Digital BW, The Print

I have just created my own grey icc profile using Roy's program. I then 
used this instead of Same as Source in PS, with ABW mode. The result is 
a better print. Excellent !! However I do not really understand why 
this works. Using the grey icc profile in PS and ABW mode means your 
using colour managment in PS and the driver - double profiling. Could 
some kind sole explain how this works.

Many thanks.

Peter.

What you have done is profile the luminance axis of the printer.  ABW is
normally a same as source workflow which means the image's file values are
sent unaltered to the Epson driver.  You have profiled what happens when
these unaltered values are received and are now using PS colour management
to alter the sent file values in consideration of what the printer does when
it receives value x or y.  So the colour management system alters the file
value to achieve a more desirous output value (subject to black point
compensation and white point compensation).  You have not doubled up on CM.
Hope this helps.

> From: bwbonkers <PeterDLevis@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Tue, 02 Aug 2005 20:56:13 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: [Digital BW] QTR ICC Profile + R2400 ABW
> 
> I have just created my own grey icc profile using Roy's program. I then
> used this instead of Same as Source in PS, with ABW mode. The result is
> a better print. Excellent !! However I do not really understand why
> this works. Using the grey icc profile in PS and ABW mode means your
> using colour managment in PS and the driver - double profiling. Could
> some kind sole explain how this works.
> 
> Many thanks.
> 
> Peter.

Steve

Many thanks for the reply. As far as I can see printing using a 
custom grey icc profile instead of Same As Source seems to linearise 
the printer. I tried the following experiment:

I printed the IJC 21 step file first with Same As Source as print 
space profile in PS CS and then with my own custom grey icc profile. 
I then measured both and saved the L values as a text file. I then 
imported them into IJC linearisation page (MAC version) just to see 
what the values looked like. Result the grey icc profile values 
produced a virtually perfect -4 down curve. The Same As Source values 
were not as accurate.

Form this it seems that using a custom grey profile instead of Same 
As Source is certainly beneficial.

Any comments.

Peter.

I guess I am not sure what you mean by "-4 down curve".  The idea is that
you linearize the printer or it is done for you as part of the ABW driver,
as one step, and then you profile this output.  For example, Black Only
output is far from linear (terrible in fact) but we can still profile it.
Then colour management is used to manage the difference between file values
and the print "space".  In essence, for B&W this amounts to just tone
compression due to the different black and white points.  In the current
incarnation of the Grey ICC profile generator, Roy is doing the black point
compensation rather than leaving it to PS (this is why you can't proof for
the weaker print black at the moment).  All the data is adjusted for white
point (ie it is media white point relative).  In essence Roy has created a
file that scales input data (file values) according to the media black and
white points.  A Same as Source workflow just takes file data and maps it
directly to the (linear or, in the case of Black Only, non-linear printer
output).  If the printer is perfectly linear there is a straight line in L*
from dMax to dMin and hence the vast majority of pixels are printed lighter
than one would prefer.  What the CM does with the profile I do not fully
understand (it is very hard to get clear answers from colour people when you
ask them to think in terms of just greyscale ICC profiles).  I think it is
in essence simply a transfer curve which has been scaled as I described
above.  So the file data gets adjusted by this curve before hitting the
linear output table of the printer (in lay terms).  Open up your grey ICC
profile in Colorsync and take a look at the various tags.

