Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "David Wroblewski" 
<dawroblewski@y...> wrote:
> Hi Costa, that's interesting. Coupla questions.
> 
> First, how do you set up an image to print a single-ink
> tone ramp as you describe? I know this must be basic
> but I've never seen it explained in my half dozen photoshop
> books. QTR even has a file, inkpattern.psd, that prints 7
> different single-ink step wedges, but it uses some CMYK magic
> that I just cannot get my head around. Sorry to be the
> class dunce.

David - 
  Remember that this is using QTR and gimp-print. The final files produced by Roy's
calibration procedure just tell the driver how much ink to put out for each pixel value. 
These are just text files, so, it is easy enough to just replace all values with zero for 
all inks except the one of interest. Easier yet, you should also be able to just do the 
calibration pretending you have just one ink.  

I started doing it this way when I could not get a smooth multi-ink ramp. Then  I 
figured out that the bad transitions were at points where only the lightest ink was 
being put out. To test this idea I turned off the other inks. The result was still bumpy. 
Roy helped me figure out which dot value in the gimp-print driver needed 
adjustment. After I got the right value the ramp became very smooth. Of course this 
is just the part of the ramp where the lightest ink is being put out, but smooth as can 
be. Then I added the other inks, linearized, and got a great curve. I think the value of 
using one ink is that you don't get distracted by ink transitions. 

> Second, doesn't your method assume that all the inks have the
> same transition point(s)? Is that a safe assumption?

  The gimp-print software only lets you specify the dot value once. It is not per ink. 
That's why a single value of 0.45 or 0.46 is being mentioned in this thread. I agree it 
seems like the optimal value could be different for each ink.  It is possible that there 
is a different best value for the different inks and that the overall best single value is 
a compromise. However, I think all that is really needed is a decent value where there 
are no obvious discontinuities. The linearization process can then take over and mix 
the inks to give you a smooth ramp. 

hope that helps.

Costa
> 
> -david