QTR-Quadtone RIP

Hi Leping,

If you want to get into understanding the internals of ICC profiles 
there are a couple of
terms and definitions that are worth getting used to.

Y = Luminance,  the gray part of XYZ,  usually 0...1
    this is a basic physical property, the fraction of photons reflected 
from a surface
L = Luminosity or L*, part of LAB,  usually 0...100
    this is linear with human perception.

some standard formulas:
density = -log10(Y)
Y =  ((L+16)/116)^3   for L >= 8
Y =  L/903.3  for L<8

A good website for more of the math and some explanations is:
    http://www.brucelindbloom.com/

The ICC profiles can be based on either XYZ or Lab.  You can see this 
in the header.
-- the generic qtr icc's are XYZ,  but the custom ones from 
QTR-Create-ICC are Lab.

When you have a gamma based profile, its simply:
    datavalue = Y^gamma    or  Y=datavalue^(1/gamma)
where datavalues are logically 0...1  (in 8 bit that would be stored 
0...255)

------------------------------------

When people talk about "linear" they imply linear in Y or L but don't 
typically say which.
QTR curve linearization is linear in L values.  In the ICC files CM is 
does
interpolation with linear L values, but expansion/contraction of dMin & 
dMax is
done with linear Y values.  Why all the complication?  Mostly because 
that's
what works the best.

If you are comfortable with lots of math conversions this is not as bad 
as it may sound
but most people would rather just see it all as a magic black box.

Roy

On Friday, May 4, 2007, at 12:26  PM, Leping Zha wrote:

> Hi Roy,
>
> Thank you so much for responding, and confirming that:
>
> 1. The linearization targets should be printed "raw" without
>    any color management.
>
> 2. The linearization results (after inserting the linearize
>    line to the source text file of the curves, remaking and
>    installing the curves, and restart Photoshop) shoule be
>    also measured "raw" without color management, for the
>    objective of Lab linearity.
>
> Since my densitometer is borrowed, I have been concentrating
> in understanding the linearization process.  As you pointed
> out, what really matters is how the real prints look, or match
> the display on screen on different types of paper, the area
> I have not explored yet.  It is then more in the area of
> artistic preference, or personal taste, wheather to use the
> QTR Matte/Photo Paper curves for printing, which is no more
> in the domain of analytical discussion.  Some of my friends
> have told me they have been happy printing with QTR curves
> without the profile conversion, but some may perfer to do so,
> I guess.
>
> The only question remaining right now to me is that, with the
> only tool I have to exam the ICC profiles, the OS X ColorSync,
> your QTR Gray curves simply contain a gray transfer curve that
> do not look linear.  They are probably not pure explential,
> but I measured the interceptions and found it is close to
> a Gamma 2.5 curve from calculations.  I put the curve side
> by side to the Gamma 2.2 and 1.8 curves trying to understand
> the differences, and certainly they look much closer to the
> 2.2 curve, with the exception of the small interception at
> near the origin in the Matte Paper profile.  Then I measured
> the difference of the QTR curve relative to Gamma 2.2, and
> predicted it would crush in some extremely low shadows to
> pure black (from the interception), but expand the other
> shadow values, and compress the mid-highlight a little bit so
> the prints would be a bit brighter in midtones.  Next, I
> watched the spiked histogram line shifts with a gray step
> wedge image before and after converting (from the untagged
> file under my working space that is Gamma 2.2, to the QTR
> Matte Paper profile), that confirmed these predictions.
>
> If a profile is identical to Gray Gamma 2.2, for instance,
> the conversion from the Gray Gamma 2.2 to it does nothing.
> If your profile is a "linear" curve, a conversion to it from
> the Gamma 2.2 work space changes the numbers a lot before they
> are fed to the QTR engine.  Since converting to the QTR
> profile in your specified QTR printing workflow only changes
> the print's looking little bit, as you described, can we say
> that the profiles are closer to the Gamma 2.2 ones than being
> "linear", even they are linear to the Lab values?
>
> Many thanks again,
> Leping
>
> --- In QuadtoneRIP@yahoogroups.com, Roy Harrington <roy@...> wrote:
>>
>> Hi Leping,
>>
>> You've got a lot of inter-related issues going on and I think
>> they tend to confuse the situation.
>>
>> In QTR there are two similar but different features.   First,
>> there's the Linearization in the QTR profiles and second,
>> there's the ICC profiling with either generic QTR Gray
>> Matte/Photo Paper profiles or custom Create-ICC profiles.
>
>> QTR profiles don't use a gamma function which is a pure
>> exponential.  QTR linearization uses the Lab L* function.
>> The idea being to make a straight-line in L measurements.
>> The line always goes from dMin to dMax and the goal is a
>> simple straight-line.  xGamma functions are a different shape.
>
>> This is specifically for printing targets that will be used for
>> linearization or ICC profiles.  You need to get "raw" density
>> measurements so that a curve can be fitted to correct/linearize
>> the output.
>
>> It sounds like what you are doing is measuring a stepwedge for
>> Lab linearity.  Since Lab linearity is what the basic QTR profile
>> goal is then naturally you'll get the best linearity with just the
>> basic QTR profile and No Color Management conversions.
>>
>> Color Management and ICC profiles however have a different
>> purpose and goal.  It's much harder to measure the precise result
>> because the goal is matching of our perception not a specific
>> linearity.
>>
>> When you print on matte paper there's a mapping of dMin's and
>> dMax's from the file to the paper.  In general you lose more at
>> the shadow/dMax end.  So with straight QTR profiles this will
>> usually mean a lighter print because the straight-line pushes
>> the whole range lighter.  The idea of Color Management is to
>> do a mapping that better (but not perfectly) matches our
>> perception.  This sacrifices some of the shadow separation
>> in favor of midrange density.
>>
>> In addition to this issue you also have to consider the source
>> embedded profile (or working space for untagged files).  Say
>> you have a stepwedge and look at K=95, it's really the
>> corresponding L value that determines how dark it is.  So for
>> Gamma 2.2 there is very little separation between K=100 and
>> K=95.  Look at the eye-dropper in Photoshop, the difference
>> is only L=0 to L=1 -- much less than the separation between
>> any other steps.
>>
>> The bottom line with all color management issues is how good
>> the print and the screen match with our perception not
>> measurements with a densitometer.
>>
>> Roy
>
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-
Roy Harrington
roy@...
Black & White Photo Gallery
http://www.harrington.com