> From: bwbonkers <PeterDLevis@aol.com>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Wed, 03 Aug 2005 14:46:39 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: Re: [Digital BW] QTR ICC Profile + R2400 ABW
> 
> Steve
> 
> Many thanks for the reply. As far as I can see printing using a
> custom grey icc profile instead of Same As Source seems to linearise
> the printer. I tried the following experiment:
> 
> I printed the IJC 21 step file first with Same As Source as print
> space profile in PS CS and then with my own custom grey icc profile.
> I then measured both and saved the L values as a text file. I then
> imported them into IJC linearisation page (MAC version) just to see
> what the values looked like. Result the grey icc profile values
> produced a virtually perfect -4 down curve. The Same As Source values
> were not as accurate.
> 
> Form this it seems that using a custom grey profile instead of Same
> As Source is certainly beneficial.
> 
> Any comments.
> 
> Peter.
> 
> 
> 
> 
> 
> 
> Please visit the Group Homepage to check the Files, and other resources as
> they are often being updated.
> 
> http://groups.yahoo.com/group/DigitalBlackandWhiteThePrint
> 
> If you wish to receive no emails or just a daily digest, or you wish to
> unsubscribe, please edit your Membership preferences by visiting this same
> page.
> 
> Please follow these basic guidelines:
> - As threads develop, trim off excess portions of earlier messages to keep
> them short.
> - Good manners are required at all time. No personal attacks or flames.
> Hostile, aggressive or argumentative users may be removed from the membership
> without notice.
> - Keep your posts and threads related to the group topic of digital B&W
> printing. Users who persistently make off-topic posts may be removed from the
> membership.
> - By posting on this forum you agree to abide by the group rules and
> guidelines, and to abide by the actions and decisions of the group Owner and
> Moderators. See ³Group Topic, Rules and Guidelines² in the Files section:
> http://groups.yahoo.com/group/DigitalBlackandWhiteThePrint/files/
> 
> BY PARTICIPATING IN AND/OR POSTING MESSAGES TO THE DIGITAL BW, THE PRINT
> YAHOO! GROUP YOU EXPRESSLY UNDERSTAND AND AGREE THAT THE ³OWNER² AND
> ³MODERATORS² OF DIGITAL BW, THE PRINT YAHOO GROUP SHALL NOT BE LIABLE TO YOU
> FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, CONSEQUENTIAL OR EXEMPLARY
> DAMAGES, INCLUDING BUT NOT LIMITED TO, DAMAGES FOR LOSS OF PROFITS, GOODWILL,
> USE, DATA OR OTHER INTANGIBLE LOSSES (EVEN IF THE  ³OWNER² AND ³MODERATORS² OF
> DIGITAL BW, THE PRINT YAHOO GROUP HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH
> DAMAGES), RESULTING FROM: (i) THE USE OR THE INABILITY TO USE THE DIGITAL BW,
> THE PRINT YAHOO GROUP; (ii) UNAUTHORIZED ACCESS TO OR ALTERATION OF YOUR
> TRANSMISSIONS OR DATA; (iii) STATEMENTS OR CONDUCT OF ANY THIRD PARTY ON THE
> DIGITAL BW, THE PRINT YAHOO GROUP; OR (iv) ANY OTHER MATTER RELATING TO THE
> DIGITAL BW, THE PRINT YAHOO GROUP.
>  
> Yahoo! Groups Links
> 
> 
> 
>  
> 
> 
>

I have had and still have the same trouble understanding the QTR ICC 
effect. In essence my understanding of a traditional icm is that it 
reflects/includes both the printer-paper-ink caracterization and the 
conversion needed to map from one space to another thus including the 
needed linearisation calculation. In the case of QTR caracterization 
and linearization are done in the QTR curve creation process and 
curves are called from the printing interface (this is why curves are 
refered to as profiles in the manual and the soft). So while at this 
first stage you can't proof you do have a linear output.

ICC generator adds accurate soft proofing without a doubt : 
Well,to me at least, and it seems to be the feature Roy wanted to 
add, this would, to me again, explains why before the Generator he 
created a gray-lab space and generic mat/photo grey icc in the hope 
one would go for a first basic soft proofing ability, but I tend to 
believe BP and WP were then only generic and not describing the real 
ones while he probably wanted true soft proofing with actual BP/WP.

Back to the new ICC, it can also serve a conversion in PS from RVB to 
Grayscale. Besides, from memory, using this Grey ICC as a printing 
space profil also produces a B&W image from a RVB colored file in the 
OEM driver (though I did not print, I just visualised the preview). 
So I would tend to conclude it is a true ICC generator that can be 
used like any other profile except that instead of mapping source 
space colors to destination space "available" colors it maps s.s.c. 
to destination space grey values, probably only retaining the L* 
values to do so. And Roy recently informed he wants to provide full 
icc support including colors to reflect toning and white paper color 
if I'm not wrong.

Now since the read target is an already linearised stepwedge (this is 
how I understood you have to create the ICC not from the non-linear 
setpwedge), the profile just becomes a description of the printer-ink-
paper BP and WP with evenly distributed grey values.

