paul@ppgbms (Paul Evan Matz) (06/13/89)
We are in the process of trying to print a grey-scaled image on
an HP-laserjet, which has an output characteristic of 300 dpi, monochrome.
The image file contains 8 bit/pixel grey scale values. Although we can
get a printable image using NeWS to do the half-toning, it appears that
a half-toning algorithm used for a CRT display device isn't really cut out
for producing an image tailored for a 300 dpi output device. Anyone out
there know of a public domain filter that may do this? A posting or direct
email response would be much appreciated.
(I did see the recent posting by Ed Falk, containing the two
dithering filters "odither.c" and "fsdither.c". However, the real question
is, has anyone studied the characteristics of the HP-laserjet and tweeked
a half-toning algorithm accordingly?)
Thanks (wishful thinking)
Regards,
Paul Matz
PPG Biomedical Systems
One Campus Drive
Pleasantville, NY. 10570
914-741-4685
path ppgbms!moe!paul@philabs.philips.com
PS: Sorry if this gets posted twice. Something is not quite right with
my postnews.paul@hpldola.HP.COM (Paul Bame) (06/24/89)
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
cb@sequoia.UUCP (Christopher D. Brown) (06/29/89)
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'd like to add that, in a similar situation, I convinced myself of the following: 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. Nice story ... Chris Brown
tuna@athena.mit.edu (Kirk 'UhOh' Johnson) (06/29/89)
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
cliff@ficc.uu.net (cliff click) (06/30/89)
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).