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From: laser-lovers@uw-beaver
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Subject: PostScript and Interpress: a comparison
Message-ID: <889@uw-beaver>
Date: Fri, 1-Mar-85 19:08:05 EST
Article-I.D.: uw-beave.889
Posted: Fri Mar  1 19:08:05 1985
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Organization: U of Washington Computer Science
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From: Brian Reid <reid@Glacier>

This essay offers a comparison of two modern schemes for controlling
what laser printers print. One scheme, called PostScript, is offered
by Adobe Systems, Inc.; the other scheme, called Interpress, is
offered by the Xerox Corporation. A discussion of these two schemes
has provoked a considerable amount of interest in this forum
recently.  I have for some time been promising (threatening?) to
provide my interpretation of the difference between the two systems.
It is long enough and detailed enough that you will certainly never
want to read another word on the topic after you read it, but given
the nature of computer mail systems you almost certainly will be
given the opportunity.


To a first order, PostScript and Interpress are indistinguishable.
What I mean by that is that by comparison with all other current
techniques for page image representation, the two can be considered to
be nearly identical. I believe that it is worth looking at how they got
to be that way; their similarities and differences can best be
understood with a proper historical perspective.

			Part I: History

The Evans and Sutherland Computer Corporation has for quite a number
of years sold very expensive, very powerful graphics devices for
CAD/CAM and for real-time simulation. The CAD/CAM machine is called
The Picture System; the simulation machines are custom-built for each
application. Custom simulation graphics machines are used for such
purposes as providing the windshield graphics for military flight
simulation systems--emulating what a pilot would see if he were
looking out the window of a real airplane. These graphics systems use
a very clever graphics model, developed by Ivan Sutherland and others,
which is based on coordinate system transformations and line

Although the Evans and Sutherland company is primarily in Salt Lake
City, they had a small research office in Mountain View (California)
in the early 1970's. John Warnock was in charge of it, and John
Gaffney worked for Warnock. One of the activities of the Mountain
View office was to develop software for producing 3-dimensional
graphical databases both for the Picture System and for the
simulation machines. Working with Warnock, Gaffney had by 1975
programmed and documented and released the first version of a
programming language that was called "The Evans and Sutherland Design

Gaffney came to E&S from graduate school at the University of
Illinois, where he had used the Burroughs B5500 and B6500 computers.
Their stack-oriented architectures made a big impression on him. He
combined the execution semantics of the Burroughs machines with the
evolving Evans and Sutherland imaging models, to produce the Design
System. Like all successful software systems, the Design System slowly
evolved as it was used, and many people contributed to that evolution.

John Warnock joined Xerox PARC in 1978 to work for Chuck Geschke.
There he teamed up with Martin Newell in producing an interpreted
graphics system called JAM. "JAM" stands for "John And Martin". JAM
had the same postfix execution semantics as Gaffney's Design System,
and was based on the Evans and Sutherland imaging model, but
augmented the E&S imaging model by providing a much more extensive
set of graphics primitives.  Like the later versions of the Design
System, JAM was "token based" rather than "command line based", which
means that the JAM interpreter reads a stream of input tokens and
processes each token completely before moving to the next. Newell and
Warnock implemented JAM on various Xerox workstations; by 1981 JAM
was available at Stanford on the Xerox Alto computers, where I first
saw it.

In the meantime, various people at Xerox were building a series of
experimental raster printers. The first of these was called XGP, the
Xerox Graphics Printer, and had a resolution of 192 dots to the inch.
Xerox made XGP's available to certain universities, and by 1972 they
were in use at Carnegie-Mellon, Stanford, MIT, Caltech, and the
University of Toronto.  Each of those organizations produced its own
hardware and software interfaces. The XGP is historically interesting
only because it is the first raster printer to gain substantial use by
computer scientists, and was the arena in which a lot of mistakes were
made and a lot of lessons learned.

To replace the XGP, Xerox PARC developed a new printer called EARS,
and then another newer printer called Dover. After the agony of
converting software from XGP to EARS, various Xerox people realized
that applications programs generating files for the XGP or for EARS
should not be tied to the device properties of the printer itself.
Bob Sproull and William Newman, of Xerox PARC, developed a relatively
device-independent page image description scheme, called "Press
format", which was used to instruct raster printers what to print.

As part of an extensive grant program to selected universities, Xerox
donated Dover printers and made documentation of the Press format
available under a nondisclosure agreement. As far as I know, that
nondisclosure agreement has never been lifted, though information about
Press format has been widely enough distributed that by 1982
researchers at the Swiss Federal Institute of Technology (EPFL) at
Lausanne had given conference papers about their own independent
implementation of Press format.

Press format was a smashing success; it revolutionized laser printing
technology in the academic and research communities, and stimulated a
large number of people to think about issues of device-independent
print graphics. Nevertheless, Press format had its limitations, and
various people felt the need to revise the basic design.

Sproull left Xerox in 1978 to become a professor of computer science
at CMU.  Newman returned home to England to become an independent
consultant. Martin Newell left Xerox to join Cadlinc Corp.
Warnock and Geschke remained at Xerox.

While at CMU, Sproull began making plans for a new version of Press
that would combine the graphics model of JAM with the page image
description properties of Press. Sproull returned to Xerox for a
sabbatical leave in 1982, and enlisted the help of Butler Lampson in
the creation of the new page image description language that Warnock
dubbed "Interpress". The name caught on.

While it is difficult to separate the contributions made by Sproull
and Lampson, it is not incorrect to say that Lampson and Warnock
produced the execution model of Interpress while Sproull and Warnock
produced the imaging model. It is also approximately correct to
characterize this first version of Interpress as being derived from
the graphics model and execution model of JAM with additional
protection and security mechanisms derived from experience with
programming languages like Euclid and Cedar, and a careful silence on
the issue of fonts. The trio worked under Geschke's direction, and
Geschke was responsible for refereeing disagreements and for making
certain that the resulting design was acceptable to the rest of Xerox.

My own involvement with the Interpress effort is difficult to explain.
Sproull was my thesis adviser at CMU; we had discussed many of the
issues in page description languages at length. As a consultant to
PARC during the Interpress design work, my primary activity was one of
writing or rewriting the Interpress materials. I also represented a
"consumer" point of view rather than a "designer" point of view,
and often complained about aspects of the evolving language. 

