Half-tone images and photo-copying...
Thomas E. Jones
never figured out.
Greyscale on low-end 300 bpi devices have a large "grain" size that
makes them photocopy *GREAT* on any lousy photocopy machine. On the
nice typsetting devices this grain gets very small, so the resulting
original looks much more greyscale, and less half-tone, but you can't
photocopy this well. Is there any way to manipulate this? I know
a Apple Laserwritter can't be made to have a smaller grain size, but
can the typsetting machines be made to have larger grains on half-tones?
A smoother greyscale could probably be made by dithering black and white
images on a Laserwriter to get a smoother looking original (not so grainy)
that of course, won't photocopy well.
Going the other way, I suppose we could make larger grain sizes on a
1250 dpi typsetter by purposely making black/white images with large
grains, but this would be painfull and awkward.
Has anyone tried either of these two methods? Does anyone out there try
to do grey-scale themselves by dithering and writing black/white images
to get smaller grain size? Does anyone do the opposite to get larger
grain sizes on a typesetter? Do the expensive typsetters have any
options to make larger grain sizes so the letters of the original are
sharp, but the photos can be photo-copied easilly? Please enlighten
me to this area I've never understood.
- tj.
--
t...@xn.ll.mit.edu or t...@ll-xn.arpa (one of these should work)
Thomas E. Jones, home (617) 924-8326 work (617) 981-5093
William Roberts
>Has anyone tried either of these two methods? Does anyone out there try
>to do grey-scale themselves by dithering and writing black/white images
>to get smaller grain size? Does anyone do the opposite to get larger
>grain sizes on a typesetter? Do the expensive typsetters have any
>options to make larger grain sizes so the letters of the original are
>sharp, but the photos can be photo-copied easilly? Please enlighten
>me to this area I've never understood.
WHat you refer to as a "grain" is more properly called the
halftone spot size (at least, that's what the PostSCript
manuals call it). The reasoning is as follows:
1) The printer pixels can only be either black or white - no greys
2) The human eye is responsive to average intensity over quite
large areas (which is why those line printer posters work)
So
3) If we group pixels together into "spots" we can turn on
parts of the spot and get a range of average intenisties over
the area of the spot; the eye will see this as a shade of grey.
The parameters of this kind of halftoning are the different
spot shapes (corresponding to different intensities), the size
of the spot (usually arranged on a rectangular grid, but could
be hexagonal grid etc) and an angle. The angle is to do with
reducing interference effects between the printer pixel grid
and the "spot grid"; think of it as the angle by which the spot
grid is sheared along the vertical axis.
The resulting sheared grid of spots is called a "halftone screen".
PostScript gives you all of this machinery (handy eh?) in the
form of
a screen frequency (in spots per inch)
a screen angle
a spot function
The spot function is a way of specifying all of the spot shapes
at once, and fulfilling a constraint that darker spots should
just have more pixels set to black (not different pixels set to
black).
The PostScript operator for this is setscreen, and a very good
explanation is given in
Real World PostScript
Steven Roth, ed.
Addison Wesley, 1988, ISBN 0-201-06663-7
PS. This book is also good for people who need to know a lot
about PostScript and who would otherwise be pestering the Net
with stupid "tell me everything" requests [[ You know who you are ]]
--
William Roberts ARPA: li...@cs.qmw.ac.uk
Queen Mary & Westfield College UUCP: li...@qmw-cs.UUCP
Mile End Road AppleLink: UK0087
LONDON, E1 4NS, UK Tel: 071-975 5250 (Fax: 081-980 6533)
Liam R. E. Quin
>Greyscale on low-end 300 bpi devices have a large "grain" size that
>makes them photocopy *GREAT* on any lousy photocopy machine. On the
>nice typsetting devices this grain gets very small, so the resulting
>original looks much more greyscale, and less half-tone, but you can't
>photocopy this well. Is there any way to manipulate this? I know
>a Apple Laserwritter can't be made to have a smaller grain size, but
>can the typsetting machines be made to have larger grains on half-tones?
You can set the transfer function, of course. A better way is to get
one of the contrast-enhancing sheets sold by Letraset (for example). I
think it's called Letracopy:PT or something. This is a transparent sheet
of film with lots of little white dots over it, and it can improve the
quality of photocopied images dramatically, especially if grays or colours
are involved.
This won't satisfy your curiosity about gray-scale stuff, but it might
help with photocopying!
Lee
--
Liam R. E. Quin, l...@sq.com, {utai,utzoo}!sq!lee, SoftQuad Inc., Toronto
``When I talk to the people who are the trees that grow in the city...
Rob Bradlee
In article <22...@sequent.cs.qmw.ac.uk> li...@cs.qmw.ac.uk (William Roberts) writes:
>>to do grey-scale themselves by dithering and writing black/white images
>>to get smaller grain size? Does anyone do the opposite to get larger
>
>halftone spot size (at least, that's what the PostSCript
I find it helps somewhat (well, at least a tiny bit) to call these halftone
DOTS which are made up of device SPOTS. So our film recorders can output
at 1200 spots per inch which can make nice dots of 65 to 150 lines per
inch.
>be hexagonal grid etc) and an angle. The angle is to do with
>reducing interference effects between the printer pixel grid
>and the "spot grid"; think of it as the angle by which the spot
and interference between different layers of ink when printing a color
separated page.
>The spot function is a way of specifying all of the spot shapes
>at once, and fulfilling a constraint that darker spots should
>just have more pixels set to black (not different pixels set to
>black).
The spot function determines the order that spots are darkened when going
from smaller to larger dot.
--
Rob Bradlee w:(508)-658-5600 X5153 h:(617)-944-5595
AGFA Compugraphic Division. ...!{decvax,samsung}!cg-atla!bradlee
200 Ballardvale St. bra...@cg-atla.agfa.com
Wilmington, Mass. 01887 The Nordic Way: Ski till it hurts!
J. Deters
>Greyscale on low-end 300 bpi devices have a large "grain" size that
>makes them photocopy *GREAT* on any lousy photocopy machine. On the
>nice typsetting devices this grain gets very small, so the resulting
>original looks much more greyscale, and less half-tone, but you can't
>photocopy this well. Is there any way to manipulate this? I know
>a Apple Laserwritter can't be made to have a smaller grain size, but
>can the typsetting machines be made to have larger grains on half-tones?
>Going the other way, I suppose we could make larger grain sizes on a
>1250 dpi typsetter by purposely making black/white images with large
>
>Has anyone tried either of these two methods?
>Thomas E. Jones, home (617) 924-8326 work (617) 981-5093
The "grain" you notice that copies well vs. the grain size that doesn't
is used extensively in the engraving business. Examine your next payroll
check, or any 'business' check or gift certificate you receive. Most
of the modern checks have "check protect" features designed to enhance
this effect on a copying machine. The payroll and accounts payable
checks where I used to work came printed on stock that had the word
VOID written across the face in a different halftone pattern. To the
naked eye, the face of the check was simply patterned with horizontal
lines. Closer examination showed that most of the lines were pastel
solid yellow, whereas the word VOID was superimposed over the face in
a larger grain size. Our current gift certificates here have more than
three (!) such devices embedded in the face. One of the cute tricks they
use is to print "DH" using narrow vertical lines over a circle of
horizontal vertical lines of the same width. Either copiers can't
discern the two dimensions equally, and therefore highlight the word, or
they can't resolve the fine lines and only print a black circle.
I've never really had the time to play with this in PostScript yet, so
I'm afraid I can't post an example of such. Knock yourself out.
-j
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