QTR-Quadtone RIP

On Jan 8, 2008 2:04 AM, Olivier <odesmais@...> wrote:

> > OK,  the conversion to the QTR printer profile does about
> > the same to the tonal range of the image.
> > The linearity of the printer isn't changed but the tone
> > range of the image is adapted in the conversion to that
> > profile for what is possible in tone range on that paper.
> > But I see you got that idea already.
>
> Fine, but the way it's done I'm not so sure.
> I'm having a desperate time trying to properly output MPS K7 on glossy
> media.
> I somehow managed (finally) a proper linearization using the 51
> stepwedge, but when the profile is created and used with percep. w/ BPC-
> on in CS3 to convert the piezography.psd target, the
> 100/99/98/97/96/95% patches get clustered together in a
> 100/99/99/99/98/98% ramp given a L*14.5 Dmax. What this leads to is a
> blocked shadow area at the end of the greyscale.
> Perceptual intent compresses the tonal range but here the compression
> is very agressive.
> Olivier
>
>
Olivier and others,

With a weak dMax, yes the shadows are compressed somewhat -- that's
inevitable.

But your mentioning only the K values of the ramp you are using misses the
point that it's the L values that are important.  Color Management is all
about L-values.
If you happen to have a GrayGamma 2.2 stepwedge then K=100 gives L=0 but
K=95 gives L=1.
So the difference between K=100 to K=95 is already very small -- only 1 unit
of L.
It's the stepwedge itself that is very compressed in the shadow area.
(don't fall into the trap:  "but it's untagged and I said no color
management".   If you look
at L values or you Convert-to-Profile, remember the file ALWAYS inherits the
working space)

"Thinking in color management" requires a switch in concept from thinking K
(or RGB) values
to Lab values.  This really is the crux of the issue with CM discussions.
 It's a "light-bulb" moment.

Virtually all stepwedges are evenly spaced in K values but not necessarily
even in L values.  So any CM discussion of values MUST MUST MUST include the
L values
or the profile that is being used so one can convert.

Roy


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