I feel uncomfortable discussing the issues involved in the transition
of Interpress from an artifact of the research lab to a marketable
product.  I shall therefore not discuss them. During this transition
phase Geschke and Warnock left PARC (December 1982) to start Adobe
Systems, Sproull returned to CMU (June 1983), and Lampson left PARC to
join DEC Research (November 1983).

Warnock had various philosophical differences with the final
Interpress design, and he voiced those differences to the rest of the
Interpress group at every opportunity.  At Adobe, Geschke and Warnock
saw the opportunity to try again, with a design group composed of
people who shared his ideology. They enlisted Doug Brotz, a Xerox
PARC researcher who had had no involvement with any of the
Press/JAM/Interpress world, to join them in developing a new page
description language named PostScript, based on combining the
execution model and imaging model of JAM with a protection structure
more reminiscent of C or the Unix shell than of Euclid or Cedar.
While not at all a copy of JAM, PostScript resembles JAM more than it
resembles Interpress.  PostScript also embraced various Unix notions,
such as the use of text streams to convey information.

On March 15, 1984, Adobe shipped its first PostScript manual to a
potential customer. That PostScript manual was printed on a PostScript
printer using a Times Roman font licensed from Allied corporation and
digitized by Adobe.

At that time all aspects of the Interpress project were still very
proprietary, and it appeared to me that Xerox had no interest in
releasing them. However, on April 25, 1984, I received a Xerox press
release announcing the availability of Interpress documentation. I
finally managed to get my hands on a copy of the Interpress
documentation in February of 1985, and was quite surprised to discover
that the Interpress documentation had not been printed on an Interpress
printer, but was instead printed on a Press format printer, using the
same Times-like and Helvetica-like fonts that I had become familiar
with at CMU and Stanford on the Dover printers.


			Part II: Comparison

Part I outlined the history of PostScript and of Interpress, as I have
been able to determine it. With that historical background, I now offer
a comparison of the two languages.

While there are quite a number of extant schemes for the description of
printed images, most of them are better described as "data structures" than
as "languages". In particular, only PostScript and Interpress are 
directly executable.

Languages can be compared at several different levels.  Languages have a
lexical representation, a syntax, a semantic model, an intended style of
usage, and implementation considerations.


The lexical properties of a language define the way the tokens of the
language are represented in terms of bits, bytes, or characters. The
FORTRAN language was defined in terms of a particular character set,
which the implementor was expected to use. The ALGOL language was
defined in terms of keywords and symbols, and the language definition
left the implementor free to choose how he would represent those
keywords in terms of characters available on his computer. For example,
the FORTRAN definition of a "DIMENSION" statement is that it is the
letter "D" followed by the letter "I" followed by the letter "M", etc.
The ALGOL definition of the "BEGIN" keyword was merely that it was a
keyword; the ALGOL standard document used boldface to identify
keywords. When ALGOL is implemented on computers whose character sets
include boldface, the implementors normally use the boldface characters
as a way of identifying keywords. When ALGOL is implemented on other
computers, the implementors choose other schemes for identifying
keywords, such as putting them in quotes or putting them in all capital

Both PostScript and Interpress have an operator called MOVETO, and in
both languages it does exactly the same thing, which is identical to
what the MOVETO operator did on the Evans and Sutherland hardware that
spawned this graphics model. Let's look at how that operator would be
represented in the two languages.

The PostScript language is defined in terms of characters, like
FORTRAN. The definition of the PostScript operator "MOVETO" is the
letter "M" followed by the letter "O" followed by the letter "V", etc.
The Interpress language is defined in terms of keywords; the definition
of the Interpress operator "MOVETO" is that it is a keyword in the
ALGOL sense. The Interpress 2.1 standard suggests that MOVETO can be
represented with the serial number 25 in a standard encoding that the
standard provides, but the definition of the MOVETO keyword is
independent of the choice of encoding.

Since PostScript is defined in terms of sequences of characters, it is
always possible to assume that a PostScript file can be transmitted
over any link capable of sending characters, and can be stored in any
device capable of holding characters. Since Interpress is defined more
abstractly, it is not necessarily possible to make any assumptions at
all about a particular Interpress file. However, any Interpress
encoding can be translated into any other Interpress encoding, so it is
always possible to take an Interpress file and translate it into a
stream of characters which will then have properties identical to
PostScript's. Conversely, it is always possible to translate a
PostScript program into a tokenized keyword form, though the PostScript
standard does not suggest any particular tokenization scheme.

It is worth mentioning that the word "token" is slightly overloaded
here.  A "tokenization scheme" is a means of doing data compression,
wherein a sequence of characters is called a "token" and is replaced by
a token number, which will occupy less space. However, a language can
have tokens without having a tokenization scheme. Both PostScript and
Interpress have an execution semantics that is defined in terms of
things called "tokens". The Interpress tokens are normally represented
by tokenization schemes--i.e.  replaced with integers--while the
PostScript tokens are normally left as sequences of characters. In
later sections of this message the word "token" will be used to mean
either the PostScript kind of token or the Interpress kind of token; by
the time they get to the interpreter they are roughly the same thing.

The Interpress 2.1 standard defines a particular encoding of
Interpress, and gives bit and byte formats, decimal integer operator
numbers, and so forth.  This encoding is a full binary encoding, using
all 8 bits of each byte, which means that it cannot always be sent
over a serial character link. The Interpress standard encoding of a
page description normally occupies a smaller number of bytes than the
equivalent PostScript character representation. This is possible
because binary encodings make more efficient use of the bits.

Interpress files are clearly intended to be transmitted via XNS
protocols over Ethernet. In its current form, without further
processing or re-encoding, Interpress is not suitable for transmission
over character-protocol lines. PostScript files are clearly intended
to be transmitted over character-protocol lines. Like all character
stream protocols, PostScript can also be transmitted over Ethernet,
but a PostScript file will use more bytes than the corresponding
Interpress file.

Text files such as PostScript sources are highly redundant (i.e. they
make inefficient use of their bits) and can be run through data
compression programs (such as the Unix "compact" program) to reduce the
amount of space they occupy in storage and during transfer. Data
compression techniques will probably not yield much further compression
of Interpress files, because the information is already quite tightly
packed. After compression of both, the PostScript and Interpress
representations of an image will likely occupy approximately the same
number of bits.


