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

[Kai-Uwe Behrmann]

> 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 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.

> 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.

> 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.

> Stephan Maier


kind regards
Kai-Uwe Behrmann
-- 
developing for colour management 
www.behrmann.name + www.oyranos.org