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From: laser-lovers@uw-beaver
Newsgroups: fa.laser-lovers
Subject: Re: PostScript printers vs. ImPress printers
Message-ID: <577@uw-beaver>
Date: Fri, 25-Jan-85 17:17:54 EST
Article-I.D.: uw-beave.577
Posted: Fri Jan 25 17:17:54 1985
Date-Received: Sun, 27-Jan-85 07:15:11 EST
Sender: daemon@uw-beaver
Organization: U of Washington Computer Science
Lines: 13

From: Mabry Tyson <Ty...@SRI-AI.ARPA>

Thanks for the good description of the differences between PostScript and
Impress.  I have a question though.  A significant portion of our printing is
bitmap dumps of 1Kx1K screens.  Since you say that the Impress description
is characters, how much bigger would the description of the screen be (assuming
it is fairly dense in turned on bits).

The other question would be in how flexible is the description language.  If
I have a complicated display (not generated by some simple plotting commands),
would the only reasonable way to send it to a PostScript printer be the
equivalent of sending a bitmap?
-------

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uw-beaver!laser-lovers
From: laser-lovers@uw-beaver
Newsgroups: fa.laser-lovers
Subject: Re: PostScript printers vs. ImPress printers
Message-ID: <579@uw-beaver>
Date: Fri, 25-Jan-85 18:11:19 EST
Article-I.D.: uw-beave.579
Posted: Fri Jan 25 18:11:19 1985
Date-Received: Sun, 27-Jan-85 07:21:25 EST
Sender: daemon@uw-beaver
Organization: U of Washington Computer Science
Lines: 18

From: Mike Caplinger <m...@rice.ARPA>

Perhaps I am confused, but Brian seems to be saying that PostScipt uses
a size-independent outline description for a character in a particular
font, and can arbitrarily rotate it before conversion to a bitmap.

I was under the impression that there was no such size-independent
representation.  Isn't that what the whole hoopla over bit-tuning is
all about?  Certainly MetaFont is incapable of producing a uniformly
great font at 300 dpi resolution, 10 point.  We have used both a Xerox
2700 and an Imagen 8/300, both 300 dpi, but the character of one-pixel
line drawing on the two engines is so different that CMR fonts that
look great on the Imagen were awful on the 2700.

Or is the outline description some incredibly sophisticated
representation that takes all into account?

	- Mike

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houxm!vax135!cornell!uw-beaver!laser-lovers
From: laser-lovers@uw-beaver
Newsgroups: fa.laser-lovers
Subject: Re: PostScript printers vs. ImPress printers
Message-ID: <589@uw-beaver>
Date: Sat, 26-Jan-85 01:51:02 EST
Article-I.D.: uw-beave.589
Posted: Sat Jan 26 01:51:02 1985
Date-Received: Mon, 28-Jan-85 07:37:04 EST
Sender: daemon@uw-beaver
Organization: U of Washington Computer Science
Lines: 41

From: Brian Reid <reid@Glacier>

    Perhaps I am confused, but Brian seems to be saying that
    PostScipt uses a size-independent outline description for a
    character in a particular font, and can arbitrarily rotate it
    before conversion to a bitmap.
Nope, that is what I am saying. You are not confused.

    I was under the impression that there was no such
    size-independent representation.  Isn't that what the whole
    hoopla over bit-tuning is all about?  Certainly MetaFont is
    incapable of producing a uniformly great font at 300 dpi
    resolution, 10 point.

It may or may not be impossible, but the PostScript printers do it and
do it well. If you want to see an example, look at the recent POPL
conference publicity. The flyer announcing the conference announcement
was set with Scribe on an Apple LaserWriter in 8-point Times Roman
using a font described with ordinary outlines. One of the recent issues
of CACM (I think it was the November issue) had a 2-page advertisement
for the POPL conference somewhere towards the back. That advertisement
was also set on an Apple LaserWriter in PostScript. It isn't quite so
remarkable because it uses 10-point letters and does not use any
rotated fonts or graphics, but it is an example of the PostScript
raster conversion. Remember that it has been through a printing press.