The syntactic issues (or issues of syntax, if you will) of a language
are the means by which an interpreter for the language distinguishes
variables from operators from constants from function calls from quoted
strings, and by which it determines whether or not a certain sequence
of characters or tokens is in fact a "legal" construct in the language.

As languages in general go, both PostScript and Interpress are
remarkably free of syntax. As token-oriented postfix languages, each
token of the language is "executed" as soon as it is identified, and
that execution will either succeed or fail depending on the state of
the execution environment at that point.

Nevertheless, both languages have a small amount of syntax, though they
differ radically in the nature and application of this syntax. In fact,
the primary area in which the PostScript language and the Interpress
language are incontrovertibly and irrevocably different is in their

As explained above (Lexical Issues) PostScript is defined in terms of
character sequences. A PostScript program is a series of character
tokens, separated by white space characters. That program is fed to an
interpreter to be executed; the interpreter reads in the characters and
assembles them into words (i.e. tokens), then looks up the tokens in
dictionaries to determine their meaning. In this regard PostScript is
similar to many other programming or command languages: if the
PostScript interpreter sees the command "MOVETO", it finds the current
definition of that string, and then performs whatever action is
requested in that definition.

By contrast, Interpress is defined in terms of byte codes, which behave
more like the instruction codes of a hardware interpreter than like a
traditional programming language. Instead of the letters "MOVETO", an
Interpress file will have a byte whose binary value is 25; the number
25 is then used to index an operation code table which directs the
interpreter to the program implementing the MOVETO operation.

The byte codes of Interpress can be viewed as a compiled form of the
character codes of PostScript. One could imagine a translator that
passed over a PostScript file, looked up each name, and produced an
output file whose contents was the binary identification of the thing
found during the lookup. In fact, the Interpress standard document
explains that the two forms are equivalent, and the Introduction to
Interpress document explains how to write a program to convert one to

There is, however, a crucial difference between the PostScript and
Interpress naming schemes that makes them very different, and makes
impossible the above-mentioned imagined compiler to translate
PostScript into Interpress. That difference is best understood as a
semantic difference, and will be explained in the next section.

Returning to syntactic issues, an Interpress file has what is called
"static structure" or "lexical structure". This means that you can
look at an Interpress file and make structural assumptions about what
you find there.  For example, an Interpress file is defined to be a
sequence of "bodies"; each body is a sequence of operators and
operands. The first body is the "preamble", or setup code; all
following bodies correspond to printed pages. If an Interpress file has
11 bodies, then it will print as 10 pages. 

By contrast, a PostScript file has no fixed lexical structure; it is
just a stream of tokens to be processed by the interpreter.  PostScript
prints a page whenever the SHOWPAGE operator is executed.  If a
PostScript file contains a loop from 1 to 10, with a SHOWPAGE operator
inside the loop, then it will print 10 pages even though there is only
one actual call to SHOWPAGE in the file. However, since PostScript is a
textual language, and since it has a "comment" facility like the C
/*....*/ or Pascal {...}, it is possible for the creator of a
PostScript file to represent whatever additional information is desired.
It is a slight misnomer to call this a comment facility, because the
normal use of the word "comment" in programming languages implies
that the contents of the comment are irrelevant. PostScript comments
are irrelevant in the sense that they do not affect the image produced
by a PostScript file, but they do convey machine-readable information
about the structure of the document.

A PostScript client is free to choose any structuring scheme that he
wants, and the tool that he has available to implement this
structuring scheme is the PostScript comment.  There is a particular
"standard" structuring convention documented along with PostScript
by which page boundaries and other lexical information can be marked.
A PostScript file that follows that convention is called a
"conforming" file, but it is a convention and not a rule; the
printed image produced by a nonconforming PostScript file will be
identical to that produced by the equivalent conforming PostScript
file. Conversely, the structure of a PostScript file, as represented
by the structuring convention, is completely independent of the
appearance of the page images--the actual PostScript text appears to be
a series of comments as far as the structuring systems are concerned.

The technique of mixing two different languages in one file, so that a
processor for one language sees the text of the other language as
comments, is not new. Perhaps the most widely-known instance of this
scheme is Don Knuth's "WEB" system, in which Pascal and TEX are
woven together in such a way that the Pascal program looks like a
comment to the TEX interpreter and the TEX  source looks like a
comment to the Pascal compiler.

This absence of fixed lexical structure in PostScript is a two-edged
sword. On the one hand, it offers more flexibility in creating page
images, especially repetitive ones; on the other hand, it provides more
opportunities to make mistakes.

One final syntactic issue is perhaps worth mentioning, though it could
also be considered a semantic issue. Interpress does not support
"variables" so much as it supports "registers", in the hardware sense.
All storage in Interpress is accessed by address and not by name. What
would be called a "local variable" in a programming language is
represented in Interpress by an integer subscript into the procedure's
frame. All programming languages must ultimately reduce their variable
names into memory locations; Interpress asks that this translation be
performed by the creator of the Interpress file and not by the
interpreter. An obvious benefit of this approach is efficiency--no
name lookups need be performed as the file is being printed. An
obvious drawback of this approach is the restricted name space
available to the programmer and the extra care that must be taken to
manage addresses instead of names. By contrast, PostScript supports
ordinary named variables.


Since both Interpress and Postscript derive their semantics from the
same source, it stands to reason that the semantics would be similar.
Both use similar graphical semantics, the same imaging model, and both
use very similar execution semantics. The differences are minor,
though one could imagine that the consequences of those differences
might be major.

There are two substantive differences between the graphical semantics
of PostScript and Interpress 2.1, namely that Interpress has no
facility for describing curves, and the Interpress standard is
completely silent on the issue of fonts.

A curve can of course be approximated with a series of line segments,
and if the line segments are short enough the resulting appearance
will be identical, but many classes of curved lines, such as those
appearing in fonts, can be described very succinctly in terms of the
PostScript CURVETO operator while requiring a tedious collection of
short line segments to describe in Interpress.  Because of the
importance of fonts to printed images, this seemingly minor omission
could possibly have major consequences.

On the issue of fonts, the Interpress standard states only that a font
is an operator that will be executed for you when appropriate, and
that the operators for that font are defined "in the Environment". A
PostScript font is just an ordinary PostScript defined operator, and
the PostScript manual gives explicit instructions for creating
user-defined fonts and making those font definitions be part of a
PostScript file. One could imagine that it is possible to write an
Interpress composed operator (in Interpress, of course) to behave like
a user-defined font, but the Interpress implementations do not
currently have any mechanism for recognizing that an operator is in
fact a user-defined font and should therefore receive any kind of
special treatment. This is not a deficiency in Interpress, just a
silence, accompanied by a deficiency in current implementations (this
and other implementation issues are discussed in the last section).

