[Openicc] Drop size calibration

[Gerhard Fuernkranz]

Graeme Gill wrote:
> Robert Krawitz wrote:
>   
>> I'm experimenting with another approach to drop size calibration.
>> This uses the new segmented dither algorithm to print stripes using
>> different drop sizes.
>>     
>
> I guess I'm a bit puzzled as to why you need such a calibration.
> By definition a larger drop is going to result in higher density
> than a smaller drop. So as long as the dither/screen progresses
> through the dots in order, the result should be monotonic, even
> if it's not very linear. As long as the input precision to
> the screen/dither is high enough, and the resulting raw screen
> transfer curve is smooth enough, the calibration will linearize the
> result.

The calibration algorithm just must be prepared of course, that the
theoretical raw transfer curve of a typical multi-level screen which
dithers between two adjacent drop sizes, may have discontinuities of the
1st derivatives at the boundaries between the drop sizes, i.e. the
theoretical raw transfer curve is segmented, where each segment is
continuous. But I think linearizing such curves should still be no
problem, if the channel response is measured with sufficiently small
steps (if necessary iteratively with two linearization passes). In
practice one likely does not even notice these discontinuities as sharp
corners in the measured raw response curve, due to noise, so in practice
I'd assume that the raw curve is smoother than theoretically anyway.

So I'm not so sure, whether it is really necessary to tune the dither
algorithm just on order to give a smoother raw response (if this tunig
does not improve other issues too), but it is likely more important to
keep the algorithm's behaviour monotonic as long it is granted that drop
size 1 < drop size 2 < drop size 3 ... The dither algorithm should be
designed in a way, so that even wrong assumption about the absolute
sizes of the drops should never result in a non-monotonicity.

Regards,
Gerhard

> Of course to avoid banding artefacts it is important to
> crossover from one dot side to the other (ie. there needs to be
> overlap of the different dot sizes), so that the pattern never
> "fills up" at intermediate dot sizes.
>
> Graeme Gill.
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