QTR-Quadtone RIP

On Dec 26, 2007 5:56 AM, ferdinand_paris <ferdinand_paris@...> wrote:

> I regret to say that I seem to have been getting inconsistent results.
>  I've double checked and rechecked them all and posted a revised
> version of my histograms chart:
>
>     http://www.ferdinand-paris.com/BPC-Qimage_V2.png
>
> It seems that the only combination that causes a big dMax loss is the
> one that you really want to work: Convert to custom in Qimage with BPC
> (bottom left).


The "big dMax loss" is just one symptom that happens to be most visible.
But since they appear to all be little different, they all produce different
results
than Photoshop.


>
> I fully appreciate that it is hard to find the problem when it only
> happens when converting to a QTR custom profile in Qimage.  Is the
> profile at fault or the Qimage CMS?


The bottom line is that I don't really know.  I've used Photoshop as my
standard
but it's not an absolute either.

The fact that the Q-CMS handles
> other profiles ok suggests the profile.  That PS handles the profile
> ok suggests the Q-CMS.


How we actually know whether any profile is done "correctly"?   I admit the
dMax
issue shows the problem, but other color profiles "could" be done wrong and
we
might not know.

I'd be more than happy to be corrected in all this but as Eric mentions:
>>And now that Mike has released v200, all your tests may change with the
next
>>upgrade. Darn frustrating : (

It's very weird to me that different CMSs with exactly the same input can
produce
different results.

Roy


> I wish I could help.
>
> F_P
>
>
> --- In QuadtoneRIP@yahoogroups.com, "Roy Harrington" <roy@...> wrote:
> >
> > Thanks Ferdinand.  It's interesting to see how many different
> > transformationsshow up.  Unfortunately solving it all seems to be
> elusive.
> >  The CMS of Qimage
> > is done by yet another person, so it becomes very difficult to pin
> down the
> > problems.
> > In fact even to agree on the existence of something wrong is hard.
> >
> > The difference between my generic profiles and now custom profiles
> is mostly
> > an
> > accident of the evolution of my understanding ICC profiles.  There
> are two
> > major formats
> > for curves in ICCs -- one uses a "transfer function" and the other uses
> > "lookup tables".
> > The transfer functions are simpler and what I did first -- they have
> one set
> > of curve values
> > and the same curve is used for both directions of a profile -- i.e.
> K to L
> > and L to K with the
> > CMS doing the inversion.  I used this first and that's what the generics
> > use.  Later on
> > I used the lookup tables which allow lots of curves for different things
> > ---each direction,
> > each intent and full color as well.  This allowed softproofing with
> the hue
> > of the print.
> >
> > So the custom ICCs use the tables and I have separate K to L curve
> and L to
> > K curve.
> > L=0 in your file maps to K=100 to the driver, but in softproofing
> K=100 maps
> > back to L=dMax
> > so you can see "Simulate Black Ink".  I think somehow the
> softproofing curve
> > is getting
> > used during the print conversion but I haven't any idea where or
> why. (maybe
> > BPC isn't
> > even the real issue).
> >
> > You can actually see some of this and play with conversions using
> Photoshop
> > and softproofing.
> > I think there are some free programs that you can download to graph the
> > innards of ICC profiles.
> >
> >    *  Ernst's point about the PS CM never being fully off is well taken,
> >       but these are large changes.  All programs than can display the
> >       histogram (e.g.  BreezeBrowser Pro), display the same dMax loss.
> > I think you are safe if you look at histograms -- the info palette is ok
> > ONLY for the native type
> > of the file.  I.e. Don't look at RGB values for a grayscale, only
> the K is
> > valid for a grayscale.
> >
> > Roy
> >
> > On Dec 24, 2007 4:43 AM, ferdinand_paris <ferdinand_paris@...> wrote:
> >
> > > CLARIFICATIONS:
> > >
> > > *  All conversions were perceptual (but see below).  CM_OFF was done
> > > in PS.  Others were done in Qimage.
> > > *  All conversions were to my newly created custom RGB profile.
> > > *  Ernst's point about the PS CM never being fully off is well taken,
> > > but these are large changes.  All programs than can display the
> > > histogram (e.g.  BreezeBrowser Pro), display the same dMax loss.
> > > *  I was using Qimage 2008.104 but have since upgraded to .109 (latest
> > > - see below).
> > > *  I tested Ernst's idea of using 8-bit input to Qimage to no avail -
> > > same problem.
> > > *  Since the initial QTR-related problems with the CMS in Qimage were
> > > discovered in early 2006, I have generally followed Ernst's workflow:
> > >  i.e. create a copy and do the CM in PS, and do all other printing
> > > manipulations in Qimage with CM off.  But this partly defeats one
> > > purpose in using Qimage, the ability to print from master files and
> > > not have to create special versions just for printing.  It works for
> > > colour, but not it seems for B&W.
> > >
> > > FURTHER RESULTS
> > >
> > > I upgraded to 2008.109 and one histogram changed.  The one with
> > > profile conversion done in Qimage but no BPC ("CM On, BPC off in Q")
> > > now looks exactly the same as the one done in Qimage *with* BPC ("CM
> > > on, BPC on in Q").  Now in both cases the black point shifts from 1 to
> > > 27 with an associated loss in dMax.
> > >
> > > This tests my sanity.  I am fairly sure that I used all perceptual
> > > conversion when I produced those histograms.  But I can't now
> > > replicate the middle histogram ("CM On, BPC off in Q") using *any* of
> > > CM options in Qimage.  This suggests that something has change in
> > > Qimage, but the revision log says nothing about any changes to the CM.
> > >
> > > I decided to push the tests a step further.  I also produced
> > > histograms for conversions in Qimage to generic profiles.  Results
> are at:
> > >
> > >  http://www.ferdinand-paris.com/BPC-Qimage_V2.png
> > >
> > > CONCLUSIONS
> > >
> > > *  BPC is not the problem per se when I convert to a QTR custom
> > > profile, the reading of the custom profile by the whole Qimage CMS is.
> > > *  There is something about the custom profiles that Qimage can't
> > > handle, but can with the generic profiles.  I don't know if this has
> > > been discovered and commented on before.
> > >
> > > @Roy:  I still don't understand the differing treatment of BPC between
> > > custom profiles and the generic profiles that you mentioned in post
> > > 5728.  What practical difference would I expect to see (other than the
> > > fairly obvious ones in Qimage)?
> > >
> > > F_P
> > >
>


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