There are three consequential differences between PostScript execution
semantics and Interpress execution semantics: user-defined operators,
the nature of the "firewalls" between pieces of the program, and
error recovery.

In Interpress, a user-defined operator is syntactically different from an
intrinsic operator, and requires an explicit "DO" operator to call it.
In PostScript a user-defined operator is syntactically identical to an
intrinsic operator, and in fact any intrinsic operation can be
redefined by simply making a new entry for that operator's name in the
appropriate dictionary. This is stylistically similar to the difference
in lexical structure: Interpress guarantees that if a byte code 25--the
MOVETO operator--is found in a file, that it will when executed perform
a standard MOVETO. PostScript guarantees nothing because it enforces
nothing. If you want to redefine the meaning of MOVETO, then you can do
so, and when the characters "M O V E T O" are found in a PostScript
file, the redefined operator will be executed instead. To execute a
PostScript user-defined operator you just include its name, the same
way you execute any other operator. To execute an Interpress
user-defined operator, you execute the DO operator (or a variation of
it), after pushing onto the stack the thing that you want to execute.

Analogously with the static structural issues, The PostScript
user-defined-operator scheme offers more flexibility than Interpress
but carries with it more dangers. Like the old saw about giving one
enough rope to hang himself, the additional flexibility of the
PostScript scheme requires discipline on the part of the user.
Furthermore, just as PostScript has a convention for the voluntary
inclusion of static structure in a file, it has a mechanism by which a
PostScript program can reference the true built-in version of an
operator and not the current, possibly user-redefined, version of an
operator. From the point of view of language design, this scheme is
not terribly elegant, but it is quite practical, as it provides a
mechanism for the solution of all of the problems associated with
operator redefinition and the prevention thereof.

It is this ability to redefine builtin operators that makes the
compilation of a textual Postscript file into an encoded Interpress
file (mentioned above under Syntax) impossible. A static analysis
cannot determine the operator that will be executed when the textual
token is interpreted. By contrast, it is easy to translate Interpress
into PostScript, because all of Interpress' semantic capabilities
have direct equivalents in PostScript, and the lexical translation is

Interpress has a distinction between "bodies" and "operators". A
"body" is a sequence of Interpress tokens. The Interpress operator
"MAKESIMPLECO" (make simple composed operator) translates a body into
an operator. Like all other Interpress operators that reference
bodies--referred to in the Interpress standard as "body
operators"--the MAKESIMPLECO operator is prefix and not postfix.
This was done to make it easier for small computers to implement
Interpress interpreters; it has the interesting side-effect of making
it impossible for an Interpress program to generate and then execute
a piece of Interpress source code. I would guess that the entire
reason for the distinction between Interpress bodies and operators is
to enable a clean prefix implementation of body operators while at
the same time permitting the more conventional postfix use of
expressions of type "operator".

By contrast, PostScript represents operator bodies as arrays of
PostScript tokens. The PostScript lexical scanner processes a body by
building an array out of the tokens that it finds in the input
stream; that body is then handled as an ordinary data value in the
language, and it can be stored into variables, executed, modified,
searched or searched for, etc. The translation of a body into
something like an Interpress operator consists merely of returning
the address where the body is stored; that can be handled by the
PostScript type system and does not require a special conversion
operator. Consequently, a PostScript program is able to generate an
array of PostScript operators, however it so chooses, and then
declare that array to be a new PostScript operator and have it be
executed just like any other PostScript operator.

The second important semantic difference between PostScript and
Interpress is the set of mechanisms that they offer for protecting one
piece of the file from side effects in another. As you might be able to
guess if you have read this far, the Interpress protection mechanism is
static and mandatory while the PostScript protection mechanism is
dynamic and optional. This kind of mechanism is often referred to as a

An Interpress file consists of a series of bodies. Each body is
executed completely independently of each other body. In particular, at
the beginning of each page body, the execution environment is restored
to the state that it had at the end of execution of the preamble, so
that each page body is executed as if it were the only page in the
document. There is absolutely nothing that the code in one Interpress
page can do that will have any effect on the execution of the code in
any other Interpress page, and the Interpress language guarantees that
independence. This permits, for example, the pages to be executed or
printed in any order, front to back or back to front, or in folios of
16 pages at a time, with complete confidence that the appearance 
of the pages will not change.

By contrast, a PostScript file has no static structure, so there is no
convenient place to build automatic firewalls. PostScript provides,
instead, two pairs of operators by which a PostScript user can build
his own firewalls wherever he wants them. There is an operator called
SAVE, and another operator called RESTORE. The RESTORE operator
restores the execution state of the machine back to what it was when
the last SAVE operator was executed. Thus, if a PostScript user wants
to have pages that are firewalled against each other, then he puts a
SAVE operator at the beginning of the page and a RESTORE operator at
the end of the page. If the PostScript user wants to play tricks, and
build PostScript files that do bizarre things with the execution state
between pages, he is free to do so by leaving out the SAVE and RESTORE.

By now you can probably see the fundamental philosophical difference
between PostScript and Interpress. Interpress takes the stance that the
language system must guarantee certain useful properties, while
PostScript takes the stance that the language system must provide the
user with the means to achieve those properties if he wants them. With
very few exceptions, both languages provide the same facilities, but in
Interpress the protection mechanisms are mandatory and in PostScript
they are optional. Debates over the relative merits of mandatory and
optional protection systems have raged for years not only in the
programming language community but also among owners of motorcycle
helmets. While the Interpress language mandates a particular
organization, the PostScript language provides the tools (structuring
conventions and SAVE/RESTORE) to duplicate that organization exactly,
with all of the attendant benefits. However, the PostScript user need
not employ those tools.

Before taking a stand on this issue, you must remember that neither
Interpress nor PostScript is engineered to be a general-purpose
programming language, but rather to be a scheme for the description of
page images, so it is not necessarily valid to apply programming
language lore to these two systems.