This is where I am confused and additional testings need to be run :
1. convert a RVB file with the grey profile and send it as such to 
the Epson driver or print with "print space" profile being the grey 
icc to measure linearity. If unlinear (and it must be) the profile 
simply act as a PS grey mode action in this direction.
2. Create a profile from the non-linear stepwedge and use it in the 
destination space profile from an RVB file : though it will 
demonstrate nothing but the linearisation is achieved by the profile 
and could serve in ABW mode for instance.
3. print with QTR with and w/o converting to the profile (oh yes I 
run Windows as an OS) adn mesure the stepwedge.

I understood that without QTR, the EOM driver will interpret whatever 
comes gray into a CMYK output thus blending the color ink in a non-
linear way (the same with a quadtone setting). So the whole QTR 
profile generation would turn to be mainly a soft proofing feature : 
whether you convert or not a gray file before sending it to QTR will 
have no effect (as long as you create the profile from the linear 
stepwedge).

If not wrong in the above, the "better" B&W experienced using a QTR 
ICC would come from teh fact that actual BP and WP would have been 
measured and grey values re-distributed accordingly, proving QTR ICC 
provides a true B&W profiling feature.

But still they are questions :
1. Do you need to(or should you) use this profile when printing with 
QTR for anything else than softproofing ?
2. Would you consider converting a RVB file into a Grayscale one in 
PS using this profile ?
3. Would you use it in an ABW workflow with a 2400/4800... and if so 
do you create from a linear stepwedge or non-linear one in the case 
of the 2400 ?

I'm sorry all this sounds confused and probably repeating part of 
what Steve wrote, but to me... it is very confusing.

Now one question to Steve :


>In the current
> incarnation of the Grey ICC profile generator, Roy is doing the 
black point
> compensation rather than leaving it to PS (this is why you can't 
proof for
> the weaker print black at the moment).  

This I don't understand : "can't proof the weaker print black" ? In 
the (little) testing I did I would have said the proofing also adjust 
the BP. This would also be demonstrated by Roy's instruction 
mentionning you need to soft-proof using perceptual and BPC (the BP 
being thus embeded in the grey ICC). I'm sure I wrongly understood 
the sentence.

The linearization of a printer is separate from profiling the printer.
Typically as part of a RIP you have the ability to linearise the output - in
effect, in our case, smooth the greyscale ramp so that there is a constant
increase in L* as we go from dMax (pixel value 0) to dMin (pixel value 255).
In the case of the Epson driver you do not have the ability to relinearize
the output.

Once linearization is complete and, in the case of a RIP, embedded in the
curve that tells the printer what inks to fire and when, you can profile
that output with an ICC profile.  A greyscale ICC profile only profiles the
luminance axis (not hue).


> From: odesmais <odesmais@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Wed, 03 Aug 2005 16:48:16 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: [Digital BW] Re: QTR ICC Profile (MAINLY) + R2400 ABW (MUCH LESS)
> 
> I have had and still have the same trouble understanding the QTR ICC
> effect. In essence my understanding of a traditional icm is that it
> reflects/includes both the printer-paper-ink caracterization and the
> conversion needed to map from one space to another thus including the
> needed linearisation calculation. In the case of QTR caracterization
> and linearization are done in the QTR curve creation process

You tell QTR what inks to use in the greyscale, to what limits and in what
order etc.  You then linearize this output by including the linearize
function as you mention so that there is a smooth ramp in L*.  I would not
call this "characterization" - certainly not in the profile sense.  It is
also confusing that the term profile has been introduced here.  We used to
refer to this process as the process of building ink "curves".  They are
still called curves in the QTR driver.  They are not profiles in the ICC
profile sense.


>and 
> curves are called from the printing interface (this is why curves are
> refered to as profiles in the manual and the soft). So while at this
> first stage you can't proof you do have a linear output.

