[argyllcms] Re: How does the targen do the black generation withpre-conditioningprofile?

Nikolay Pokhilchenko wrote:

It seems to me, targen computes grid for device space in Lab(?), then lookups 
the CMYK value in B2A table from given pre-profile. It is right?


Graeme Gill wrote:

No, it depends on the distribution chosen. For the -m points, the grid is in 
device space (ie. a 4 dimensional hyper-cube grid for CMYK).



But what about -I key?


The perceptual based distributions lookup the Lab value in the profile, yes. 
This
doesn't determine the geometric arrangement though, just the space in which it
is computed.I think the K value is computed as the K only L* value,
although there are some tweak factors that may reduce K space density compared
with the other colorants.


Okay, for the present time I'm get the gray axis patches by OpenOffice calc and

xicclu or icclu. But I pay attention for black generation, therefore I was 
forced
to ask "How does the targen do the black generation". How can You map L*a*b to
CMYK without subaudition of the black generation? It is multivariant problem,
especially for gray axis. I think the black generation options would be useful 
too.

It doesn't do black generation, it explores the 4 dimenstional CMYK space. For 
the
perceptual space distributions, the K space is treated as an orthogonal 
dimension
to L*a*b*, and measured on an L* scale as if it was K only output. Naturally
there is interaction between the L*a*b* value and the K target though.
It may be useful to add Jab perceptual distribution of the data points in targen 
with viewing conditions too.

I'm not sure if this would be worthwhile, since I suspect the difference between
L*a*b* and Jab is too subtle to have any noticable positive effect on the final 
profile.
It's hard enough seeing a reproducable trend between device space and 
perceptual space,
or between the different geometric arrangements.

Graeme Gill.

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