Digital BW, The Print

Steve,

By the way, I'm starting to use Lab b axis to judge my simple profiles --
it's easier.

I'm also half considering a full-on Lab low gamut inkset controlled by one
of the affordable B&W rips.

Paul
www.PaulRoark.com 
_____________________

-----Original Message-----
From: Steve Kale [mailto:stevekale@...] 
Sent: Tuesday, December 07, 2004 4:25 AM
To: DigitalBlackandWhiteThePrint@yahoogroups.com
Subject: Re: [Digital BW] Tonal range and linearization


Hi Tyler and others following along

No maths, just managing image tones to print tones - what a B&W RIP is all
about. 

With regard to LAB - you sold me.  Even as a (final) edit tool.  In a colour
world LAB is not intuitive - what's with the a b mumbo jumbo?  Too hard!
But in a B&W world it couldn't be easier... a and b are 0!  Cool.  A simple
scale from 0 (white) to 100 (black) with 50 in the middle - what a pity it
isn't 0 black to 1 white but never mind.  And, LAB=50 equals my Kodak grey
card! Cool.  And to boot it supposedly matches the way the eye "sees".
Cool.  I'm sold.  Let's pick it and stick to it. Where can I get a step
wedge in LAB?  

At a minimum with LAB I can place a grey card in the scene, expose to it,
check to make sure it has a value of LAB=50 in the image file with my
eyedropper (a test of my exposure) and with a well profiled monitor and
colorsync doing the translation to my monitor's (hardware) space - huge sigh
of relief here, thank god I don't have to do this myself - look at Kodak
grey on screen! Now for the print....ooops, no colorsync to help me here. Oh
no!

Here is where we are having difficulty.  Maybe a simple example would help.

A teacher of digital photography wishes to test his/her students' ability to
expose and print a scene correctly.  This is not a test of their subjective
artistic interpretation of the scene but of their technical method and
ability. (He/she will can't test their artistic ability just help them
explore it.) So the teacher goes to the local hardware store and buys three
paints.  The first is almost bright white.  The second is chosen very
carefully so that when it is dry it is Kodak middle grey with 18%
reflectance.  The hardware store is all out of very dark black but they have
a very very dark grey - kind of sort of almost black.  The teacher paints a
wall with the "white" paint and, once it is dry, paints two large squares on
the white wall, one Kodak grey and the other with the near black.  The
teacher wants the students to photograph the image and then print it on two
printers.  The printers are both Epson 1280s but one has Eboni ink and EEM
paper while the other has Epson PK ink and Epson Premium Semi-gloss paper.

Just before the students start their exercise the teacher takes out his
Eye-One and measures the three colours.  He gets LAB readings of 96.5 for
the white, 50 for the mid grey as expected, and 16 for the dark grey.  He
doesn't let the students know the answers but he does tell them that the mid
grey is the same as their Kodak grey card.  On the assumption that all the
students do perfect exposures how will these print using QTR on the two
printers?

The digital image file could not be simpler.  A bunch of pixels at 96.5, a
bunch at 50 and a bunch at 16, in LAB terms.

Take printer No 1.  In setting up this printer we would have printed a step
wedge, measured the printed densities and written LINEARIZE= [density
readings] in the curve .txt file.  It is EEM with Eboni and so dMin is 0.04
(LAB=96.5) and dMax is 1.68 (LAB=16), conveniently!   The printer will not
print the scene as recorded.  It will put some ink down for the "white" wall
and not enough ink down for the "dark grey" square.  I do not know how it
will print the Kodak grey.  The rendition will be better on the second
printer but is still incorrect.

But, you might say, my image file does not use the full tonal range.  I
should use levels to shift my black point to the right and my white point to
the left.  Ok so I do this and the image file is altered.  Now, and only
because the dMin of the printer equals the lower end of my adjusted image
tonal range and the printer dMax equals the high end of my adjusted image
tonal range, the first printer will print the "white" correctly and "dark
grey" correctly.  I, again, have no idea how the Kodak grey will be printed.
Now what happens if I then take this adjusted image file and print to the
second printer.  It will print the dark grey too dark, the white too white
(just), and again I have no idea how it will print the Kodak grey.  I also
can only fiddle with a soft proof to get it to look kind of sort of right.

I would simply argue that this is not a satisfactory workflow.  If I can't
rely on my workflow to print the simplest of (in-gamut) images correctly
then I am simply fiddling in the dark.  It's no wonder everyone breathed a
massive sigh of relief when Carl figured out a way to soft proof with the
Eye-One! At least then a partial light came on in the room and we could
fiddle with the before and after on screen.

Now, yes, to get the workflow to a more satisfactory position does require a
little math.  If you followed the methodology I have been proposing the
image would print perfectly on BOTH printers without adjustment to the image
file WHICH WAS CORRECTLY EXPOSED.

This doesn't fly in the face of "maintaining some sort of middle grey" - it
achieves it.  