The third area in which there are significant semantic differences
between PostScript and Interpress is in error handling and error
recovery. The Interpress 2.1 standard is slightly vague as to what
happens when various error conditions occur; one assumes that the
implementors of Interpress printers will do something reasonable. The
PostScript language provides a user-extensible error-recovery mechanism
that is keyed on PostScript's ability to redefine intrinsic operators.
Whenever an error of any kind occurs in PostScript, be it the printer
out of paper, the file asking for a font that doesn't exist, or a
division by zero, the PostScript interpreter responds by executing an
"error operator". If the error operator has not been redefined, then
some standard action is taken; sometimes the standard action is to do
nothing, while sometimes the standard action is to abort or to retry.
The standard action is merely the execution of the error operator.

The Interpress documentation does not offer much explanation, one way
or another, of error handling. The Interpress standard describes
certain kinds of error conditions that can occur, such as "appearance
error" or "master error", but does not specify exactly what will
happen if those errors occur. I assume that the reason the standard
is vague is to provide leeway to the implementors in error handling.
The Interpress language standard does not describe any technique by
which an Interpress master can control or modify the error recovery

When a PostScript error occurs, an error operator is executed.  There
is a set of built-in error operators provided as part of PostScript,
and documented like all other operators.  If a PostScript user wants
to change the error handling of a PostScript printer, he simply
changes the dictionary entry for the relevant error operator.
Depending on the relative position of that redefinition with respect
to SAVE and RESTORE operators in the PostScript file, the
redefinition will have a certain lifetime.  A SAVE and RESTORE pair
is wrapped around each separate file printed by a PostScript printer,
so that the redefinition does not carry over to other jobs. The
manager of an installation can change the overall default of the
printer by sending it a redefinition, during printer startup, before
entering the SAVE/RESTORE loop around each print job.

Like so much of PostScript's flexibility, the ability to redefine
operators is a two-edged sword. Redefining an operator can be used to
advantage by clever and knowledgeable users, and it can be used as a
technique for fixing bugs in a PostScript implementation. For
example, if an accounting package were not provided as part of a
PostScript implementation, the owners of a PostScript printer could
add page accounting to their printer by downloading a redefinition of
the SHOWPAGE operator that kept accounting information. However, a
user might be able to disable that accounting by doing yet another
redefinition that disabled the installation's accounting. To
circumvent this class of problem, PostScript provides a mechanism for
declaring certain objects to be read-only, or execute-only. The
management of a shared PostScript printer can specify that part of
its power-up or restart sequence is to load a configuration file;
that configuration file can redefine certain operators--for the
purpose of bug fixing or accounting or any other reason--and then, if
desired, mark the redefined operators read-only so that they cannot
be further redefined. As a language mechanism this is very clumsy,
but as an operational technique it is effective.


The implementation considerations are the most difficult to review and
compare, because it is next to impossible to determine the reason for
some annoying property of an implementation; it is also not entirely
proper to criticize a language for the state of its implementation.
Nevertheless, the history of programming languages has repeatedly shown
that good implementations of languages have longer-lasting impact than
good designs. For example, I quite commonly encounter people who choose
to run VMS on their Vax systems instead of Unix and who offer the
explanation that they do this because the VMS implementation of Fortran
is so good that their programs will run a lot faster. Naturally, other
people have other reasons; this is just an example.

The Interpress documentation is peppered with "fine print" explaining
the possible limitations of various possible Interpress printers, and a
chapter of the Interpress standard is devoted to a discussion of the
various ways to subset Interpress so that stripped-down versions of the
language can be implemented. Indeed, as of today (March 1, 1985) I am
not aware of the existence of any printer that implements the full
Interpress 2.1 language defined in the standard. Certainly none is
offered now as a product, and if one has been announced the
announcement has not yet reached me. The Xerox 8044 "Star" printer and
the 5700 and 2700 printers all implement various subsets of Interpress.
Perhaps there are others. The only one of these that I have used to any
extent is the 8044. It implements a textual subset of Interpress, with
the capability of a certain amount of line graphics, and has some
unknown capacity for more sophisticated graphics. It does not implement
very many of the features that distinguish Interpress from the older
Press format, and in fact has some surprising limitations. For
example, Interpress provides the ability to get rounded ends on line
segments. The 8044 implementation of Interpress that I experimented
with faked the circular arcs with sections of a 9-sided polygon. The
Interpress standard promises the ability to rotate the coordinate
system through arbitrary angles; all of the existing implementations
of Interpress limit coordinate system rotations to multiples of 90

Xerox quite likely has been developing true Interpress printers,
which implement the full documented language, but none has been
demonstrated or announced.

By contrast, the PostScript documentation makes no mention of any
subset, or of any implementation restrictions.  The entire PostScript
language was fully implemented before any PostScript documentation
was distributed or any printers shipped.  There are four PostScript
printers announced and demonstrated by three OEM vendors: the Apple
LaserWriter (300 dots/inch) the QMS 1200A (300 dots/inch), the
Mergenthaler P300 phototypesetter (2540, 1270, or 635 dots/inch), and
the Mergenthaler P101 phototypesetter (1270 or 635 dots/inch). The
Apple printer has been shipped to customers, the QMS printers are in
Beta test, and the Mergenthaler machines will be shipped to
customers by Fall of 1985.

All implementations of PostScript printers can print any PostScript
file, with no restrictions save the availability of fonts as licensed
to that manufacturer.  Circles come out as circles.  A PostScript
file that has been proof-printed on an Apple LaserWriter can be
typeset on a Mergenthaler P101 without making any changes to the
file. Naturally all device-independent page representation schemes
have this ability as their goal, and many claim to be able to do it,
or claim that they could do it if they had all of the necessary fonts
available in all of the requisite sizes.  The current set of
PostScript printers actually do it.

Given that Xerox has been working on Interpress for about twice as
long as Adobe has been working on PostScript, and many of the
graphics techniques necessary for the implementation are copiously
described in the open literature, I find it surprising that there are
no true Interpress printers on the market. I am puzzled by this, and
as a student of programming languages I am very interested in
learning whether or not there are any properties of the Interpress
language itself that are somehow contributing to this difficulty, or
whether this is just the usual sluggishness that one expects from all
large companies.

Brian Reid				R...@SU-Glacier.ARPA
Computer Systems Laboratory		decwrl!glacier!reid
Stanford University			415/323-6100

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From: laser-lovers@uw-beaver
Newsgroups: fa.laser-lovers
Subject: Interpress and PostScript, A Second Comparison
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Date: Sun, 3-Mar-85 16:15:23 EST
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From: mendelson...@XEROX.ARPA

I've been scooped!!  I was about to send the message that is presented
below the dotted line when Brian Reid's masterful message "PostScript
and Interpress: a comparison", appeared in my mail.  So I'll preface my
original message with these remarks.

