Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!sun-barr!decwrl!hplabs!hpfcdc!hpldola!paul
From: paul@hpldola.HP.COM (Paul Bame)
Newsgroups: comp.graphics
Subject: Re: Need Half-Toning Algorithm suited for HP-Laserjet
Message-ID: <11390020@hpldola.HP.COM>
Date: 23 Jun 89 17:15:32 GMT
References: <8022@ppgbms.UUCP>
Organization: HP Elec. Design Div. -ColoSpgs
Lines: 30

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
	   bad at <= 150dpi.  Even though the ratio of pixels is correct to
	   simulate a certain grey level, the perceived level is different
	   from the predicted one in my tests.  A possible solution I did
	   not try is to backlight the printout.  There are definite
	   perception differences between images made of emitted light
	   (CRTs) and those of reflected light (printers).  As a fun test of
	   this, try bringing up two terminal windows on a single
	   (moderately-high resolutio n) display using the same font.  Make
	   one Black on white and the other White on black.  They often
	   appear to be using slightly different fonts (e.g., the black font
	   on white appears "skinnier").  If you can get your printer to do
	   this same test (a window dump at a high dpi might work) you
	   should see the reverse happening.


	2) All laserjets are not equal.  There are adjustments which vary
	   contrast - especially in the LJ2000.



		-Paul Bame