#9593 - Part 21 - Jean MAAS

@ Luc DUPUIS - Éditions Chantraine (#5590) [Mactivity 02/96]
Concerne : Language PostScript

<< Comment interpréter les textes "Info PostScript" qui se créent après une impression ? Y a-t-il une bible du PostScript ou plus simplement un guide sommaire pour la traduction des situations les plus courantes ? >>

Voici un autre texte qui peut aider à comprendre:

<< PostScript
PostScript is a page-description language developed by Adobe Systems. It tells computers how to render pages of type and graphics at any resolution on a wide variety of screens, printers and other devices. Virtually all quality PostScript fonts use what's called the Type 1 font format. That's a proprietary format, and until March, 1990, only Adobe and companies who purchased licenses from Adobe could create Type 1 fonts. [The same was, and is, true of PostScript printers. They also require a license from Adobe. Printer licenses are expensive, adding hundreds (it used to be thousands) of dollars to the cost of PostScript printers.] Adobe finally released their specifications for Type 1 fonts in response to the threat posed by TrueType. After the Type 1 specifications were released, every font vendor started releasing Type 1 fonts. You can create your own Type 1 fonts using tools like Altsys's Fontographer and Letraset's FontStudio.
To use a Type 1 font, you need two files: a bitmapped screen font, (required for screen display and Font menus), and a Type 1 outline file. The second file, the one with the actual outline data, is often called a printer font, because it's used mainly for producing output on a PostScript printer or other PostScript output device.
You install the screen font in normal fashion for bitmap fonts, either by dropping it onto the System file icon or using a font manager (like Suitcase II) to install it in the System temporarily. Generally, Type 1 printer fonts are installed by simply dropping them onto the System Folder icon. More elegant solutions are available, and I'll discuss them in the article on font hints, tips and utilities on Power User's Tool Kit 15.0.
When they first appeared, Type 1 fonts had a serious drawback. Their bitmapped screen fonts were poor imitations of the actual output. Performing typographic adjustments on the typically jagged text was often a horror. Adobe solved this problem by developing Adobe Type Manager (ATM). You place this PostScript-based INIT (Extension) in the System Folder. It uses the outlines stored in the Type 1 printer fonts to produce sharp, legible text on-screen at any size. ATM eliminates the need for most screen fonts (you need install only a single size for each font you want on the Font menu). This wonderful tool not only makes your Type 1 fonts look good and scale accurately on-screen, it also lets you output good-looking text on non-PostScript printers, such as the ImageWriter and DeskWriter.
ATM has some drawbacks. It tends to break easily when Apple issues new versions of the System software and you should always use the latest version (3.0 when this was written). It also tends to conflict with other INITs, Extensions and Control Panels. If you're having System problems, try running without ATM installed.
Adobe has just released SuperATM (technically ATM version 3.5). This utility extends functionality and is not a necessary upgrade. I'll discuss it in more detail on Power User's Tool Kit 15.0.
You might still have, or be offered, PostScript fonts in a format called Type 3. Font vendors used this format to create PostScript fonts before Adobe released the Type 1 specifications. Type 3 fonts usually don't look as sharp as Type 1 fonts, their files are larger, and they don't work with ATM. Type 3 is a dying format. Stay away from Type 3 fonts, no matter how cheap they are.

TrueType
TrueType font rendering technology is an integral part of System 7 (also available as an INIT for Systems 6.0.5 and 6.0.7). It was developed as part of the TrueImage page rendering system that Microsoft intended to compete with PostScript. Microsoft appears to have given up on TrueImage, but TrueType doesn't require it and is thriving in the Windows arena, especially as part of Windows 3.1.
TrueType fonts are easier to install and use than Type 1 fonts, as they don't require bitmapped screen fonts and separate printer fonts. A single outline font file provides all the information necessary for both screen display and printing, in every point size, and at every resolution. To install a TrueType font in System 7, just drag the font icon over the System Folder icon - the font is automatically installed in the System file or, in the case of System 7.1, in the Fonts folder. You can even see a type sample by double-clicking on the font icon. To install TrueType fonts in Systems 6.0.5 and 6.0.7, use version 4.1 of Font/DA Mover. Earlier versions will not work.
If you plan to use TrueType fonts in documents that were originally created using PostScript Type 1 fonts, and you don't want slight differences in character spacing to cause lines of text to break differently, install the original bitmap fonts that came with the Type 1 fonts, along with the TrueType versions. The font data stored in the bitmap files will control character widths, kerning, leading, and other formatting details.
It's a good idea to install one or two bit-mapped fonts in commonly used sizes (10 and 12 points, for example) in both PostScript and TrueType. Bitmapped fonts often look better on-screen, particularly at small point sizes, and the Mac can display a bitmap much faster than it can build the image of a font from an outline file.

