QTR-Quadtone RIP

--- In QuadtoneRIP@yahoogroups.com, "Tom Husband" <thusband@s...> wrote:
> I'm struggling with the math required to come up with the percentages
> of black ink relative to light black.  On page 14 of the User Guide it
> says to find the black patch that matches the 100% light black patch.
>   Say it's 40%.  I then measure the 45% black patch and determine that
> percentage needed to produce the same density of the 100% light black
> and so on.  It says it's bit of simple math to figure the percentage
> but I can't get my simple mind around it.  What is that formula?
> 
> Thanks,
> 
> Tom

The idea is called interpolation.  In the example:
we are looking for is what would match 1.29 (the 100% light-black)
so in the black ramp we have: patch 40 is 1.22   and patch 45 is 1.33
imagine that we had intermediate patches 41, 42, 43, 44 -- which is
most likely to match the 1.29?  

mathematically what you doing is:  5 levels (i.e 45 - 40) is a difference 
of 1.33 - 1.22 = 0.11 but we only want 1.29 - 1.22 = 0.07
so  (0.07 / 0.11) * 5 = 3.2 levels i.e. 40 + 3.2 = 43.2

Or:
   wanted-diff-levels = total-diff-levels * (wanted-diff-density / total-diff-density)
         3.2   =      5    *    (   0.07  /  0.11  )

(notice that with the L values there are some negative values).

Do a simple sanity check -- obviously we want a number between 40 and 45.
the 1.29 we want is a little closer to 45 than 40.  Just guess 43.  Since you will
linearize later this is probably close enough.

Roy