QTR-Quadtone RIP

First I apologize for having posted the same on the BWprinting group 
too : Steve has been quite helpful on this group, and I have not seen 
him intervening on QTR group. So please do not be mad at me for those 
double posting...

Quote :

I have been trying hard to understand the QTR ICC tool. I thought I
got it but... no!!!

When the stepwedge is printed linearised and profile converted I got
values far lighter than when non profil-converted :
(L* only)
DEN Linear Linear+converted
0 96,45 96,14
5 92,9 93,59
10 89,24 90,35
15 85,27 86,92
20 81,34 83,99
25 77,4 80,57
30 73,61 77,03
35 70,04 73,91
40 66,63 70,39
45 62,3 66,99
50 58,03 63,17
55 53,54 58,76
60 50,04 55,38
65 45,97 51,15
70 41,94 46,6
75 37,92 42,89
80 34,08 38,01
85 30,44 31,62
90 25,85 26,12
95 21,73 21,11
100 17,7 17,89

Basically it looks like it applies a higher gamma...

I just took the QTR stepwedge, opened it with no color managed
conversion (CS then assumes it will have the default grey profile
e.g. 15% dot in my color settings), converted to QTR ICC and print it
in QTR.

Anyway, when visualizing the kTRC tag of the QTR profile, the curve
does look like it maps destination values lighter than the source
ones in a nice below the 45° line curve.

Last to make it easier : kTRC seems (from ICC spec.) to be build from
XYZ. I don't have PM, so I just past Lab values in a PM measurement
file I got. PM XYZ look pretty strange with double digit figures
while XYZ range from 0 to 1.99... ?

So 2 questions :

1. Do I need to also input XYZ in PM file to feed QTR create ICC and
if so how do I convert "real" XYZ to the PM XYZ format ?
2. Is that normal QTR ICC lightens a file ?

Any help would be more than welcome...

Unquote

Just to add also to it, Steve mentionned BPC is included in the 
profile, not allowing perfect soft-proofing : I have not experienced 
this was limiting the soft proofing... And may have missed something.

Olivier

Hi Olivier,

See comments below.

--- In QuadtoneRIP@yahoogroups.com, "odesmais" <odesmais@y...> wrote:
> First I apologize for having posted the same on the BWprinting group 
> too : Steve has been quite helpful on this group, and I have not seen 
> him intervening on QTR group. So please do not be mad at me for those 
> double posting...
> 
> Quote :
> 
> I have been trying hard to understand the QTR ICC tool. I thought I
> got it but... no!!!
> 
> When the stepwedge is printed linearised and profile converted I got
> values far lighter than when non profil-converted :
> (L* only)
> DEN Linear Linear+converted
> 0 96,45 96,14
> 5 92,9 93,59
> 10 89,24 90,35
> 15 85,27 86,92
> 20 81,34 83,99
> 25 77,4 80,57
> 30 73,61 77,03
> 35 70,04 73,91
> 40 66,63 70,39
> 45 62,3 66,99
> 50 58,03 63,17
> 55 53,54 58,76
> 60 50,04 55,38
> 65 45,97 51,15
> 70 41,94 46,6
> 75 37,92 42,89
> 80 34,08 38,01
> 85 30,44 31,62
> 90 25,85 26,12
> 95 21,73 21,11
> 100 17,7 17,89
> 
> Basically it looks like it applies a higher gamma...
> 
> I just took the QTR stepwedge, opened it with no color managed
> conversion (CS then assumes it will have the default grey profile
> e.g. 15% dot in my color settings), converted to QTR ICC and print it
> in QTR.

This is where the "lighter" output is being done.  Dotgain 15% is a fairly
light profile so when you convert you are changing all the internal values
to much "lighter" numbers.  This is what Convert to Profile does -- it matches
the gray values visually.  Check out the K values after your conversion you'll
see they are quite a bit lower.

The big difference with using the QTR ICC is that the gray profile of the 
source data file is used by the CMS (color management).  If you just print
with "Same As Source" the numbers are passed through without any notion
of their "meaning" i.e profile.  So K=50 just results in whatever QTR prints
for that value.  (Actually it prints (Lmax+Lmin)/2).  With the QTR ICC used
the K=50 is converted from source profile to output profile using a Perceptual
Intent method.  

> 
> Anyway, when visualizing the kTRC tag of the QTR profile, the curve
> does look like it maps destination values lighter than the source
> ones in a nice below the 45° line curve.
> 
> Last to make it easier : kTRC seems (from ICC spec.) to be build from

For grayscale, kTRC can use either Y values or L values.  I've actually done
it both ways but the current code uses and shows the L values which are
a bit easier to tell what's happening.

> XYZ. I don't have PM, so I just past Lab values in a PM measurement
> file I got. PM XYZ look pretty strange with double digit figures
> while XYZ range from 0 to 1.99... ? 

The Y values from PM are scaled 0-100 i.e. percent rather than 0-1.

> 
> So 2 questions :
> 
> 1. Do I need to also input XYZ in PM file to feed QTR create ICC and
> if so how do I convert "real" XYZ to the PM XYZ format ?

No, the L values are what are used.

> 2. Is that normal QTR ICC lightens a file ?

As above: its you dotgain 15% that is lighter.

> 
> Any help would be more than welcome...
> 
> Unquote
> 
> Just to add also to it, Steve mentionned BPC is included in the 
> profile, not allowing perfect soft-proofing : I have not experienced 
> this was limiting the soft proofing... And may have missed something.

The ICC spec doesn't address BPC at all.  Adobe added the feature to
their CMS.  Softproofing for grayscale pretty much doesn't do anything
because there isn't a reduced gamut.  However it turns out that if the
kTRC curve is based on the blackpoint rather than (0,0) you can turn
off BPC in photoshop.  Then you can see a reduced dmax in the softproof if you
turn on Simulate Ink Black.   In a sense I think this is really outside the 
ICC spec so if you use something other than PS it may not work.  I'm
planning on trying to make this an option.

Roy


> 
> Olivier

Roy,

Thanks for this explanation : I never bothered to think about the 
gray color setting in PS which is a mistake.

I'll take an other chance to properly set my workflow.

Apparently "grey : dot 15%" is equivalent to a 1,51 gamma. The 
stepwedge is no tagged, but at creation time it had to have a working 
space, am I correct assuming it had a grey 1.8 gamma ? This is just 
for informative purpose : I would just like to measure the actual 
output of the stepwedge once QTR-ICC-profiled by applying a proper 
working space for conversion (I guess it is useless for actual 
editing-printing).

Working under Windows and in RVB color space for BW transformation 
(though ultimately PS grey mode is used for space saving sake) when 
editing and both being gamma 2.2, I plan to now set grey at gamma 
2.2 : minimum conversion should now take place. Does this make sense ?

Last, I haven't followed you at all on BPC : 
1. before converting to QTR-ICC profile and when soft-profing I 
go "personalised" and check perceptual/BPC/white paper simul. which 
also turns on black. What you mean is that black is not simulated ? 
2. the correct procedure would be (also before converting to QTR ICC 
profile) soft-proofing->personalised-> QTR-ICC name and check 
perceptual then no BPC and white paper (+ de facto Black) ?
Do you suggest any alternative soft-proofing procedure ?

Thanks for your help.

Olivier