All Databases MacTech Vol 02-1986

Laser Print DA

Volume Number: 2

Issue Number: 2

Column Tag: C Workshop

Laser Print DA for Postscript

By Mike Schuster, Software Engineer, MacTutor Contributing Editor

Mike Schuster wins $50 for the outstanding program of the month with this

feature article. Congratulations Mike!

Using a POSTSCRIPT Server on AppleTalk

Most Macintosh applications generate printed pages by issuing a sequence of

QuickDraw calls which are intercepted and translated into printer dependent commands

by Apple's Printing Manager. This technique has the advantage that an application

never needs to know what kind of printer the user has connected to the Macintosh. It

also allows the application to write to the screen and the printer with a single device

independent routine. On the other hand, it does not provide the application access to the

extensive page description capabilities nor the wide range of printer resolutions

available in modern electronic printing systems, such as those embodied in Adobe

Systems' POSTSCRIPT page description language, Apple's LaserWriter™ laser printer

and Linotype's Linotronic™ 100 typesetter. This is because Quickdraw is much more

limited in it's abilities than is POSTSCRIPT, and hence only a fraction of the capability

of a POSTSCRIPT machine is needed to interpret a set of Quickdraw commands.

In this article, I show how your application may overcome QuickDraw and the

Printing Manager limitations by communicating directly with a POSTSCRIPT server on

an AppleTalk network. In addition, I describe the implementation of Laser Print, a desk

accessory that con currently spools POSTSCRIPT programs you create with MacWrite,

Edit or Just Text, and sends them to the Apple LaserWriter or other POSTSCRIPT

compatibile printer. You can use this printing desk accessory to explore programming

your LaserWriter directly, using an example postscript file in this article. In future

issues of MacTutor, we will expand on postscript programming techniques with a

regular postscript column and this desk accessory will be invaluable as a way of

sending your programs over Appletalk to the LaserWriter. The Laser Print DA is

written in MegaMax C.

Deep Background by The Editor

[When a Macintosh Application prints, the text and graphics that it writes to a

quickdraw port to display on the screen are sent to a special quickdraw port used by the

printing manager. You print text and graphics by drawing into this port with

Quickdraw, just as if you were drawing on the screen. The Printing Manager changes

the pointers to the low-level drawing routines so that Quickdraw calls drive the

printer rather than the screen. In this way, MacWrite can display a what- you- see-

is- what- you- get screen version of your document with one set of routines and it can

go to either the screen or the printer thanks to the clever Printing Manager. The whole

point of the Print Manager and associated device drivers is to convert Macintosh

Quickdraw text and graphic descriptions into page bit maps that can be printed. This

design works very well with the Imagewriter.

Now comes the LaserWriter, after the Macintosh. This device is very different

from anything Apple has ever produced and one can only wonder who was the

mastermind behind it. It is so much more powerful than the Mac itself, one has to

wonder if it was an outside design. In some respects the LaserWriter is a high powered

Macintosh. Substitute POSTSCRIPT for Quickdraw. Substitute 0.5 Meg of ROM for 64K

ROM. Substitute 1.5 Meg of RAM for 128K RAM. Substitute a 12 Mhz 68000 for an 8

Mhz 68000. Connect the two with the Appletalk serial port. The problem for Apple was

how the lowly Macintosh could be made to talk to this very intelligent device with a

minimum of changes to the Mac itself.

The answer was since all Macintosh software writes to the screen (and hence the

printer via the Print Manager) via Quickdraw, those same routines could drive a

LaserWriter by converting Quickdraw descriptions into POSTSCRIPT. The purpose of

the Laser Prep file is to convert quickdraw commands to POSTSCRIPT. The Laser Prep

file is downloaded into the LaserWriter as a POSTSCRIPT application running within

the LaserWriter. Normal Macintosh applications present Quickdraw documents to the

Printing Manager, which invokes the LaserWriter device driver, downloads the Prep

File into the LaserWriter (Initializing Printer...) which in turn converts quickdraw to

POSTSCRIPT, which is interpreted by the built-in POSTSCRIPT interpreter within the

LaserWriter (the 0.5 Meg of ROM) and finally converted to a 300 dot per inch page bit

map (stored in the 1.5 Meg of RAM not taken up by Laser Prep, the POSTSCRIPT page

description, and fonts) and printed! This double conversion is one reason the

LaserWriter takes so long to print initially. It also explains why the LaserWriter often

runs out of memory with some applications like Pagemaker, which uses a larger Prep

file. In fact, the LaserWriter may be even more memory bound than the 512K

Macintosh, if you can believe that! So limited is memory space that the new Laser Fonts

from Adobe will probably require the new LaserWriter II to be useful, which is

supposed to have 2 meg of RAM. A conversion kit is rumored to be available for

upgrading current LaserWriters.

