Digital BW, The Print

>I was making a rather simple point that contrast is not "dynamic range".
So
>your point:

>> the gamma adjustment in QTR doesn't affect the overall
> contrast of the print (i.e. the dynamic range)

>is technically wrong.  . . . So gamma, ie contrast, is a function of the
rate of
change in density as you move from dMin to dMax - I can have the same gamma
but two completely different dynamic ranges.

Actually I was just describing what happens when you adjust the gamma slider
in QTR. I made a bunch of prints of 21 step wedges with the slider in
various different positions from minus 15 to plus 15. The overall contrast
of the step wedges remained the same (i.e. the darker blacks and whiter
whites didn't visibly change regardless of where the slider was placed).
Only the midtones changed. Whether that's "technically wrong" or not I don't
know, it is however an accurate description of what happened when the
sliders were moved and a 21 step wedge was printed.

I think you must be using the term gamma differently than it has
historically been used in photography because gamma isn't simply "contrast."
In photography the term "gamma" has been used to describe the slope of the
straight line portion of a curve. Gamma is determined by dividing density
range by subject brightness range.  It therefore would be impossible to have
the same gamma but two completely different dynamic ranges (unless, of
course, you're using the term "dynamic range" to mean something other than
"density range," in which case I don't know what you mean by "dynamic
range)."

Dmin and dMax don't enter into the determination of gamma except in the
indirect sense that density range is one of the numbers used to derive gamma
and dMax and dMin enter into the determination of density  range (under the
ANSI standard IIRC density range equals 90% of dMax minus 0.04% of paper
white).  So I don't know what you mean when you say that "gamma i.e.
contrast is a function of the rate of change in density as you move from
dMax to dMin." What you've described by that statement is more akin to
density range than it is a desription of gamma. As noted above, density
range enters into the determination of gamma but it isn't "gamma," at least
not in the traditioal photographic meaning of that term.

Since we're getting pretty far afield from the OP's original question, and
from the topic of this group, I'll stop here. Thanks for responding to my
questions, both in this thread and in others. I've learned a lot from
reading your various messages in the time I've participated here.


----- Original Message ----- 
From: "Steve Kale" <stevekale@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Friday, March 25, 2005 2:44 PM
Subject: Re: [Digital BW] Quadtone RIP Faded print



I was making a rather simple point that contrast is not "dynamic range".  So
your point:

> the gamma adjustment in QTR doesn't affect the overall
> contrast of the print (i.e. the dynamic range)

is technically wrong.  You can't say "i.e. the dynamic range".  The gamma
adjustment will affect contrast, the rate of change of density as you move
from dMin to dMax.  But it won't affect the dynamic range ie dMin and dMax
remain the same.  In constructing the QTR curves we typically linearise Lab
L values.  Lab does not have a fixed constant gamma but 2.2 is a pretty good
approximation.  You can alter the way you get from dMin to dMax but the end
points are fixed.  So gamma, ie contrast, is a function of the rate of
change in density as you move from dMin to dMax - I can have the same gamma
but two completely different dynamic ranges.

I don't know what David's exact problem is but it seems to be a jumble of
things.  The perceptual intent rendering should take care of the
"gamma/contrast" issue (after all, it is a process common to the Epson and
QTR workflows) - at least enough for the two prints to look comparably the
same. (I would still typically accentuate contrast further with a "s-curve"
for matte paper but would do more or less the same for either driver.)
Maybe the issue is a more mechanical one ie not enough ink on paper with QTR
(less dynamic range) but that too seems unlikely - at least to an extent
great enough to provoke a cry for help.

Before Roy came up with using perceptual intent rendering and his ICC
profiles for matte and photo paper, gamma was a big issue with QTR.  The
print space had a completely different gamma than that of the work space.
You can plot this out for yourself by calculating the density values of Lab
vs those of a printspace where 0 (black) was mapped to L=16 and 256 (white)
was mapped to L=96 and the line in between was straight.  The latter is a
linear QTR curve.  Presumably (someone with a greater knowledge of colour
ICC profiling than I can jump in here), when we profile a printer for colour
there is a measurement of paper white and full black and points in between
are linear in some way (regardless of what inks are used) - ie not TOO
dissimilar from the manual process we go through with QTR or IJC/OPM. So
then each has a linear or relatively linear grey scale and we use, for both,
perceptual intent rendering to manage the transition from workspace to
printspace.  A long way of saying that if your workflow is right there isn't
that much difference between the two  - except that with QTR (or IJC/OPM)
you get to decide which inks are used and how in the construction of the
grey scale.




> From: "B. Ellis" <bellis60@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Fri, 25 Mar 2005 11:06:47 -0500
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: Re: [Digital BW] Quadtone RIP Faded print
>
>
>> Just as an fyi, contrast IS gamma.  You mean dynamic range which, yes, is
>> determined by dMin and dMax and not gamma/contrast.
>
> Thanks but I'm not sure what your point is. I know what gamma is. I was
> saying that since the gamma adjustment in QTR doesn't affect the overall
> contrast of the print (i.e. the dynamic range) and instead affects only
the
> contrast within the midtone areas, it was unlikely that changes in it
would
> cure the flat print problem the OP was having (i.e. it wouldn't change the
> overall contrast or dynamic range of the print).  Are you saying that was
> inaccurate (or just that instead of using the term "overall contrast" I
> should have used the term "dynamic range")?
>
> Or were you perhaps saying that since "gamma" by definition is a
measurement
> made only on the straight line portion of a curve (ignoring the toe and
> portions beyond the shoulder) my statement about it affecting only the
> midtones was redundant? I do know how gamma is measured and I'd agree my
> statement may have been redundant. However my basic point was just that
> QTR's gamma adjustment was unlikely to cure the OP's problem of having an
> excessively flat print with washed out shadows and highlights, I wasn't
> trying to explain what gamma was or how it was measured or why other
> measurements (e.g. average gradient or contrast index) are perhaps
> preferable when describing a print's overall contrast or dynamic range.





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