[Openicc] Drop size calibration

[Graeme Gill]

Robert Krawitz wrote:

> I think you're right here.  We'll need to figure out error
> diffusion/EvenTone type algorithms that use three drop sizes also.

My experience was that it was more difficult to get dot size
overlap with error diffusion. I managed it, but it wasn't as
smooth and controllable as a stochastic threshold array.
At the end of the day there was little to choose between these
two approaches, with the stochastic approach being faster and
slightly smoother, and the error diffusion being better for
dot proofing where you are trying to reproduce microscopic
detail.

>    Sufficient overlap should smooth some of the worst of this out,
>    since it's the near 100% fill that has the highest dot gain, and
>    calibration should be capable of taking care of the rest.
> 
> Yup, 50% overlap seems to work well.

Maybe this depends on how randomized the screen is. I don't
recall using such a great overlap, and still got a perfectly
acceptable raw transfer curve to use with calibration.

Graeme Gill.