Date: Sun, 6 Oct 91 14:14:18 PDT From: kehoe@fortuity.sf.ca.us (Daniel Miles Kehoe) To: timbl@nxoc01.cern.ch (Tim Berners-Lee) Subject: Interchange PostScript Format Tim, I'm sending you the complete text of an article from the Seybold Report on Desktop Publishing. I believe you'll find it very interesting. Incidentally, it is copied without permission. Regards, Daniel Kehoe P.S. Any new developments with WWW? [Note this is copyright material: Subscribe to the Seybold report rather than print this! -Tim]

Adobe unveils editable PostScript interchange format

Combines Multiple Master fonts, Illustrator 3.O file format, viewer

Adobe Systems announced that it has developed a PostScript-based technol- ogy for exchanging formatted elec- tronic documents. Adobe's approach to this problem draws on several existing technologies, including the Illustrator file format, Multiple Master fonts and a newly announced font rasterizing chip. However, Adobe is not announcing any specific product at this time.

When Adobe's vision is fully fleshed out with deliverable products, it promises to let you move documents freely around a network, viewing them on a wide range of computer platforms. Although these documents will be in final form and ready for presentation, they will continue to be editable.

As John Warnock was expected to sketch out in a speech to the Seybold Computer Publishing Conference this week, much of the software needed to do this already exists. The remaining problems are partly legal and licensing issues, and partly a continuing process of developing standards.


At the heart of Adobe's approach is the Interchange PostScript Format. This is based on the current Illustrator 3.0 file format, which is a subset of the PostScript language. The existence of Illustrator demonstrates that it is possible to have a file format capable of describing and outputting complex pages without losing the abil- ity to edit those pages. You can think of Illustrator as a complex editing pro- gram whose native file format expresses the exact appearance of a document.

Illustrator 3.0 does not handle all document types, of course. It knows nothing of multipage documents, style sheets, reference constructs or scanned images. In IPF (and presumably in fu- ture Illustrator-like products), Adobe clearly intends to extend the range of objects to cope with these needs.

Platform independent

A major Adobe goal is that the author or reader of a document should not have to care what kind of computer other readers or editors usc. That is, IPF needs to be independent of any particular display, central processor or operating system.

This is relatively straightforward; PostScript was from the beginning de- signed to be device independent. At the Seybold Conference, Warnock was expected to demonstrate the new tech- nology on a Mac, PCs running DOS and Windows, and a Unix workstation.

Font woes

If a remote viewer is to read a formatted text document you have just created, it needs to have the same fonts you do, right? And since it probably doesn't have all of your fonts, you either must restrict yourself to using least-common-denominator fonts or send the fonts along with the docu- ment. That, in turn, swells the size of the file and raises the hackles of the font vendors. It probably also gives your corporate counsel an ulcer just thinking about the potential copyright- infringement lawsuits.

Adobe's solution here is two- pronged. First, Adobe says, all you need to send is the font metrics--the character widths and kerning tables. At the receiving end, a document viewer program can use Multiple Master fonts to generate good-looking type that ex- actly matches the formatting of the original text. In fact, Adobe claims that for most normal text faces, the on- screen appearance of the synthesized fonts will be indistinguishable from the original fonts.

However, there is a fall-back approach: send the fonts. Adobe is work- ing with other major font vendors to modify the shrink-wrap font license to permit distribution for read-only or print-only use. Adobe will license this use for its own fonts, and the ITC (the licensing agency for a substantial fraction of everybody's font libraries) has agreed to do so as well. The other font houses will probably go along sooner or later.

(A third method--sending along the name of the font and its metrics- depends on a standardized naming scheme and presumes font-substitution smarts by a viewer at the other end. This method is being considered for ISO interchange formats.)

New PostScript drivers

A true interchange solution must work for all document-creation programs: word processors, spreadsheets, CAD and illus- tration packages, database report genera- tors and so on. Just about all of these products can print to a PostScript de- vice, and they usually go through the system printer driver.

Adobe announced last winter that it would be developing the PostScript drivers for future Apple and Windows operating systems. So it will simply add to its drivers the ability to generate an IPF file. At that point, every existing application will be able to create inter- change-ready documents.

There is an installed base of older applications that generate their own PostScript code and so will not benefit from Adobe's new device drivers. A program that converts the full range of PostScript into the more restrictive IPF would be tantamount to a complete PostScript interpreter. This is exactly how Adobe plans to assure backward compatibility.

However, Adobe points out that you wouldn't need this interpreter in every workstation. You could run it as a task at one station on the network-- in the mail server, for example.

Viewer apps

The final part of Adobe's strategy is to develop viewer applications to display and print IPF documents. There might also be appli- cations that can annotate or edit these files. Clearly, these programs will be specific to a particular processor and operating system. Each user interface will have to be separately developed. However, a great deal of the code can be shared across platforms.

Adobe has not given any hints about when such viewers might be released. However, the core code has already been written and demonstrated, so we don't expect to wait years. We might hope for an announcement next spring; but Adobe has politely declined to speculate on that subject.

ATM on a chip

Among the products Adobe is announcing this week is a Type 1 font rendering chip that can rasterize 2,000 roman characters per second. No end-user products using this chip exist yet. But like Destiny Technology's RIDA chip (see p. 40), it could be used in graphics boards, print- ers or computer motherboards.

Adobe says that text rendering is so fast with its new "ATM chip" that you can continuously zoom a page with no perceptible jerkiness--new full- page bitmaps can be generated at speeds comparable with the monitor's frame rate. This kind of hardware accel- eration will clearly be vital in gaining consumer acceptance of PostScript for viewing documents.

Capturing the semantics

Post-Script, as it stands, can only describe the appearance of a page; it can express all the QuickDraw or Windows GDI information, but it cannot show the relationships of objects (the formulas in a spreadsheet or the element groupings of a CAD drawing, for example). IPF can be extended to express object se- mantics. However, this partly depends on extending the application programs themselves to support document inter- change, and so it will be an evolution- ary process.


Adobe is in a better position than any other vendor to promulgate a document interchange format. It is a tireless champion of PostScript- based solutions, and it is better positioned to deliver products to the mass market on multiple platforms than any of the platform vendors. (DEC, IBM, Microsoft and Apple all have some stake in developing interchange formats.) Aside from formats from these vendors, the only other document interchange format is the Document Style Semantics and Specification Language (DSSSL), a draft international standard that is still too new to have any products using it yet. Although DSSSL is comprehensive, it has no vendor with a financial stake in its success to champion its cause. It is also very complex, so it may never have an implementation in mass-market products.

Adobe's format will be implemented immediately by Adobe, and its status as a "standard" could change if it meets with popular acceptance. (After all, PostScript has become an ISO standard.) Everything depends on how well Adobe's implementations meet users' requirements for electronic document interchange.

Seybold Report on Desktop Publishing; October 2, 1991