* what is it converting? 
It is definitively not converting from nothing, i.e. just from the raw file 
without any profile, because it uses at least the input *and* the workspace 
(or monitor?) profile. I know, because changing *only one* of them in the 
ICC-settings leads to different results.

So, IMHO actually there *is* as profile assigned to the picture, namely the 
input profile.

I don't know, but maybe the correct sentence would be "there is no color 
profile *embeded* in the picture" - this at least would be true in the given 
case... And then, shouldn't both, the names of the applied input *and* output 
profile be displayed, just for making things clearer? (Or even better 
[later]: asked, which ones one wants to use in the given case, with defaults 
from settings?)

4) differences between programs

IMHO one main purpose of using ICC profiles is that a picture looks the same 
in any program that is capable to apply profiles. But when I use the same 
profiles in digiKam and in UFraw I get very different results... 

I tend to think that this has something to do with the "Workspace"-setting 
discussed above, as UFraw only asks for in- and output profiles (seems more 
logic to me), but of course I have no idea about the internal or programming 
differences between those applications.

Well, that's it for now. I hope I'm not the only one who finds these questions 
interesting...

A nice weekend to everybody!

Daniel
-- 
Daniel Bauer photographer Basel Switzerland
professional photography: http://www.daniel-bauer.com
special interest site: http://www.bauer-nudes.com