At this stage you have nothing to soft proof - you have not yet generated an
ICC profile of any sort.  You could go on from here and do one of two
things: you could follow the process to use Eye One Match to generate a
"preserve color numbers" ICC profile which would allow you to proof for hue
and luminance.  This profile merely portrays the tone change and compression
- it does NOT help you manage it.  Alternatively, you could go on to use QTR
Create ICC to accurately PROFILE the luminance axis of the greyscale created
by your QTR Curve (or Curve mix).  (It is not necessary to use gray-lab
space at all - think of this as something that was developed as we fumbled
our way to the QTR Create ICC process.)
> 
> ICC generator adds accurate soft proofing without a doubt :
> Well,to me at least, and it seems to be the feature Roy wanted to
> add, this would, to me again, explains why before the Generator he
> created a gray-lab space and generic mat/photo grey icc in the hope
> one would go for a first basic soft proofing ability, but I tend to
> believe BP and WP were then only generic and not describing the real
> ones while he probably wanted true soft proofing with actual BP/WP.

Here is where you are getting confused.  When Roy first pottered around with
profiling the luminance output with an ICC profile he made two GENERIC ICC
profiles - one for an average matte paper and one for an average photo
paper.  (There were a few things wrong with these and ideally you would not
use them anymore.)  In the course of developing on this idea, Roy enhanced
QTR Create ICC to read actual data rather than hypothetical data.  This
allows us to create an ICC profile of our ACTUAL output (ie our actual QTR
Curve, ink, printer, paper combination - or in fact any combo that generates
a greyscale).  So QTR Create ICC is a refinement or development of the
generic to actual data.


> 
> Back to the new ICC, it can also serve a conversion in PS from RGB to
> Grayscale. 

I would not recommmend using these profiles as a means of converting from
colour to B&W.  That is niot their intent.  They are intended to profile the
luminance of the greyscale produced by a particular workflow.

>Besides, from memory, using this Grey ICC as a printing
> space profil also produces a B&W image from a RVB colored file in the
> OEM driver (though I did not print, I just visualised the preview).
> So I would tend to conclude it is a true ICC generator that can be
> used like any other profile except that instead of mapping source
> space colors to destination space "available" colors it maps s.s.c.
> to destination space grey values, probably only retaining the L*
> values to do so. 

You need to understand colour management a little more.  At the end of the
day a printer just gets numbers and knows what to do for each number.
Colour management allows us to know what is meant by a particular number and
so translate between numbers.  When Same as Source is used the numbers are
sent unadjusted.  When we use PS to do the colour management the numbers are
altered by PS before being sent to the printer.  When we ask the Printer to
do the Colour Management the raw numbers are sent along with the profile so
that the printer driver alters the numbers.  In the recommended QTR workflow
the file numbers are altered by PS before being sent to the QTR driver.

>And Roy recently informed he wants to provide full
> icc support including colors to reflect toning and white paper color
> if I'm not wrong.

I would say this is a very long way off.  At this point you cross into the
realms of the full colour world.  We all know how poorly ICC colour managed
workflows perform for B&W.  They do not yet allow the compressed gamut
functionality that we want so dearly.  But using colour management to help
manage luminance compression is a huge step forward.  We can get the
luminance axis right and manage hue the old way - in altering the amounts of
the inks selected to make up the ink curve.
> 
> Now since the read target is an already linearised stepwedge (this is
> how I understood you have to create the ICC not from the non-linear
> setpwedge), the profile just becomes a description of the printer-ink-
> paper BP and WP with evenly distributed grey values.

Yes

> 
> This is where I am confused and additional testings need to be run :
> 1. convert a RGB file with the grey profile

As I noted above there are far better methodologies for getting the image
into greyscale in the first instance

>and send it as such to
> the Epson driver or print with "print space" profile being the grey
> icc to measure linearity. If unlinear (and it must be) the profile
> simply act as a PS grey mode action in this direction.
> 2. Create a profile from the non-linear stepwedge and use it in the
> destination space profile from an RVB file : though it will
> demonstrate nothing but the linearisation is achieved by the profile
> and could serve in ABW mode for instance.
> 3. print with QTR with and w/o converting to the profile (oh yes I
> run Windows as an OS) adn mesure the stepwedge.

You have lost me here.
> 
> I understood that without QTR, the EOM driver will interpret whatever
> comes gray into a CMYK output thus blending the color ink in a non-
> linear way (the same with a quadtone setting). So the whole QTR
> profile generation would turn to be mainly a soft proofing feature :
> whether you convert or not a gray file before sending it to QTR will
> have no effect (as long as you create the profile from the linear
> stepwedge).

