[sane-devel] Rendering Intent for Color Management in XSANE Setup

[Stephan Maier]

  Hello Kai-Uwe,

Thank you for following up on my comments.
I am by no means an expert in this area, but
have experimenting long enough with the
Argyll software to roughly understand what it does for
my purposes, i.e., color profiling a scanner.
This is where I stumbled over this discrepancy.


On 07/28/10 13:28, Kai-Uwe Behrmann wrote:
>> Date: Wed, 30 Jun 2010 13:59:33 -0400
>> From: Stephan Maier <stephan at bwh.harvard.edu>
>
>> I have been verifying the Color Management in XSANE.
>
> Great. Do you have a link to a web page with your results?
>
I have not really assembled anything and right now I am on vacation.
I could try to pull the data off my other computer.

>> I found a clear discrepancy between the profiling results
>> achieved with XSANE and the Argyll (www.*argyllcms*.com)
>> converting routine cctiff. The Argyll profiling results
>> agree perfectly with the expected values for my target image.
>> I was able to reproduce the XSANE results with Argyll's cctiff
>> by using the same rendering intent, i.e. perceptual, for both input
>> profile (XSANE: Scanner default color ICM-profile) and output profile
>> (XSANE: Working color space ICM-profile).
>
> ICC profiles typical connect to the Profile Connection Space (PCS). 
> This is eigther Lab or XYZ encoded. During a colour trasnformation 
> your input profile connects to PCS and then goes to output profile, 
> the working or editing space. For a colour trasnformation is only one 
> rendering intent possible.
>
I agree with the first part. Thus we have

raw data (scanner) --> PCS --> output

Now, as I understand it, each arrow represents a color transformation with
its own rendering intent. In XSane does the rendering intent apply to both
transformations or only the second one? One could of-course set
the first to 'absolute colorimetric' by default and the user defined
rendering intent would then only apply to the second one.

>> This leads me to believe the Rendering Intent choice in XSANE setup
>> applies to both in input and output profile. This approach is not 
>> correct,
>> however. The rendering intent of the input profile, which corrects 
>> errors
>> of the scanner, has to be set to "Absolute colorimetric", since all 
>> other
>> choices perform a white point correction. XSANE lacks the option
>
> "Absolute colorimetric" results in a native input whitepoint on the 
> output media. E.g. the colour cast of the device would be projected 
> into D50 or whatever the editing space is and would appear there in. 
> This is useful mostly in pedantic reproduction. Normaly relative 
> colorimetric with black point compensation performs more desirable.
Yes, I agree that 'absolute colorimetric' is the least useful for output and
that it keeps the whitepoint coordinate and that perceived whites appear
no longer white. However, for the input calibration profile it is different
since we indeed need a pedantic transform, since the raw data is not
in a color space yet.

>
>> to set this rendering intent independently of the rendering intent
>> for the output profile (typically the desired output profile intent is
>> perceptual or relative colorimetric).
>
> If there is no thierd profile involved in the colour workflow, there 
> can by definition be no second rendering intent.
This is may be my misunderstanding, but as I outlined above, the raw 
data to PCS transform
I consider to be separate profile, which should have it's own rendering 
intent
or at least a suitable default value, like "absolute colorimetric".

With kind regards

Stephan Maier
>
>> Stephan Maier
>
>
> kind regards
> Kai-Uwe Behrmann