I want to compliment Brian on a piece of work of outstanding excellence.
I appreciate its objectivity.  I agree almost completely with his
technical characterizations of the differences between the two
languages.  I believe that that agreement will show up clearly in what I
wrote, but my characterization is not nearly so scholarly as his.  I
hope it is as objective as his.

We do have some differences of opinion and of knowledge, but I do not
propose to address them here.   We'll leave them for another day, after
both comparisons have been presented.

Jerry Mendelson


The following information was prepared by Jerry Mendelson, and is
submitted to Laser Lovers as a contribution to the general discussion of
the characteristics of the two printing languages, Interpress and

Let me state up front that I am a retired Xerox  Engineering Fellow, and
that I am currently working as a consultant to Xerox.  This report was
commissioned by Xerox, but these are my observations, not those of
Xerox.  Xerox did not exercise technical control over the content, nor
editorial control over the presentation, of the following information.
I have tried to be as analytic, objective, and honest as I possibly
could, but I know far more about Interpress than I do about PostScript.
I welcome equally honest and objective responses to this discussion
containing corrections, additions, other perspectives, what have you.  I
trust that these discussions will lead to well reasoned recommendations
on a standard for the description of documents for representation on
imaging devices. 


A comparison of Interpress and PostScript is perhaps most appropriately
begun by recounting the history of the development of the two languages.
The following additional material on the history of the developments of
Interpress and PostScript is based on my first hand knowledge of events
that have transpired within Xerox during the past ten years, plus direct
conversations with the key participants in the developments of these two
languages.  It is factual and complete to the best of my knowledge.
Others might be able to provide additional information of which I am


A FORTH like graphics/printing language was developed by, among others,
John Warnock before coming to Xerox/PARC.  After coming to Xerox, John
put together another similar language which he called JaM.

Long before JaM was developed another printing format (Press) had been
developed and placed in use at Xerox.  It was largely the brainchild of
Bob Sproull who is now at Carnegie-Mellon.  Variants of this effort run
most of the Xerox internal printers on the original 3 MBit/Second
Ethernet which is still operational within Xerox.  A graphics research
program known as Cedar Graphics was also pursued at PARC during this
same time frame.

Finally, leveraging off of all of these earlier activities, what I
designate as Research Interpress was created as a combined effort of a
number of people at PARC under the management direction of Chuck
Geschke.  The principal developers were Bob Sproull, Butler Lampson,
John Warnock, and Brian Reid,  with significant participation from Bob
Ayers.  (There may have been others whom I have unintentionally slighted
because I am simply unaware of their roles.)  The resulting Interpress
language contains benefits which derive from all of those very talented

During the course of these developments Xerox permitted publication of
some of the Cedar Graphics research effort in a paper authored by John
Warnock and Douglas Wyatt titled "A Device Independent Graphics Imaging
Model for Use with Raster Devices", in the July 1982 Computer Graphics
Volume 16, number 3, pp. 313-320.  Xerox also permitted Dr. Leo Guibas,
a Stanford professor, to use some of the Interpress related research
work as course material for a course at Stanford.

I led the engineering effort that extracted a suitable subset of the
Research Interpress language, and pushed it through the corporate
standards activity and into the product line.  The first subset of this
language was published internal to Xerox in a Xerox System Integraton
Standard (XSIS 048201), titled Interpress 82 Electronic Printing
Standard, dated January 1982.  Subsequent backward compatible revisions
have been internally released.  The current externally  released version
of of Interpress is designated as Interpress Electronic Printing
Standard (XSIS 048404),  Version 2.1, dated April 1984, and is contained
in the documentation set available from Xerox.  The Printing
Instructions portion of Interpress were added to Research Interpress,
and included in the  internally published Version 2.0 in June, 1983.

After Interpress 82 had been extracted, pushed through the corporate
standards process, and incorporated into products, but before
Interpress, Version 2.0 had been created, Chuck Geschke and John Warnock
left PARC and formed Adobe Systems.  They developed PostScript by
extending a combination of the work that Warnock had done prior to his
work at Xerox, plus the material that Xerox had permitted to enter the
public domain through the publication of the above referenced paper,
plus the material that Xerox had released to the Stanford course.  They
carefully avoided including any of the advanced features that Sproull,
Lampson,  and Warnock had incorporated in Interpress but that Xerox had
not publicly released.  


Again, some caveats about this general overview.  It is not presented as
fact, but rather as a set of observations as interpreted by one reviewer
who has more familiarity with Interpress than with PostScript.  Another
reviewer might reach a quite different set of conclusions.  Also, please
note that this discussion deals with attributes of the languages, and
does not address issues related to particular implementations of the

With their thread of common history it is quite natural that Interpress
and PostScript turn out to be very similar languages.  Their fundamental
concepts and structures are substantially the same.  I can only
speculate about the causes for their differences, but it appears to me
that the following issues are fundamental:

     1.  The developers of the two languages had different perceptions
of the application environments in which they were intended to operate.

     2.  Adobe was precluded from pursuing some of the elegant Xerox
proprietary techniques that were not part of the public domain.

As a consequence Interpress appears to be richer in higher level
structure.  PostScript appears to be richer in its more basic
structures.  These issues are discussed below.

Application Environment Considerations

The languages appear to have distinctively different viewpoints of their
application environments, and of how they properly fit into those
environments.  The designers of Interpress took the position that the
functions of creation and composition are properly the role of creation
devices.  Printers should print, and should do so in a highly efficient
and productive manner.  Interpress was designed to describe the results
of the creation process to printers in a printer independent fashion.
The language recognized that there were some things in the printing
domain that the printer could best deal with, and that the creator would
not necessarily have knowledge about.  It, therefore, made provision for
the creator to describe how the document description should adapt itself
to the specifics of what it finds in the printer's environment, without
knowing a priori what that environment would be.

