QTR-Quadtone RIP

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