Re: Hi-Fi inksets
Re: Hi-Fi inksets
- Subject: Re: Hi-Fi inksets
- From: Robert L Krawitz <email@hidden>
- Date: Sun, 8 Feb 2004 21:55:53 -0500
Date: Mon, 09 Feb 2004 13:31:17 +1100
From: Graeme Gill <email@hidden>
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 <email@hidden>
Tall Clubs International --
http://www.tall.org/ or 1-888-IM-TALL-2
Member of the League for Programming Freedom -- mail email@hidden
Project lead for Gimp Print --
http://gimp-print.sourceforge.net
"Linux doesn't dictate how I work, I dictate how Linux works."
--Eric Crampton
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