Interpress can be used with a single printer that is tightly coupled to
the creator source, but it is also designed so that it can operate in a
networked environment in which there may be a multiplicity of
printer/servers.  In this latter environment the creator may be highly
decoupled from the destination printer.  Further, in such an environment
the printer/server must at all times be in control of its own actions,
and an Interpress document may not be allowed to cause the execution of
operations that interfere with that control.  Interpress is designed for
machine to machine communication with no human interaction.  In fact,
Interpress is designed under the assumption that the document may be
printed on a wide variety of printers over an extended period of time,
i.e. stored over an extended period of time, later retrieved, and then

The designers of PostScript appear to have taken a different viewpoint.
PostScript appears to assume an environment in which there is not so
clear a separation between the creator and the printer.  The creator
appears to be in much closer contact with a specific target printer, and
possesses much more information on the details of that printer's
environment.  In fact, a PostScript master appears to have the ability
practically to take over complete control of the printer's resources,
e.g. open, close, read, and write files.  PostScript enables an almost
interactive environment with a human creator in the loop.   While the
language is printer independent, a specific PostScript master appears to
be more closely coupled to a target printer than does an Interpress
master.  PostScript enables much greater print time program control to
the PostScript master, and relies on that master to perform many of the
actions that Interpress defers to the printer.  Further, Adobe has
chosen to enrich the more basic aspects of their language to make it a
more general purpose composition language.  An example of an ideal
application for PostScript would be that of a fully functioned Graphics
Composition station working in conjunction with a designated printer of
known characteristics.  It would clearly be a better language for that
application than would Interpress.

Computing Load Allocation

The printing of any complex page creates a significant computing task.
Interpress's design tends to force this task to the creator side.
PostScript's design tends to push this task to the printer side.  As a
result Interpress masters tend to enable a high printer throughput
capability.  PostScript masters tend to impose higher computing loads
that lead to slower throughput capabilities at the printer.  Note the
use of the word "tends" in those last two sentences.  What I am trying
to say is that the normal use of the natural characteristics of the two
languages would lead to the suggested results.  However, these are very
general observations about the inherent characteristics of the
languages.  It is clearly possible to create a PostScript master that is
highly efficient, and that does not force heavy computation loads on the
printer.  It is also clearly possible to create an Interpress master
that does force heavy computation loads on the printer.


The two languages are strongly similar in so many ways that an
exhaustive presentation is beyond the scope of this brief analysis.
Here are some major points of identity:

Both Interpress and PostScript are document description languages.  They
use a language to describe how to construct the image they want printed
on the page.

They are device-independent.  The description of the page image is in
terms of the image, not of the device that the image is to be created
on.  It is up to the printer to interpret this description, and to
create an image that matches it to the best of the printer's capability
to do so.

Both languages contain two distinct parts, a general purpose programming
portion, and a special purpose image generating portion.

They both use a FORTH-like postfix notation language, i.e. a reverse
Polish notation in which the operands precede their associated operators
in the presentation sequence.

They both use a stack-oriented processing structure.  Operands are
pushed into a stack when they are received.  Operands are popped from
the stack when their associated operator is received.  Operator
execution results are generally pushed back onto the stack.

They both transmit their document representations to the printer in byte
streams.  These bytes streams are broken down into "tokens", explicitly
defined in the Interpress implementation, implicitly defined in the
PostScript implementation.

Both token streams can be generated on the generator side, and executed
on the printer side, in one pass implementations.

Both language operate on "typed" operands.  Operand types are generally
equivalent in the two languages.

Both languages have a generalized array processing capability.  Such an
array is a collection of objects, not necessarily of the same type.
Both languages provide means to access arrays by an integer index that
designates a specific array entry by its relative position in the array.
Arrays may also be organized by using key, value  pairs in arbitrary,
but paired locations.  A value is then located by designating its
associated key.

Both languages use a universal coordinate system as a reference
framework.  Both languages are heavily dependent on the use of identical
forms of  transformation matrices.  These are matrices that represent
the transformation from a user coordinate system to the reference
framework coordinates, and thence to the printer coordinate system.
Both use transformations to rotate, scale, and position objects on the

Both languages enable the construction of procedures that can be
repetitively invoked.  Procedures can be invoked by mechanisms that save
and restore the printer environment so that their effects can be
isolated from the rest of the page.

Both languages use very similar imaging models.  The model is one in
which the image is incrementally built starting with a blank page.  A
page image element, e.g. a character, a pixel array, or a geometrically
defined graphic structure, is added to the page by the following
process.  A "stencil" representing the object is obtained, scaled to
size, rotated for orientation, and positioned at a desired location on
the page.  An opaque "ink" of any color is then "painted" through the
stencil, overwriting anything that is currently present on the page.
Colors do not mix, they overwrite.

Both languages maintain a set of imaging control parameters in an array.
Both provide operators that are used to change the state of these

Both languages provide for formally structured representations of fonts.
These representations include font name and font metrics as well as font
shapes.  Both languages enable the creation of fonts with arbitrary
character generation techniques, subject only to the constraint that the
generation process is describable within the language.  PostScript
includes language mechanisms for the control and use of a font cache
which dynamically stores raster encoded character instances.


In spite of their common ancestry the two languages do contain some
major and significant differences.  These are catalogued in the
following paragraphs.  The general sequence of presentation is to
present the strengths of Interpress first, followed by those of

Interpress Strengths

Total Environment

Interpress has all of the essential qualities of a stand-alone language.
However, Interpress has been designed so that it also fits well into an
applications and network environment such as the total XNS environment.
PostScript is a purely stand-alone language.  It makes little, if any,
explicit provision to deal with its environment.  However, it does
contain a number of capabilities which would enable it to do so.  Their
explicit use for this purpose is not described within the available
Adobe documentation.

Examples of the integrability of Interpress with its environment abound.
It contains a sequenceInsertFile function that enables the inclusion of
files accessible to the printer within its total environment.  Such
files can contain Interpress masters, fragments of interpress masters,
or even printer dependent object code that represent previously
decomposed Interpress masters, e.g. forms.  The printer can utilize the
full XNS path name so that an Interpress master can reach out into the
entire XNS universe that is attached to the printer network.  Interpress
takes explicit steps to establish a universal name space and to provide
for a central registry mechanism for the distribution and control of
names in this space.  This provides a means for establishing a uniform
environment for an extended family of distributed printers.

