[Openicc] Krita color adjusting

[Jan-Peter Homann]

Hello Caspar, hello list

Today, I want share some ideas about coloradjustment and colormanagement 
with you. As you explainend to me in private mail, your goal is a 
coloradjustment, which is as much as possible independent from the 
actual colorspace /colormodel you are actually using in Krita.

As I explained to you, my goal is to use abstract-profiles or 
devicelink-profiles as universal exchange format for color-adjustments.
In this case, a coloradjustment could be applied to all applications, 
which use lcms or argyllcms.

Workingspace, Adjustmentlayer, Simulationspace, Outputspace
------------------------------------------------------------
Colormanagement makes it possible, to work in very complex color 
environments. I will first start with easier ones, and go later to the 
more complex versions.

1) The easiest model: workingspace and the outspace.
----------------------------------------------------
The workingspace can be e.g. sRGB and the outputspace could be the 
colorspace of the monitor or the printer.
All coloradjustments are made directly in the workinspace. For getting 
the right color at the monitor, the CMM converts constantly from 
workingspace to the monitor-colorspace.
After finishing the work, the data can be saved directly, and the 
profile of the workingspace is embedded. For printing the color is 
converted from workingspace to printer-colorspace.
If an file should be used in another workingspace, it is converted to a 
new workingspace. E.g. the sRGB data is converted to SWOP (CMYK).
Now SWOP is the new workingspace, but the overall configuration:
- workingspace->monitorprofile
- workingspace->printerprofile
- coloradjustments directly in workingspace
- Save with embedded workingspace profile
stay always the same

2) Workingspace, Simulationspace, Outputspace
----------------------------------------------
This is a little bit more complex.
The workingspace is mostly a widegamut RGB-colorspace or Lab/LCH 
colorspace. The goal is to work on color independent from the later 
output intention.
Color-adjustments, retouches and composings are made in the workinspace 
RGB or Lab/LCH.
During work, it is possible to simulate different colorspaces for the 
intended output.
The profilechain is:
workingspace-simulationspace-monitorprofile
workingspace-simulationspace-printerprofile
workingspace-Save with transformation and embedding the simulation 
colorspace

3) Workingspace, adjustmentlayer, outputspace
------------------------------------------------
Caspar, if I understand you right, your plan is to use an abstract layer 
  for coloradjustments, independent from the actual workingspace. This 
could be realized with abstract profiles.
But be aware, that your workinspace must be a valid ICC-profile.
If you are using special multichannel workingspaces for simulation of 
naturl-painting, you have to calculate a multichannel ICC-profile.
For doing this, you have to build a model for opacity, mixing of the 
channels etc. defined in the Lab colorspace.
The you have to build a matrix from different mixtures of the 
devicecolors with their Lab-equivalents. From this matrix, you can 
calulate a multichannel-profile, which can be used as workingspace for 
multichannel colormodels.

Another way are RGB-based models for simulation of natural-paintings
In this case, you have a formula, which are converting the colors from 
your natural-painting-model to a RGB-workingspace.
In this case, you have to take care, what RGB-workinspace you are using.
If you use AdobeRGB instead sRGB, you will get much more saturated 
colors on the monitor. If you use photogamutRGB - wich is part of the 
standard profiles installer from Kai - you will get a comparable result 
to sRGB, but the primaries and secondaries are orientated on printable 
colors and are very similar to the reference medium gamut of the ICC.
Photogamut is may be one of the best possible RGB-workinspaces for 
natural-painting modeling.

If you made a decision for an RGB-workinspace, you can use it as an 
intermediate step to build a mulitchannel-profile. A matrix 
(ASCCI-Numbers) of the devicecolors of your natural-painting model are 
first converted to RGB and from there to Lab ASCII numbers.
 From the resuting matrix

ASCII-File
Device-colors->Lab-colors

you calculate the multichannel profile.

So the krita-team has to made following decision:
- should there be a Master RGB-workingspace ?
- If yes, how should different workinspaces be evaluated ?
- Should multicolor-profiles for natural-painting be implemented ?

If you go the multicolor-way, it makes sense to concentrate on 
coloradlustment based on abstract profiles
If you concentrate on a Master RGB-workinspace, it makes more sense to 
implement RGB-based coloradjustment.

Greetings from warszaw (actual place for this week)
:-) Jan-Peter











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homann colormanagement ------ fon/fax +49 30 611 075 18
Jan-Peter Homann ------------- mobile +49 171 54 70 358
Kastanienallee 71 ------- http://www.colormanagement.de
10435 Berlin --------- mailto:homann at colormanagement.de