Digital BW, The Print

Steve,

I have some test strips at http://home1.gte.net/res09aij/Test-files.htm that
show some relationships I find helpful.  This is just my approach, however.
I've never seen a real scientific discussion of the theoretically "correct"
way to "linearize" a gray scale.  Just what the midtone density should be is
not 100% clear to me.  

However, you noted the range of 0.04 for the paper white to 1.65 for the
100% black.  That just happens to be a typical EEM range.  So, reading a
0.04 EEM paper white I get a Lab L reading of 97.  Taking a reading of the
1.65 black with my X-Rite, I get L = 16.7.  The committee that made the Lab
scale did a tremendous amount of work on how we see.  So, I take their scale
as rather persuasive.  So, if you average the EEM black and white Lab
Luminosity values you get about L = 56.9.  You'll see on my test strips that
I put 50% at 0.61 visual density.  This is about Lab L=56.9.  (I use the
readouts of the X-Rite for these equivalents.  I think there is a formula,
but I don't have it.)

Perhaps not by coincidence, this was also about the average PiezoBW 50% I
measured some years ago when I was still printing with that system, and I
wanted my files to be able to print on either system.

When I profile other papers with a higher dmax, I don't change the 50% spot,
because I think (perhaps incorrectly) that the midtone density is what most
people see.  That seems to determine how bright or dark the overall image
is.  A great black and bright white just give more dynamic range.  Our
ability to see into those 2+ black densities, however, is limited.  So, I
use 0.61 for my target 50% density for all papers.

On the other hand, the Kodak standard gray card is L = 50.  I think one
could also argue that we ought to standardize on that as a uniform 50%
density no matter what the paper white and ink black happen to be.  If I
were starting from scratch, I'd be tempted to do just that.  I think using
the L=50 midpoint might make the average monitor closer to the print.  L=50
is about density 0.74 on my X-Rite system.

I'm not sure this is what you were looking for, but this is what I do.

Paul
www.PaulRoark.com

__________________________________

-----Original Message-----
From: Steve Kale [mailto:stevekale@...] 
Sent: Tuesday, November 30, 2004 5:26 PM
To: DigitalBlackandWhiteThePrint@yahoogroups.com
Subject: Re: [Digital BW] Tonal range and linearization



How do I relate and convert a measured paper/ink density value back to the
0-255 scale used in PS's curves?

If I were to print a 255 step wedge out for a particular printer/paper/ink
setup I will have in front of me the tonal range (in 8 bits) of that setup.
The 0% wedge with a density of say 0.04 is off-white rather than pure white
and the 100% wedge (or 255 wedge) is also a dark dark grey rather than pure
black - let's give it a density of 1.65.  I have a compressed tonal range
versus the input values of the image on the display (with a good monitor
with great tonal range).

Furthermore, if I understand the process of printer linearization properly,
my "output space" is linearized and so there is a straight line relationship
between steps in input values and the steps in output values.  And so if the
printer is linearized properly this compression of tonal range can be
"replicated" by a curve which has the zero output level shifted up and the
255 output level lowered.  The slope of the curve is less and hence the
contrast or gamma has been lowered.  Worse yet, if, as is most commonly the
case, there is greater tonal compression at the 255 level than at the 0
level, ie we have better paper whites than we can achieve ink blacks, then
the mid-gray value has shifted lower also.  These two factors, I understand,
are why unadjusted prints print "lighter than they appear on screen" - a
common complaint.  

If I can relate my printer (paper and ink) output range, dmin and dmax, to
the 0-255 range in PS's curves then I have a more precise mechanism for
dealing with output tonal range compression and readjusting the mid grey
point (and any other points on either side of it) - I can use a curve to
compress the tonal range before it gets to the printer with much greater
control over how that compression is done (or in other words I can get away
from linearized situation that exists in the printer domain).

Thoughts?






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