Digital BW, The Print

John

In essence what we have been talking about is to what extent the printer
space (think density output for each possible pixel value, in 8 bit 0-255)
can more closely resemble our workspace.  That together with how QTR is
calibrated.  

Think of a step wedge printed with QTR.  QTR is calibrated such that is
produces steps from 0 (pixel value 255 in 8 bit) to 100% (pixel value 0)
with equal changes in Lab's L value from dMin (minimum density or paper
white) to dMax.  If you were to plot each step's L value you would have a
straight line.  As an example, EEM has a dmin of around 0.04 which is Lab
L=96 and a dMax with Eboni ink of around 1.68 which is Lab L=16.  With Lab,
L goes in a straight line from value 100 (perfect white) at pixel value 255
to 0 (perfect black) at pixel value 0.  I think you can see that if one puts
paper white at 255 (L<100) and dMax at 0 (L>0), even if it is a straight
line it is not the same as the Lab line - it has a different slope, it is
flatter.  So even if we were editing in Lab, our editing space is not the
same as the print space (which would vary depending on the different dMax
and dMin of various papers).

We were then also talking about the benefits, if any, of editing in Lab
rather than Gray Gamma 2.2.  One of the problems with Lab for B&W is that it
is 3 channels rather than just one so we would use a lot more disk space
than necessary.  So Roy came up with the single channel greyscale-Lab - LAB
without the a and b.  Exactly the same as Lab but lower file sizes.

If you have an image on screen and want to bring it into greyscale Lab
without changing the way it looks then use Convert to Profile.  You can
then, if you like, continue editing in greyscale Lab.  When it comes to
printing, however, you will still face the same issue I mentioned in the
second paragraph above.

I hope this helps

Steve  


> From: Chris Hargens <chargens@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Tue, 14 Dec 2004 20:39:21 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: [Digital BW] Re: Lab Step Wedge, grayscale LAB Space & tonal
> Linearization simplification request
> 
> 
> 
> As a starting point, download the LAB profile that Roy posted -- see
> message/52160 -- install it with your other ICC profiles, open up in
> PS one of the images you've printed in QTR (open it in your default
> grayscale, i.e., working gray) then go to
> Image, Mode, Assign Profile and assign the Lab Grayscale profile. You
> can preview the difference it makes. If the Lab Grayscale profiled
> image corresponds more closely to your print, then it makes sense
> that the profile would be the better one to work in when editing your
> images for QTR. In my case it does seem to come closer to what I'm
> getting out of the printer. Hope this helps as a first step.
> 
> Chris Hargens 
> 
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Mr_Misty_44"
> <jharvey@i...> wrote:
>> 
>> Being a semi computer illiterate photographer who uses QTR I am not
>> completely at home with the nuts and bolts of  QTR and LAB
> discussed 
>> in these threads. If someone in the group could take much of this
>> discussion and the conclusions and work flow adjustments and boil
>> them down into a bit simpler explanation and practical application
>> when the dust settles, I'd appreciate it. Sorry to sound so
>> dense.Perhaps it could be placed in the FILES.
>> John H
>