No.  You can use QTR Create ICC for Black Only printing for example.  BO
printing generates a greyscale which likely differs from printer to printer.
It is also unlikely to be particularly linear and you have no way of
linearizing it - the Epson driver doesn't allow you to.  But you can profile
it with QTR Create ICC.  Print a step wedge using BO, read it into QTR
Create ICC to generate a profile of that output.  You can then, rather than
printing Same as Source, convert to this profile on the fly at printing in
PS.  The file values going to the printer will be altered to reflect the
print space.  This has nothing to do with soft proofing but you can also
create a soft proof with this ICC profile to see how the image will look
when converted to this profile at printing.

The situation is a little confused by the fact that Roy is doing the black
point compensation in the profile (in a similar way to how he is required to
do the white point compensation - required by the ICC spec).  When you do a
soft proof using an ICC profile generated by QTR Create ICC at the moment PS
sees that it has perfect black, ie that no black point compensation is
necessary (because Roy has altered the kTRC to do BPC there and so it goes
from full white to full black).  Remember this is just a beta version and
things are subject to change!!

> 
> If not wrong in the above, the "better" B&W experienced using a QTR
> ICC would come from teh fact that actual BP and WP would have been
> measured and grey values re-distributed accordingly, proving QTR ICC
> provides a true B&W profiling feature.
> 
> But still they are questions :
> 1. Do you need to(or should you) use this profile when printing with
> QTR for anything else than softproofing ?

Yes.  The idea is to convert to the profile from your workspace at printing.
Think of it as exactly analogous to doing colour managed colour printing -
except the QTR ICC profile only cares about luminance.

> 2. Would you consider converting a RVB file into a Grayscale one in
> PS using this profile ?

No.  If you mean a straight conversion to this profile then NO.  First there
are better ways of making the conversion to B&W.  Secondly remember that
this ICC profile is not a workspace but rather a printer output profile - it
describes a very narrow situation.

> 3. Would you use it in an ABW workflow with a 2400/4800... and if so
> do you create from a linear stepwedge or non-linear one in the case
> of the 2400 ?

Yes.  With ABW you can only print a step wedge.  The linearization is
already done and embedded in the driver.  SO print a step wedge and profile
it.  Convert to this profile on the fly from PS so that file values are
altered by PS before being sent to the ABW driver.


> 
> I'm sorry all this sounds confused and probably repeating part of
> what Steve wrote, but to me... it is very confusing.

Again just think of a colour colour-managed workflow.  You work in ProPhoto
or Adobe RGB and you files are tagged as such.  When you come to print you
select the printer profile for you printer, paper, ink combination.  You
might first do a soft proof.  When you print you are likely to have PS do
the colour management (the new terms in CS2 really are much more
informative) and hence PS converts (read: alters the file numbers so that
colors look the same in the print space and manage out of gamut colors
according to the rendering intent selected) the image file to the print
space before sending it to the driver where you have selected no colour
management (you don't want to convert the file twice!!)
> 
> Now one question to Steve :
> 
> 
>> In the current
>> incarnation of the Grey ICC profile generator, Roy is doing the
> black point
>> compensation rather than leaving it to PS (this is why you can't
> proof for
>> the weaker print black at the moment).
> 
> This I don't understand : "can't proof the weaker print black" ? In
> the (little) testing I did I would have said the proofing also adjust
> the BP. This would also be demonstrated by Roy's instruction
> mentionning you need to soft-proof using perceptual and BPC (the BP
> being thus embeded in the grey ICC). I'm sure I wrongly understood
> the sentence.


Read up above.  Black point compensation was designed by Adobe to plug a gap
in the ICC spec.  When Roy was looking at this ICC stuff he decided to code
the BPC into the profile generator (ie into the profile) rather than leaving
it to PS - "it has to be done anyway" right?  In essence the curve that
describes the printer output is scaled so that it runs all the way down from
the measured black point to perfect black.  (If you open one of the ICC
profiles in Colorsync and look at the kTRC tag you can see that the curve
starts at the bottom left corner 0,0 instead of say 0,10.)  So when PS looks
at the profile when you check Black Point Compensation it sees that no
compensation is necessary ie the profile says that the printer can print
perfect black.  And so the black you see on screen (which is a file perfect
black displayed as the best black your display can produce according to the
intent you select in your colour management preferences) is not altered to
reflect the weaker black of the printer.  Checking and unchecking BPC makes
no difference to the image.  Roy and I have chatted about this and it is
something he will take another look at whenever he gets a chance.  I'm sure
he has a lot of other things to do other than just push forward the B&W fine
art printing world!