Interpress contains printer instructions (see below) that are merged
with those transmitted by the Printing Protocol that is used to invoke
the printing of a document.  This enables the document to contain some
printing instructions that are document dependent, and the transmission
protocol to provide some printing instructions that are user dependent.
Interpress makes provision for the interrogation of the printer
environment from the master so that the master can adapt itself to that

Page Independence

Probably the most important difference between the two languages is the
fact that an Interpress document possesses a well-defined structure.
Among the attributes of this structure is that it guarantees page
independence.  Page independence means that the language description of
each page is totally independent of that of any other page.   Ensuring
page independence is critically important for a number of reasons.  It
enables the decomposition and printing of documents in arbitrary page
order.  (Many printers find it desirable to print last page first.  Full
duplex printing may require unusual page printing sequences.)  It
enables the creation of utility routines to manipulate Interpress
documents.  Such routines can perform such tasks as creating a new
Interpress master by pulling together pieces from existing masters, or
creating two-up, head-to-toe, or signature masters from existing
masters.  Because of page independence these operations can be executed
without any consideration of the Interpress representation of the
document.  They need only parse the Interpress master to locate the page
breaks which are clearly delimited.  

PostScript documents are specifically free form and unstructured.   The
language enables page independence to be achieved through externally
imposed programming disciplines, but the language itself provides no
such guarantee.  In fact, in the absence of a carefully controlled
programming discipline it is practically certain that PostScript defined
documents will not exhibit page independence.

Printer Instructions

Interpress contains an extensive set of Printing Instructions.  These
instructions enable the master to control the actions of the printer,
e.g. to invoke two-sided printing or special finishing such as stapling.
They also provide information necessary for the effective use of the
printer within a multi-user environment, e.g. who printed the document,
what its name is, whom to charge for the printing, the provision of
passwords to control who can authorize the final printing, and so on.
Printing Instructions also enable the declaration of resources that the
document will require, e.g. the files that it will use, the fonts and
font sizes that it will use.  Not only do Printing Instructions enable
the control of the printing environment, they also enable an up-front
determination of the ability of a given printer to print a document
and/or enable it to gather the resources it needs to do so in the most
efficient fashion.  These might be used by the printer itself, or by a
printing environment manager/dispatcher who determines which of a number
of printers should receive the document for printing.  PostScript makes
no explicit provision for any of these functions.

Priority Important

The availability of multiple colors with opaque printing quality means
that rules must be established for the printing of overlapping objects
of different colors.  PostScript takes no explicit cognizance of this
fact.  The unambiguous appearance of the final page can only be obtained
if one assumes that PostScript defined objects are printed in the
sequence in which they are generated.

Some printers may not find it convenient to print objects in the order
in which they are created within the master.  Interpress contains an
imaging parameter, priorityImportant, that is used to designate when the
printing sequence must match the Interpress presentation sequence.  When
priorityImportant is not true the printer is free to image objects in a
sequence of its own choosing.  This can increase printer performance in
many cases.


Interpress specifies a compact encoding for the transmission of a
document.  This encoding reduces to one or two bytes the number of bytes
required to designate an Interpress operator.  It also includes special
encoding notations for compact representations of sequences of literals.
These encoding techniques substantially reduce the number of bits
required to either store or transmit a document.  Simple utility
routines are available for the bi-directional translation from this
compact format to full English language form ASCII character
representations suitable for human usage.  PostScript only employs this
latter form of more verbose representation.

PostScript Strengths

File Handling

PostScript contains a much more extensive and more powerful set of file
handling capabilities than those provided within Interpress.  This
reflects the viewpoint of the language that the creator is closely
coupled to a dedicated printer.  It permits the master to take over
control of the printer's file resources at print time.  The Interpress
philosophy of multiple printers, highly decoupled in space and time, and
possibly with different file handling techniqes precludes this
capability.  The Interpress philosophy of not letting the document take
over the machine resources also precludes this capability.

General Purpose Programming Capability

PostScript provides a much richer set of general purpose processing
capabilities.  It provides a programmer wth a rich set of arithmetic,
control, looping, string processing, conversion between object types,
signalling, etc., operations.  Its use of full ASCII text representation
makes it convenient for human programmers to write programs in the
language.  All of this is in keeping with the PostScript viewpoint
previously described.  Use of these capabilities can impose heavy
processing loads on the printer.

Interpress provides a sufficiently rich set of general purpose
processing capabilities to meet the needs of the environment for which
it was designed.  It has been explicitly constrained in these
capabilities so as to reduce the processing load on the printer, and, in
keeping with the Interpress philosophy, force the processing load to the

Procedure Calling

Both PostScript and Interpress provide procedure calling capabilities.
Both make provision for the establishment of a working space containing
local variables for the procedure.  Both make provision for saving and
restoring the state of the imager control parameters so that the side
effects of procedures can be well controlled.  Interpress very tightly
couples all of these effects in its procedure creation and calling
mechanisms.  PostScript provides a more flexible, and in many cases more
efficient, capability by separating these effects.

Graphic Imaging Capabilities

PostScript has a much greater set of graphic image creating capabilities
than that of the currently released version of Interpress.  These
include the ability to invoke circles and Bezier curves in either solid
or dashed representations.  PostScript also includes a clipping region
capability, i.e. a defined curve whose interior defines the region in
which the current imaging is permitted to occur.  PostScript contain a
richer set of line joining constructs, including mitering, bevelling,
and rounding.

The currently released version of Interpress only contains the ability
to generate straight line bounded graphic constructs with mitering and
rounding at line joins.  Circles, conics, and Bezier curves are included
in Research Interpress, and are scheduled for later releases of the
published language.  Thus, the two languages will ultimately be brought
into much closer alignment in their graphic imaging capabilitapabilities
now, and Interpress won't do so until later.


In summary, Interpress was designed for use by a large variety of
document creation and document  output devices, to be suitable as a
general purpose electronic printing standard, a language for printers
capable of high performance.  It was not designed as a general purpose
composition language to be used directly by a programmer.  The
Interpress model assumes that the user creates Interpress Masters via
any variety of document editing and composition systems, including
graphics composition languages.  It is clearly meant to be a printing
standard with emphasis on an organization that presents a clear
separation between theprocesses that Xerox believes belong in the
creation domain, and processes that Xerox believes belong in the
printer's domain.  Postscript, on the other hand, appears to have been
designed for use both as a programmers composition language and as a
language for printers in a tightly coupled stand-alone configuration.
It does not draw clear distinctions between creation domain processes
and printer domain processes.  It is an excellent language for full
capability graphics composition applications.Relay-V

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