I have carefully avoided learning anything about how the internals of
the PostScript font mechanism work, because I often have difficulty
keeping my mouth shut and I don't want to give away any Adobe secrets.
But you may rest assured that an ENORMOUS amount of software cleverness
and expertise in the nature of laser printers has gone into the
PostScript system. I think that I will have to doublecheck back with
the folks at Adobe to find out what I am allowed to say and what I am
not allowed to say before I give you any more information about the
font representation, but please understand, for the moment, that the
fonts are indeed represented as size-independent outlines and that they
can be artitrarily scaled and rotated before being scan-converted and
that the scan-conversion works just fine in point sizes down to 4 on a
300dpi printer.

Brian

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houxm!vax135!cornell!uw-beaver!laser-lovers
From: laser-lovers@uw-beaver
Newsgroups: fa.laser-lovers
Subject: one Page Description Language dominating another
Message-ID: <594@uw-beaver>
Date: Sun, 27-Jan-85 17:44:48 EST
Article-I.D.: uw-beave.594
Posted: Sun Jan 27 17:44:48 1985
Date-Received: Mon, 28-Jan-85 07:46:25 EST
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Lines: 66

From: Brian Reid <reid@Glacier>

Judging from the number of questions I've received, I guess I was too
obscure in my comparison of ImPress and PostScript, with respect to the
statement that "neither language completely dominates the other."

Both languages are complete, which is to say they can both describe any
image that a given raster printer can print. At some level, both
languages must fall back on the crude hack of storing rasters. There is
a very wide range of page images for which ImPress must fall back and
use rasters but for which PostScript can still use an analytical
description. I don't need to preach to you all about the benefits of an
analytical description. I am not aware of any page images for which
PostScript is forced to use rasters but for which ImPress can get by
with an analytical description. That is what I meant when I said it
would be fairly easy to translate ImPress into PostScript.  In fact,
since PostScript is a programming language, all you would have to do is
to define one PostSript operator that corresponded to each ImPress
operator, and then turn the ImPress file from prefix form into postfix
form (Impress has you say "MOVETO X Y" while PostScript has you say
"X Y MOVETO")--no big deal. No further translation would be necessary.
(these operator definitions would just be prepended to the front of the
file).

When I said that "one format does not totally dominate the other", what
I meant is that the ImPress format is superior for applications in
which speed is at a premium. In theory an ImPress file can always be
printed faster than a PostScript description of the same image, just
because the PostScript interpreter must do more computing. Our Imagen
12/240 printer, for example, can print 12 pages a minute with an 8MHz
68000 processor; the Apple LaserWriter can barely keep up with 8 pages
a minute with a 10MHz 68000 processor.

The principals of Imagen will remember, if they stretch their memories,
that they and I spent quite some time discussing page description
languages back in 1980 and 1981 at Stanford. They will also remember
that at that time (long before Adobe was invented) I told them I
thought ImPress was too simplistic and too limiting; they gave me
various reasons why it had to be that way.  Well, in once sense they
have proved me wrong, because they have built a fine company with many
happy customers by selling laser printers using this format that I
never liked; they have good reliable products, good documentation, good
customer support (at last!!), good fonts, and so forth. The Imagen
8/300 is an absolute marvel of a machine, and almost everybody that I
know who has one thinks it is wonderful.

I mean no harm or ill will to Imagen; I'm just glad that finally they
have some serious competition, and I'm especially pleased that the
competition's page description language works more or less the way that
I tried to convince Les Earnest and Luis Trabb-Pardo to go with back in
1980 or 81 (whenever it was).

