Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!usc!bloom-beacon!athena.mit.edu!tuna
From: tuna@athena.mit.edu (Kirk 'UhOh' Johnson)
Newsgroups: comp.graphics
Subject: Re: Need Half-Toning Algorithm suited for HP-Laserjet
Message-ID: <12317@bloom-beacon.MIT.EDU>
Date: 29 Jun 89 15:46:39 GMT
References: <8022@ppgbms.UUCP> <11390020@hpldola.HP.COM>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: tuna@athena.mit.edu (Kirk 'UhOh' Johnson)
Organization: Massachusetts Institute of Technology
Lines: 54

In article <11390020@hpldola.HP.COM> paul@hpldola.HP.COM (Paul Bame) writes:
>I've played with HP laserjet dithering before (logical, since I work for HP)
>and have been very pleased with the standard dithers usually used on screens
>most of the time.  The time when I was displeased was trying to have the
>images look right at 300 dpi.  A test image, white on the left and changing
>continuously to black on the right "appeared" wrong even though I counted
>some pixels and they were correct (dithered with FS dithering).  I suspect
>either or a combination of:
>
>	1) Some phychovisual quirk which is really bad at 300dpi but not so
>          ..........
>
>	2) All laserjets are not equal.  There are adjustments which vary
>          ..........

i suspect neither of these is the likely cause of the problem. in the
early part of the _Digital Halftoning_ book by Ulichney (i'm not
positive of this name, but the book has been cited here fairly
recently), the author cites the importance of measuring the physical
reconstruction function of whatever display media you are using and
correcting for it when displaying to that media. he shows a rather
striking example laser-printed corrected and uncorrected images; the
corrected image looks _much_ better.

just what _is_ the physical reconstruction function, you ask? in the
case of the laser printer, if we print nothing, we get out white. call
this intensity 1.0. if we print all the dots ("turn all the pixels
on"), we get out black. call this intensity 0.0. by turning on a ratio
N of the dots (pixels), we are attempting to achieve perceived output
of intensity 1-N. unfortunately, beacuse of dot spacing, size, spread,
overlap, paper quality, and any number of other intangibles, even the
most careful setting of pixels at that ratio may not result in the
desired intensity level.

the physical reconstruction function is a function which maps pixel
ratios (N in the above example) to the actual resulting intensity
level (presumably measured by some type of light-gathering equipment)
for the display media in question. given a desired output intensity,
the inverse of the reconstruction function can be used to determine
what pixel ratio should be used to achieve that intensity.

note that different printers, video displays, etc. are likely to have
different physical reconstruction functions.

i've never actually gotten around to tracking down the hardware for
measuring reconstruction functions myself, but the book does claim
(and, for one example, show) pretty dramatic improvement, as i recall.

unfortunately, i've loaned my copy of the book to a friend, so this is
all off the top of my head. anybody wanting more details should feel
free to drop me e-mail, at which point i can try to track down more
precise info.

kirk