Re: [Gimp-print-devel] Black generation, continued

[Thomas T. <tt...@bi...>]

Robert L Krawitz wrote:

> I'm not too concerned about having more matrices; it does mean more
> space will be consumed (although we could use unsigned shorts to store
> the matrices), but if the quality improves enough, it's well worth
> it.

I commited a second matrix under print/Matgen. I am now generating a
257x257 matrix, but that will take a while. I'll commit that too,
including the new quickmatrix source including all used parameters.

I tried making a 73x73 matrix but that is just too self-correlated.
Produces interference patters if overlaid with itself.

> If you want to put it in, please do so.  If it elicits too many
> screams, somebody will just back it out.  That's the nice thing about
> source control.

OK. I cleaned it up a bit and added an ink reduction mechanism. It
basically does what it did before, with some modifications in scaling.
These modifications make it work well if the black limits are set lower.
I reflected this in print-escp2.c. Instead of 0.4 and 0.999 there are
now values of 0.25 and 0.6. This may be a bit much for six color
printers.

Added is functionality that can produce more black than before if there
is more than 100% ink coverage. This prevents the paper from being
soaked.

I can imagine adding something that cals will reduce some ink in full
colors by replacing some with black: good for printing on real plain
paper.

> Make sure it works well with colors.tif
> (http://www.tiac.net/users/rlk/colors.tif).  That's a pretty good test
> for finding discontinuous behavior.

That works well.

> As for black level, it's very important that it start at zero and
> smoothly increase for the variable dot size printers, to allow a range
> that uses small black dots.

I wouldn't dare doing otherwise: discontinuities in curves will produce
discontinutities in output. No way one can compensate for that.

I hope the black values I set in print-escp2.c are good for 6 color
printers as well, but my feeling is that this section needs to be
expanded:

- 6 color printers get a higher threshold for beginning to use black
- variable dot size printers get a lower threshold for beginning to use
black
- cheap paper maxes out on black early
- coated paper takes more ink so the curve can be smoother

The last 2 are reflected in the current print-escp2.c.

I have confidence now that the bblack separation is useable with a wide
variation of thresholds. I tried 0.1 and 0.2 as thresholds. Grainy
result, but generally okay. 0.25 and 0.6 seem to be fine for my printer:
green midtones are mostly hidden by the black, and there is no
noticeable extra grain.



Thomas