QTR-Quadtone RIP

Roy Harrington wrote:

> Speed of algorithms is very important. The shear amount of data that has
> to get processed is quite astounding:  consider an R800 printing an 8x10
> at 2880x1440 dpi.    That's 8x10 x 2880x1440 locations on the page.
> That's 330 million locations, x 8 inks, 16bit = 2byte internals.
> Total of 5 gigabytes processed for an 8x10!
> What about the guy doing a banner on his 9600 -- 40 in x 25 feet?

According to Robert Krawitz the next version of Gimp-print aka 
Gutenberg can handle 32 channels with 16 bit input. So it 
isn't getting less complex. Will there be anyting gained for 
B&W printing if you ever base QTR on the newer versions of 
Gimp-print ?

On EvenTone and Ordered dithering there was something written 
that's interesting, EvenTone usually unloads the droplets 
often in one pass even when more passes are used:

Quote:

  Robert L Krawitz wrote:

    >Graeme Gill wrote:
    >   Good point. So to use this type of screening in the 
real world, it
    >   would be desirable to have the different color planes 
screens
    >   de-correlated once the dot density grow high enough, 
thereby
    >   getting the benefits in the highlights where it 
counts, without
    >   triggering banding and pattern issues.
    >
    >Why would this *trigger* banding issues?  At worst it 
wouldn't help
    >solve them, but I don't see how it would actually trigger any
    >problems.

    Because if you have two screens that are correlated, they 
need to
    be printed in exact alignment (to sub pixel precision) to 
avoid
    moiri type effects between them (assuming the pixel 
density isn't
    sparse). Banding effectively displaces pixels (generally 
sub pixel
    sized displacements) in a pseudo random "banding" fashion, 
so moiri
    effects amplify the banding displacements. If the screens are
    completely uncorrellated, then banding displacements will 
have much
    less visible effect.

Good point.

One other thing that I've had good experiences with (mostly for a
different reason) is to use a very good screen (Raph Levien's 
EvenTone
Screening) with some perturbation.  In this case, the purpose 
of the
perturbation is to break up screening artifacts that create
patterning, but this slight decorrelation may also favorably 
affect
banding.  The EvenTone Screen algorithm suffers from banding 
more than
the Ordered (farthest neighbor matrix) dither algorithm that's our
mainstay, and watching the printer actually print suggests 
that this
would be the case.  It's very common for something printed with
EvenTone to print the vast majority of its drops in one pass, 
even if
4 or 8 passes are being used.  Ordered dithering doesn't 
suffer from
this, and is faster.  The problem with ordered dithering, of 
course,
is the noise that's visible in the midtones.

Using the dither matrix to perturb EvenTone screening 
partially breaks
this up and reduces the banding a bit.

-- Robert Krawitz

End of Quote

                    --
           Ernst Dinkla


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