Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "odesmais" <odesmais@y...> 
wrote:
> Steve,
> 
> First I need to apologize : I have double posted the below in here and 
> QTR group which is not very ethical... In fact I just realised as you 
> below mentionned that Roy would be of help in this.
> 
> I did look at the kTRC explanation and also the color.org explanation 
> of BPC, apparently this is an issue the organisation is wanting to 
> tackle, I don't know if this is being implemented in v4 or not, but by 
> experince not all soft (profilers, don't know about editing 
> softs) "understand" v4. There's also a lack of standardised perceptual 
> intents (seems all profilers implement a proprietary algorithm) which 
> is also something they are looking at, and to make it even more 
> confusing there's now a debate about CIE Lab as a PCS (it does not see 
> the same for XYZ)due to hue shifting thus non-uniformity... Also this 
> is simply and unfortunately coming from booking reading and web 
> browsing : I try to know but seldomly understand.   
> 
> Anyway, in fact what happens is that since my PS color settings is 
> (default, I never bothered to set it properly)grey : dot 15% I work 
> with too "light" a working space and since the stepwedge is not tagged 
> PS assumes the dot 15% as a source space thus printing light after 
> converting.

It's not really that dotgain 15% is "too" light.  It's just lighter than some of
the other grayspaces.   Profiles/grayspaces just define what the numbers
in the file mean.  Think of a ruler.  It can be labeled in cm or inches.
Either works just fine.  CM are shorter than IN but labels like "too short" or 
"too long" don't make much sense.  Convert to Profile is like changing
12 in to 30 cm -- same length different units.  Assign is like stamping the
cm or in label on the ruler.

> 
> I'd like to measure the actual output of the QTR-ICC profiling but I 
> feel it's a lost search. 

I think maybe this question is a source of confusion.  The QTR-ICC profile
is basically just a conversion table.  It allows the CMS to convert from the
source profile to the printer output.  So I don't know what "actual output 
of the QTR-ICC" really means.

   When printing the raw stepwedge in QTR, datas 
> are fed irrespective of color space, so the raw L* value are dependent 
> on printer-ink-paper and does not reflect...anything else than the 
> linearised stepwedge printed on ONE printer with ONE ink set and ONE 
> paper.  

Yes, that's right.

   May be there's a way to translate the linear L* measurements 
> into a gamma 

At lot tend to be read into the word "gamma".  All it is, is the exponent in
a mathematical function.  It's a different math function than L*.

    and set it as a grey working space to perform the 
> conversion with the QTR ICC profile to evaluate, but I don't know it. I 
> asked Roy if 1.8 would make sense. If not I leave it here since anyway 
> it will not improve the workflow. I'll set color settings grey : gamma 
> 2.2 (RVB and the windows environment) to minimize conversions and that 
> will do.

If you do both gray and RGB (RVB) I'd stick with workspaces that have the
same luminance curve i.e. gamma.  GG 2.2 corresponds to Adobe RGB.
GG 1.8 corresponds with Apple RGB or Colormatch RGB.

> 
> BPC remains a mystery : I use to do the soft-proofing with 
> perceptual+BPC+white paper (which also turns on Blakc ink). However it 
> might be that you need/can soft proof not turning BPC but Black ink 
> simulation (which is done when white paper is checked)... to be 
> followed since it's only my understanding.

Softproofing with color is a lot more important than with B&W.  In fact I rarely
use it anymore.  Since there are no gamut issues with B&W mappings are
very simple white-to-white and black-to-black.   The important part is just
that all the intermediate values are linearly expanded or contracted using
Y luminance values (from XYZ).   The only exception to this is that you
may want to reduce the black on the screen (i.e. reduce the dynamic range)
so that it's closer to the DR of matte paper.  I think BPC is mainly an issue
for color where a black could be a warm black or a cool black.  But it can also
be used here for reducing the dyn range.  You can easily see this by using
the QTR Gray Matte Paper profile -- this allows selecting BPC or not.

Roy



> 
> I'm sure I'm tiring many people with some lack of competence/knowledge 
> and not-so-important issues... So, thanks Steve for your great 
> willingness to help, I do appreciate a lot.
> 
> Olivier
> 
> 
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, Steve Kale 
> <stevekale@b...> wrote:
> > Olivier
> > 
> > Roy should step in here but (1) the ICC spec discusses how the kTRC 
> data
> > should be scaled for media white point-relative colorimetry and (2) 
> in the
> > documents section of www.color.org you will also find a discussion 
> paper on
> > Adobe black point compensation.  Roy has done both the black and 
> white point
> > scaling.
> > 
> >