Path: utzoo!attcan!uunet!ficc!cliff
From: cliff@ficc.uu.net (cliff click)
Newsgroups: comp.graphics
Subject: Re: Need Half-Toning Algorithm suited for HP-Laserjet
Summary: 1200dpi dither
Message-ID: <4830@ficc.uu.net>
Date: 29 Jun 89 18:17:31 GMT
References: <8022@ppgbms.UUCP> <11390020@hpldola.HP.COM> <578@sequoia.UUCP>
Organization: Ferranti International Controls
Lines: 30

In article <578@sequoia.UUCP>, cb@sequoia.UUCP (Christopher D. Brown) writes:
>         3) Laser printing uses a charge to attract toner.  When a
>            pattern like "X X X X " (i.e. 50% grey) is printed at
>            sufficiently high resolution, the area between the pixels
>            behaves as though it is charged.  If the pattern "XX    XX"
>            draws 4 units of toner, than the pattern "X X X X " draws
>            5 or 6 units of toner.  I produced nearly linear results
>            using dither patterns which avoid alternating pixels.  E.G.
>            6 by 6 spiral.

I worked with a company producing a 1200dpi laser printer.  At those
resolutions most dither patterns fail due to:

1) Beating with 60Hz jitter inherent with the driver motor,

2) This pixel/pattern charge problem related above,

3) The non-linearity of how the eye perceives greyshade,

4) The extreme sensitivity of the eye for picking out repeating patterns.

Producing a "nice" set of dithers used some parts of trig & set theory, and
some parts of black magic.  They (the company producing the laser printer)
consider their dithers as very propriatery (sic).  Simple greyshade dithers
are not very simple.

-- 
Cliff Click, Software Contractor at Large
Business: uunet.uu.net!ficc!cliff, cliff@ficc.uu.net, +1 713 274 5368 (w).
Disclaimer: lost in the vortices of nilspace...       +1 713 568 3460 (h).