I hope this helps somewhat.


Steve

Steve,

You have been of great help in your below reply and I am very 
thankful for it. Please allow me to try to further understand the 
subject since I am still confused...sorry.

In B&W you do not face the color value conversion, you just re-map 
the L* value from L* 0-100 to a printer-paper-ink space say L*17-95. 
But this is done at the curve creation stage and at the printing one 
when you call for the curve in QTR : doesn't this curve apply what we 
can extrapolate as an icm profile (to make it simple in the wording). 
So to convert your file with the grey icm and send the converted file 
to QTR applying the linearized curve should somehow either :

1. double profile : a source L*50 would become L*56 (with a 17-95 
linearized grayscale) converting the file with the icm. And once the 
QTR curve applies it would to re-processed to become about L*61. It 
would be like printing with a destination profile in PS and having 
the EOM driver applying color managment.
This of course can not be.

2. have no effect on the output: the file is well converted but the 
icm interpret the source L50 to a L50 output (only the display values 
but not the output values are converted thus allowing soft proofing) 
and send it to QTR as such that will do the conversion with the curve.

So in terms of purely improving the output, the grey icc is not a 
must. It is only great help in terms of editing your file in PS so 
you get a perfect match between displayed image and printed one with 
from my understanding the limitation of BP (which I understood is not 
the true paper Black but a perfect one).

Understanding this may not improve QTR users' prints (you are said to 
convert with the grey icm before printing : do it and shut up!!!), 
but it may prevent some mishandling of the soft.

Thanks.

Olivier

Hi Oliver

Here's my attempt to answer your questions.  I understand where you are
coming from and hope this helps somewhat.

> From: odesmais <odesmais@...>

> 
> Steve,
> 
> You have been of great help in your below reply and I am very
> thankful for it. Please allow me to try to further understand the
> subject since I am still confused...sorry.
> 
> In B&W you do not face the color value conversion, you just re-map
> the L* value from L* 0-100 to a printer-paper-ink space say L*17-95.

If all you do is send a file Same as Source then yes file pixel value=0 gets
printed as L*17 and file pixel value=255 gets printed as L*95.  If the
greyscale has been linearised then there is a straight line, in terms of L*,
for points in between.  So a middle grey value of PV=118 gets printed at
around L*56 (instead of L* 50).  This direct mapping into a linear L* space
where the black isn't great is why prints can look a lot lighter than on
screen (you can work out for yourself where the cross-over point is between
the image printing lighter than intended and the highlight point from which
it is printed darker than intended).  Not also that it has a different slope
and hence completely different "contrast" or "gamma".  It has a lower gamma.

The question then becomes "given the print space is different from my file
space how best can I manage the compression/mapping from one to the other?".

In the past we did this with an arbitrary s curve to punch a little more
contrast into the image.  We, in effect, compressed the shadows and
highlights a little so the midtones had more contrast - this is inherent in
the shape of the curve: a stretched "s".

A long time ago Roy and I had a lengthy debate about this.  I was in effect,
on the one hand, explaining that this was why QTR prints came out looking a
little "flat" and "light" and, on the other hand, complaining that this
mapping was not explicit to the user.  We agreed as to what was going on and
Roy made what I think is a quantum leap in terms of managing the
compression/mapping by turning to colour management techniques and using
Adobe's CMM (colour management module).

The issue exists in the colour world of course and nicely linear (or not so
linear) devices are then handled by the CMM with either Perceptual, Relcol,
AbCol or Sat intents.  We profile these devices and then use the CMM to
manage the compression from one space to another.  For B&W, Roy's idea was
to try to use the Perceptual intent to handle the luminance range
compression (the L* axis) and not worry about the a and b axes because that
then required stepping into the colour management world fully which is a
step beyond us for nor as it requires tight control over gamut and requires
a driver that thinks in colour terms.  Remember that QTR is essentially
colour blind.  Your "greyscale" could be made up of just magenta ink and yet
QTR doesn't care.  We linearize the luminance axis and just do a/b by eye
(or with indirect help from a spectrophotometer).

