laser-lovers@uw-beaver (01/29/85)
From: adobe!taft (Ed Taft) <adobe!taft@su-shasta.arpa> It is very gratifying to see the interest that has been generated among laser-lovers by the announcement of the Apple LaserWriter (as well as PostScript support for the Allied Linotronic 300 and Linotron 101 typesetters and beta-test delivery of the QMS 1200A PostScript printer). Naturally, we at Adobe Systems want to disseminate as much information as we can about these printers and about PostScript. However, Adobe is a small company; we don't have the resources to devote to detailed discussions (technical or otherwise) via net mail. Furthermore, since laser-lovers is distributed via the Arpanet, we are obligated not to discuss product plans or to promote products in this forum. Adobe's participation in laser-lovers discussions, therefore, will be limited to answering technical questions that are of general interest. Periodically (probably not more than once a week), I or some other member of Adobe's technical staff will respond to technical questions that have accumulated, concentrating on those that can't easily be answered by consulting the PostScript Language Manual. We won't respond to discussions about the relative merits of printer products. While I am on the subject of documentation, I should mention that the PostScript Language Manual, PostScript Cookbook, and Adobe font information (about 300 pages total) can be had by sending $30 to: Anne Brown Adobe Systems, Inc. 1870 Embarcadero Rd. Palo Alto, CA 94303 This documentation applies to all PostScript printers and does not include any information about the LaserWriter specifically. Apple will be selling a package called "Inside LaserWriter" that includes both the PostScript documentation and some additional information about the LaserWriter, AppleTalk protocols, and some other things (including a Mac diskette with various goodies). This can be ordered by sending $75 to: Apple Computer 467 Saratoga Ave. San Jose, CA 95129 I'm told that this will be available in about two weeks. (California residents should be sure to add sales tax.) Now for some responses to issues that have arisen in the laser-lovers discussions to date. In general, the information in Brian Reid's and Richard Furuta's messages has been quite accurate. I would like to elaborate on two topics: fonts and images. The conventional wisdom has long been that you can't produce high-quality characters at 300 bpi without bit tuning. Adobe has developed algorithms for automatic font scan conversion that, in our opinion, render the conventional wisdom obsolete. The results speak for themselves. Unfortunately, I can't discuss these algorithms because they are proprietary. The algorithms can be tuned to match the properties of different printing engines, such as the Canon engine in the LaserWriter versus the Xerox engine in the QMS 1200A. That is, a given character outline is converted into slightly different bitmaps on these two machines, even though their resolutions are the same. Adobe's fonts are uniformly scaled; no attempt is made to adjust the type style when going from one size to another. Our resident typographer (Sumner Stone) tells me that the practice of designing different typefaces for different sizes, while very common in the days of metal type, has largely fallen into disuse with the introduction of phototypesetters. A major exception to this is use of "display" typefaces for headlines, posters, and the like; as far as PostScript is concerned, these are entirely different fonts. The main algorithmic transformation in common use is anamorphic scaling, i.e., scaling by different amounts in x and y. Anamorphic scaling of arbitrary graphical objects, including characters, is supported in PostScript; and nothing precludes the possibility of defining fonts that select different descriptions for different sizes. However, none of these things is done automatically for any of the standard fonts. PostScript will give perfectly coordinated results among printers of all resolutions, because the font metrics are represented as floating point numbers that are resolution-independent. Maintaining fidelity between display and printer, however, is a much more difficult problem. At a minimum, one must have display fonts that are coordinated with the printer fonts. (A set of coordinated Mac display fonts will be sold with each LaserWriter.) For best results, one must use the printer's resolution-independent metrics for making formatting decisions such as breaking lines. Then, when placing text on the display, one must keep track of the difference between display and printer widths, and distribute this error in the white space between words. The Mac graphics software does not work this way at present; so the printed results are not always perfectly faithful to the display. Next, let's discuss images, of which bitmaps are a special case. The general case is a gray-level sampled image at an arbitrary resolution. PostScript can take such an image and render it on any resolution device by means of halftoning. (The halftoning technique is entirely under user control; but the default is a halftone screen appropriate for the given printer and resolution.) PostScript's imaging capability is intended primarily for use in rendering gray-scale graphical objects and scanned images such as photographs, and only secondarily for use in rendering user-supplied device-resolution bitmaps. Nevertheless, it is possible to define characters as bitmaps. For example, the Macintosh software synthesizes bitmap characters for Mac display fonts that don't have any printer equivalent. Needless to say, the results aren't as good as those obtained from the Adobe outline fonts. In general, PostScript is designed to work best when given high-level, device-independent descriptions of the desired results. Consider, for example, the MacDraw and MacPaint graphical models. In MacDraw, all the graphical entities (characters, lines, ellipses, etc.) retain their original identities. Printing a MacDraw document on a PostScript printer involves simply generating a PostScript description of those entities, which are then rendered at the full resolution of the printer. In contrast, everything in MacPaint is immediately converted to a display-resolution bitmap. Printing a MacPaint document requires sending the entire bitmap as an image. Not only does this take a long time, but the results are crude since they are still tied to Mac display resolution. (Some interpolation or "bit smoothing" does take place, but the improvement obtained is limited.) Device independence is achieved at a cost. Producing high-quality renditions of device-independent text, graphics, and images is computationally very intensive. This is why PostScript printers such as the LaserWriter must contain such a high-powered computer. Fortunately, the programming language base enables users to take maximum advantage of this power. Ed Taft Adobe Systems, Inc. Notices: PostScript is a trademark of Adobe Systems, Inc. LaserWriter and AppleTalk are trademarks of Apple Computer, Inc. Linotronic and Linotron are trademarks of Allied Corporation