QTR-Quadtone RIP

Ken,

Using the Photoshop eye drop sampler has a lot more to it than one
would think.  In general if you "look"
at values that are the same format as the file you see the actual
values in the file (or a simple calculation).
In other works for an RGB file if you look at R G B values you do see
what's in the file.  For a grayscale
file, K values are a simple calculation.  (internally 8-bit files are
0=black to 255=white, 16-bit files are
0=black to 32768=white).  K = (255-value)*100/255.   This is pretty
simple and probably what you already
expected.   But whenever you look at values other than the same format
the calculation go though
color management.  This means that the embedded profile (if one) and
the settings under Color Settings...
are all involved in what the eyedropper reports.  For instance if you
look at R G B with a grayscale file
you see the icc conversion of the internal gray value from the
embedded profile (or working gray if its untagged)
to the working RGB profile.  It may appear that you a looking at the
internal 8-bit values but you are not.

I'm not very familiar with using HSB but like LAB you are seeing the
conversion from the grayscale profile
to those values.   So the reason you are seeing those B values is
because that's the "definition" of the
profile that is being used.  BTW, I thought I try this in PS and the B
values you mention actually
correspond to Dot Gain 20% not Gray Gamma 2.2 -- check out your file
and Color Settings...

That's the long answer -- the short answer is: for step wedge stuff
don't look at HSB or LAB or anything
but K values.   Stepwedge files are meant to be artificial -- exact
values that you want to send to the
driver, you don't want any color management involved.   The posterize
command that you probably
used is also completely profile ignorant so that's just fine for wedges.

--

This seems like a good place to mention -- the K vs R G B eyedropper
stuff is worth understanding
but its often annoying nevertheless.  So one thing that makes this
less of a problem is to pick
grayscale and RGB working profiles that are "compatible" i.e. have the
same gamma structure.
This makes the conversions "look" like you expected.

The most common pairs are:
  Gray Gamma 2.2 and AdobeRGB
  Generic Gray Gamma 2.2 and sRGB
  Gray Gamma 1.8 and ProPhotoRGB
(note that Gray Gamma 2.2 and Generic Gray Gamma 2.2 are distinctly different)

Roy


On Tue, Jan 17, 2012 at 9:28 AM, kenleegallery <kenlee333@...> wrote:
> Let's say we make a new image in 16-bit grayscale with a Gamma 2.2 profile, and create a step-wedge with 11 steps. We get an image whose K values go 0, 10, 20... 100. Fine.
>
> However, if we mouse over the other patches, we see some discrepancies between the K value and the HSB B value. For example, the middle patch shows a K value of 50%, but the B value is 58%. Similarly, where K = 10%, B = 92%.
>
> Why aren't these two measures simply the compliment of one another ?
>
> Which system is appropriate for making a step wedge, when calibrating with QTR ?
>
>
>
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-- 
Roy Harrington
roy@...
www.harrington.com