QTR-Quadtone RIP

--- In QuadtoneRIP@yahoogroups.com, "Shilesh Jani" <shileshjani@b...> wrote:
>
> I want to rationally create a PhotoShop curve to linearize output. Is 
> the linearization scheme used in QTR public domain? If so, can someone 
> send it to me. My measurements will be in density D
> 
> Thank you.
> 
> Shilesh
>

Hi Shilesh,

Conceptually, linearization is pretty simple.  Some of the details can get involved.

Basically you have a stepwedge such as K = 0,5,10,..95,100  and all the measured
densities.  Think of plotting this with (x,y) points where  x = K value and y = 
measured value converted into the K value of where that density should occur.
E.G.  if K=50 measures d=.75  and .75 should occur at K=65 you have the
point (x,y) = (50,65).   So when you plot the points out you see a graph of the
actual output versus the input.  To make a correction curve just swap x and y
for all the points.  Now for an input of 65 it gets corrected to 50 which will output
the density .75 which is what you want for an input of 65.

Pretty simple so far, but the big question is deciding where the d=.75 should
occur.  "Linearization" implies that there is a straight line somewhere so you can
interpolate and find any of the point in between.   However the units you use make
a big difference.  QTR uses L values as the units for linearization. Density values
would not produce a straight line.  In the QTR download there's a file called
ideal_density.pdf that shows typical values for various dmax's.   QTR justs converts
densities to L-value and then draws a straight line from L-min to L-max.
   formulas:  
         Y = 10 ^ (-d)
         L = (Y^(1/3) * 116) - 16   for Y >= 8/903.3
         L = 903.3 * Y                    for Y <   8/903.3

Linear L-values aren't the only choice but that's what QTR uses.

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

I'm not sure what you are doing, but another thing you might consider is using
the QTR-Create-ICC capability.   This does all the work of creating the correction
curve and putting it into an ICC profile.  Then it can be used with the color
management system to not just linearize to straight-line L-values but actually
linearize to the builtin grayspace of the source file and do the gamut mapping
of black point compensation in perceptual intent.   This is somewhat more than
your original desire but maybe you are headed that direction anyway.

Roy