A couple of people asked in private mail for me to compare the Apple
LaserWriter with the HP LaserJet, given that they both use the same
marking engine and that they look very similar. I don't think the HP
LaserJet is even worth discussing as far as its page description
capabilities. It's like comparing rockets and bicycles--why bother.
Bicycles are cheaper than rockets; the HP LaserJet is cheaper than the
Apple LaserWriter. I suppose you could throw your bicycle through the
air and call it a missile; you can also engage in ultra-intense hackery
and get your HP LaserJet to do something that you can call typesetting,
but realistically the HP machine is just a clever variation on a
daisy-wheel printer.
--
	Brian Reid	decwrl!glacier!reid
	Stanford	r...@SU-Glacier.ARPA

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laser-lovers
From: laser-lovers@uw-beaver
Newsgroups: fa.laser-lovers
Subject: Re: PostScript: comments on two issues
Message-ID: <789@uw-beaver>
Date: Thu, 7-Feb-85 03:20:51 EST
Article-I.D.: uw-beave.789
Posted: Thu Feb  7 03:20:51 1985
Date-Received: Fri, 8-Feb-85 02:19:58 EST
Sender: daemon@uw-beaver
Organization: U of Washington Computer Science
Lines: 84

From: Mendelson...@XEROX.ARPA

The purpose of this message is to correct an error about Interpress in
the recent message from Brian Reid which states:

"The Xerox Interpress format is somewhat similar to Adobe PostScript,
but it
uses raster fonts rather than outline fonts."

That is a fallacious statement.  Interpress makes absolutely no
assumptions about how fonts are represented.  It defines a very general
mechanism for obtaining character shapes that includes both raster and
outline definitions. An Interpress font contains, among other things, a
set of composed operators (procedures) each of which when executed
causes a character of a given shape to be imaged at a previously
designated position on the page.  Interpress makes no statement
whatsoever about the nature of the composed operators contained in a
font.  They can contain outline definitions of characters, or bit-map
definitions, or any other representation that anyone can create.

What is true is that current commercially available Interpress
implementations from Xerox use bit-map representations of fonts because
of advantages in performance, speed and quality.  It is my understanding
that current Adobe implementations require one second of time to scan
convert just a single outline-defined character.  By comparison, the
Xerox 9700 decompresses the bit-map fonts and prints a whole page in
that one second.

Brian Reid also makes some statements which impute capabilities solely
to PostScript, but which  apply equally to Interpress.   I substituted
the word "Interpress" for "PostScript" and "Xerox" for "Adobe" in the
following statements quoted from Reid's message, and generated equally
valid statements.  

I quote them here with our above defined substitutions indicated in
square
brackets:
 
"The important issue for contemplation on Laser-Lovers is that people
understand the difference between specification and implementation.
[Interpress] is not a program, not an algorithm; it is a specification
language. It is completely public-domain, its documentation is
available to anyone who has [$xx] to pay for the copying costs."
 
"The important concept is that [Interpress] is a way of specifying what
a
page should look like, not a particular implementation of that
specification, and that it does contain the expressive power to
describe and use fonts in terms of outlines."

"I think that the history of computer science has shown us that it is a
bad idea to adopt standards that are too tightly tied to the limits of
current technology.  [Interpress] is a page description standard that is
not limited by current raster-based technology, and for which there is
a pilot implementation ([Xerox's]) that demonstrates its feasibility
even
with today's technology.  Furthermore, it is completely public domain
and it is completely independent of the word size, two's complement
properties, instruction set, or addressing of any particular computer,
and efficient implementations have been demonstrated on several popular
modern computers."

"My summary claim is that [Interpress] is the obvious choice for a
standard for page specification, and that alternative implementations
of [Interpress] are welcome to embody the theories of their
implementors,
such as the vital necessity of bit-tuned fonts or the importance of
efficiency over generality. It is also perfectly reasonable to
implement a[n Interpress] subset, such as all of the imaging operators
but
none of the scaling, rotation, or halftone and grayscale stuff, and
document or market it as a subset implementation. Certainly all of the
different page description schemes that are in use today can be
isomorphically transformed into [Interpress] subsets. Doing that would
enable a common interchange standard for images, a shared set of image
management software, and would reduce the need for special-purpose
"drivers" at the back end of text formatting programs."
--------

I suggest an objective evaluation of Interpress and Postscript and
welcome the opportunity to participate in such an eveluation.  I will
continue to respond to questions and comments as suitable.

Jerry Mendelson