(BTW Paul is the only guy I see out there consistently targeting a
particular density for mid grey 50%.  He has been at times a little unsure
about what that mid should be but at least it is targeted.  Paul, if you
believe in LAB make it LAB=50 which will match your grey card's 18%
reflectance.  In Grey Gamma 2.2/Adobe RGB this is about a step of 54.2.  The
50% step in a LAB step wedge should, surprisingly, print at LAB=50 or a
density of 0.73.)

As for real black, I agree that there isn't any real world real black
(although Einstein would probably say it is out there somewhere).  But there
are very real world examples of near black.  Thankfully colorsync manages
this issue for our display and aligns real black to monitor black with
whatever "intent" we have selected (interesting that they called it
"intent").  In general it doesn't matter so much these days because the
display a  far broader range of tones than we can ever hope to print.  But
we don't have colorsync by our side when printing with QTR (or any other non
colorsynced RIP).  The management and repositioning of out-of-gamut colours
has to be managed manually.  Unfortunately this involves a little bit of
maths.

I hope you will persevere with me on this and I hope people like Roy are
listening in.

Like that LAB!

Steve

PS: A couple of additional comments below:


> From: Tyler Boley <tyler@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Tue, 07 Dec 2004 02:03:58 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: Re: [Digital BW] Tonal range and linearization
> 
> 
> 
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, Steve Kale
> <stevekale@b...> wrote:
> ...
>> I understood you.  I guess I was raising the counter question that
> if there
>> is little difference in grey scale between 2.2 gamma and LAB (see
> Bruce's
>> curves) then what is gained by output at LAB?
> 
> Well, it's a choice that was made. You'd have to ask Roy. But it seems
> a likely choice given that all out icc based color output it
> referenced to it too. You have to pick something right? It seems no
> one picks the same target to hit.
> 
> ...
> then a bunch of math stuff...
> Steve, not going there. NOT GONNA DO IT!
> The excedrin bottle is nearly empty, the dog needs to go out, my
> clients are screaming, my butt hurts...
> 
> How about this? If the output dynamic range is by nature smaller than
> the "abstract" space of say, LAB, then there is indeed compression, right?
> And what you are saying is that even if one "linearizes to LAB" that
> output to the smaller range that the resulting slope is no longer the
> same as LAB, right?
> These working spaces are simply that, descriptors. There is no real
> world equivalent of absolute black or white that is relevant to us
> with this work. When translated to any useful viewing of the art,
> whether monitor, paper, whatever, there is by nature always a
> compression if the image content utilizes the full range of the
> working space.
> The resulting tonal progression is something porportionally or somehow
> mathematically related to the reference, but not exactly the same rate
> of change. 
 
The existing relationship to reference is variable and changes for every
ink/paper combination.

> If we insist on maintaining that slope in some part of the
> scale, then yes, there would be something clipped, or compressed at
> the extremes, or whatever.

Only if we sent it out of gamut tones.  We can avoid this in a CONTROLLED
fashion with curves.


> But what would be the point? What is so sancrosanct about maintaining
> that slope somewhere? The whole point is to maintain the tonal
> relationships, unless of course you want come up with your own unique
> definition of "linearization".

I am not defining linearization just using it.  Linearization comes from the
fact that the first derivative (ie LOG) of the exposure/density curve of a
gamma space is linear.  (BTW note that the first derivative of LAB would not
appear to be linear else Bruce would have been able to derive the gamma with
perfect fit.)  Whoops!  Sorry I said no maths.

> 
> Also, this all flies in the face of maintaining some standard middle
> gray density value. If I linearize uncoated Somerset Velvet (low dmax)
> and PhotoRag (high dmax) middle gray value in the file will print
> differently on one paper than the other. Rightly so! I want those
> relationships I worked so hard on maintained. If I want to match some
> gray on both prints, obviously I'll have to massively compress my
> shadow values in the uncoated version. The concept of middle gray has
> been redefined. Now it really IS in the middle. Actually it always was
> for us zone freaks, gray card be damned.
> If that's what one is attempting (a specific middle gray target
> density), every paper and ink combination would have to be linearized
> to a different gamma, based on paper white and dmax, and the tonal
> relationships in a file when printed on them all would be different.
> 
> Is this getting to what you are talking about? If so, I can only say
> that to my knowledge, linearization by today's standard means a
> uniform compression as above, so then a deviation from the strict
> slope of the reference space. I think that's what you have been trying
> to get me to absorb.
> But I'm not sure there is another really useful way to go about this.
> If you come up with one, be careful who you share it with before you
> make your own trip to the patent office <G>.
> If you are attempting to point out that the process of linearization
> to a given standard like LAB is not strictly the same as the reference
> and therefore not really "linearization" then yes, you are right.

I am saying that linearization to LAB should actually do that, not some
other function whose relationship to LAB changes for each and every
ink/paper combination.

> I think I get your point now, if not, please forgive me, I have to
> take the dog out.
> Tyler
> 
> By the way, by strict definition, colormetric icc color
> transformations maintain the relationships as you suggest from paper
> white on down with Black Point Compensation off, then of course the
> shadows clip if out of gamut. Someone correct me if that's wrong.
> 





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