Hints and Other Mysteries
Both PostScript and TrueType print using outline font files and a rasterizer - software that performs the three steps necessary to interpret or rasterize the outlines: scaling, hinting, and scan conversion. The rasterizer can reside in various places: in PostScript printers; as an application (ATM, Freedom of Press, or the TrueType INIT); installed in the Mac's System file (TrueType), or within the font file itself (TrueType again).
TrueType fonts contain part of the rasterizing code whereas PostScript Type 1 rasterizers are always separate programs. No matter where the rasterizer is, it works the same way. The scaling software fetches each character's outline and sizes it properly. Then, the hinting routine matches the resized outline to a theoretical grid of pixels that's been scaled to match the resolution of the specified monitor or printer. Finally, the scan converter turns the outline into a bitmap by switching on just those bits that fall inside the character outline.
The most complex part of the process is the hinting routine. Hints are instructions for adjusting a character's outline to achieve the most accurate, legible image at every resolution. Each character has its own set of hints.
Hints are particularly important at small point sizes and at low resolutions. When only a few pixels are available, the character's outline must be moved or altered to fit the pixel grid, a technique called grid fitting. Without hints, a character's thin stems could land between two pixels and not print or display at all, or nearly identical stems might print in different weights. Hints specify the type and amount of grid fitting that can be applied to a character. As the type size and/or the output resolution increase - providing more pixels for displaying the character - the effects of hinting diminish until no effect is visible. Hints are rarely needed at resolutions above 600 dpi.
A major difference between PostScript Type 1 and TrueType fonts is in their hinting languages. Type 1 hints are simple instructions that tell the screen rendering software how the character elements should be distorted for best low-resolution appearance.
TrueType hints (called instructions) are more complex. They give greater freedom to the type designer, who can choose to alter more parts of each character with greater precision. TrueType also supports more sophisticated instructions that go beyond preserving character shapes at low resolutions.

The Printing Line Up
When you print a document containing Type 1 fonts, the printer driver checks first to see if the requested fonts are in the printer's ROM or RAM. If so, the fonts are rasterized and printed by the printer's built-in PostScript rasterizer. If the fonts aren't available in the printer, the driver downloads the necessary printer fonts from your Mac or a hard disk attached directly to the printer. If you use ATM and have the printer fonts on your hard disk, you also can print Type 1 fonts on non-PostScript printers such as the ImageWriter and StyleWriter. ATM will do the rasterizing.
Printing TrueType fonts is equally straightforward - as long as you aren't using a PostScript printer. For a 72 dpi printer (for example, an ImageWriter set to Faster), the printer driver sends the TrueType screen image straight to the printer. On QuickDraw printers with higher resolutions, the font is first rasterized at the printer's resolution and then sent to the printer.
If you're using a PostScript printer, the printer driver will do almost anything to avoid rasterizing the TrueType outlines. This frequently slows the printing process noticeably.
A PostScript printer can't print a TrueType font at all if you install and use the TrueType version of a Type 1 font that's in the printer's ROM (it came installed in the printer). Helvetica is a good example. If you install and use the TrueType version of Helvetica on your Mac, regardless of what you see on-screen, the PostScript Type 1 version of Helvetica that's in the printer's ROM is what will be printed on paper. It may be significantly different from what you see on-screen. >>

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