 
> But this is done at the curve creation stage and at the printing one
> when you call for the curve in QTR :
 
You create the greyscale at this point and without colour management you
simply map one for one to this printer output "space".  In the colour world
this is analogous to having a nicely linear printer but not profiling the
printer output space and not using colour management to control the mapping
from, say, Adobe RGB to the printer gamut space.

>doesn't this curve apply what we
> can extrapolate as an icm profile (to make it simple in the wording).
> So to convert your file with the grey icm and send the converted file
> to QTR applying the linearized curve should somehow either :
> 
> 1. double profile : a source L*50 would become L*56 (with a 17-95
> linearized grayscale) converting the file with the icm. And once the
> QTR curve applies it would to re-processed to become about L*61. It
> would be like printing with a destination profile in PS and having
> the EOM driver applying color managment.
> This of course can not be.
> 

Nope.  The CMM looks at the printer space and adjusts file values according
to the intent selected.  You are much more likely to have your midpoint
printed closer to L* 50.  If Roy was not doing the BPC in the profile and
you did not ask PS to do it then file blacks exceeding the best black of the
printer would get clipped.  (Note as I have explained before Roy is doing
the BPC in the ICC profile and so this can't happen but the downside is you
can't softproof for the printer black.  He is still looking at this.)

Think colour for a second.  Epson linearized the printer for us but we still
PROFILE the printer with an ICC profile and use colour management to manage
the movement from our workspace to the printer output space.  Same here with
just one change - we are only bothering to focus on luminance/density (or in
other words, we only care about/mange one axis rather than 3).


> 2. have no effect on the output: the file is well converted but the
> icm interpret the source L50 to a L50 output (only the display values
> but not the output values are converted thus allowing soft proofing)
> and send it to QTR as such that will do the conversion with the curve.

I can't find the tests I did to give the numbers but you should print a 21
step with Same as Source and then using the ICC profile and note the
difference.  You should, I believe, see that the second gives an "s curve
like" fit rather than a straight line. In essence, and Roy can give more
detail to the maths behind this, the image file values are scaled for two
things the lower white point and the weaker black point according to their
XYZ_Y values.
> 
> So in terms of purely improving the output, the grey icc is not a
> must. It is only great help in terms of editing your file in PS so
> you get a perfect match between displayed image and printed one with
> from my understanding the limitation of BP (which I understood is not
> the true paper Black but a perfect one).

No.  First you don't HAVE to use it and you can do things the old "Same as
Source" way and get the 1:1 straight line mapping of L*.  I would argue this
is a loss.

If you do choose to use it then you have access to a tool which nicely and
easily (ie automatically) scales all the image pixel values for the output
space's black and white points and any blips in between.  This I think is a
major step forward.  We can profile the luminance of the greyscale produced
by any workflow (QTR, IJC/OPM, BO, Roark or whatever) and use CM to manage
the mapping.

The one pity with the current beta is in the soft proof department.  As I
have said, because Roy is doing the BPC (ie scaling of image values for the
weaker printer black point) for you/Adobe in the ICC profile, when you ask
PS to do BPC in the softproof it looks at the ICC profile and sees that the
printer can print perfect black and hence does not adjust the on screen look
of the image for the weaker black.  This is something that Roy continues to
look at.  My personal view, and remember this stuff is difficult because no
matter how many questions you ask of a colour engineer about this stuff it
is hard to get straight clear answers to the very simplified B&W case - they
are simply not used to thinking in B&W, is that the ICC profile should show
the weaker black point in the kTRC and not have the kTRC scaled to run to
perfect black (ie have embedded BPC).  I might well be wrong though!!  If
this were the case then I think when you do a soft proof and check BPC then
PS will see the imperfect black in the profile and adjust the display
accordingly.  But like I said I could be completely wrong and only a test
will tell.  It's a pity we just can't get a straight simple answer to what
would seem to be a very simple question.

I hope this helps and I am sure Roy will step in if I am off-base.

:-)

Steve

Thanks Steve for a very full explanation.

Peter.

Steve,

Many thanks, it couldn't be clearer.

Olivier