By sending a POSTSCRIPT file directly to the LaserWriter, you avoid having to

down load Laser Prep, eliminate quickdraw to POSTSCRIPT conversion, free up RAM in

the printer, and generally speed everything up. Plus, you avoid limitations of trying to

represent in Quickdraw a much more extensive capability of POSTSCRIPT. Commands

within the POSTSCRIPT file tell the printer that the file is to be interpreted and not

printed. The fact that Aldus Corp. felt the need to write their own Prep File shows the

limitations of trying to go from Quickdraw to POSTSCRIPT. Aldus needed some of the

capabilities of POSTSCRIPT that the Apple Prep file would not supply. The very thin

line in Pagemaker is one such feature that could not be represented without a new Prep

file. This is not a happy situation since now you have two companies supporting two

different prep file conventions. The smoothing algorithm of Apple is an example of how

the user is stuck when two companies support a competing low level routine. The Aldus

Prep file did not support smoothing until they had obtained a license from Apple.

Bill Bates of Knowledge Engineering saw all this conversion nonsense and

recognized that the only way to really tap into the power of the LaserWriter was

directly via POSTSCRIPT. His Just Text and LaserTool products create postscript files

directly in the traditional typesetting method. The advantage of this is POWER. Direct

control over every feature of POSTSCRIPT, with no Laser Prep quickdraw to

POSTSCRIPT conversion hassles to slow things down. This brings up an interesting

confrontation. Aldus follows the Macintosh guidelines using Quickdraw and converting to

POSTSCRIPT. Would it be possible to go the other way and create a program that would

interpret POSTSCRIPT into Quickdraw so that any POSTSCRIPT file could be displayed

on the screen as a 'soft' LaserWriter Preview? We leave to the developer to ponder the

implications of such a program.

POSTSCRIPT EXAMPLE

Figure 1 shows a drawing created on the LaserWriter by the POSTSCRIPT

program listing in figure 2. As you can see, neither MacWrite, Word or Pagemaker

could possibly create an image such as this, and yet for the LaserWriter, this is a snap!

POSTSCRIPT is a programming language very similar to Forth. It uses reverse polish

notation and a stack like Forth does. Arguments are pushed on the stack first, followed

by the command that executes them. This requires the programmer to think backwards

in some respects. Adobe Systems has just published two very good books on

POSTSCRIPT programming. They are a tutorial and a reference manual, both available

in Bookstores from Addison-Wesley. Look for a blue and red book under the title of

POSTSCRIPT. This example is taken from the tutorial book on page 98. The lines

beginning with %% are comments. The first line follows the Adobe specifications of %!

to tell the interpreter within the LaserWriter that what follows are POSTSCRIPT

commands to be executed rather than a bunch of text to be printed. This file can be

executed by typing it in as a text file with Edit or MacWrite and using the Laser Print

DA to down load it to the LaserWriter. After using postscript directly, I can see that

what is now needed is some way that artistic POSTSCRIPT programs can be "placed" in

page makeup programs like Pagemaker so that they can be edited and arranged with

other text and illustrations. Of course Just Text does this, but without the advantage of a

screen display representation of the final output. Well, enough of the introductions to

the world of POSTSCRIPT programming. Let's get on with solving the hole Apple left

us-- a utility to send POSTSCRIPT files directly to the LaserWriter. Our thanks to

Mike for this very important and timely article. -Ed.]

A POSTSCRIPT Server at Work

When connected to an AppleTalk network, the LaserWriter functions as a server

for the execution of POSTSCRIPT programs sent to it by applications running on client

Macintosh workstations. The LaserWriter begins service by listening for an AppleTalk

connection request from a client. It then executes the POSTSCRIPT program sent to it

over that connection. Any error messages or other output produced by the program,

other than the printed pages themselves, are transmitted back to the client over the

same connection. When the LaserWriter receives an end-of-file indication from the

client, it sends a matching end-of-file back to the client and terminates the execution of

the program. After receiving the matching end-of-file indication, the client closes the

connection and frees the server.

While the LaserWriter is busy servicing one connection, it refuses any

additional connection requests. This refusal causes the requesting clients to queue up

and wait for the server to become free. As soon as the current connection closes, the

server accepts the request from the client that has waited the longest period of time.

[This problem shows that a LaserWriter is really of no use to more than one person

without a File Server to manage this queue of documents. Otherwise, everyone on the

network sits around waiting for the printer to free up before he can get on with his

work. Which brings us to the next great hole in the Apple Office-- a file server that

spools files to the LaserWriter, allowing networked Macintosh computers to do

something else after clicking print in the File menu! -Ed]

The communication between the LaserWriter and its client workstations is

accomplished by means of the AppleTalk Printer Access Protocol (PAP). Apple

specified this protocol in its publications Inside LaserWriter, Inside AppleTalk and the

AppleTalk Manager chapter of Inside Macintosh. [See MacTutor, September 1985 for