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This section was constructed from a posting by Johannes Schmidt-Fischer in Jun 1993.
All PostScript Type 1 fonts should contain a UniqueId. This is a number which should be, as the name suggests, unique (at least among the fonts that you download to the printer at any given time).
There are many PostScript fonts on the 'Net which have identical UniqeIds. If two of these fonts are downloaded to the same printer at the same time, attempts to use either font may cause the wrong characters to be printed.
In a nutshell, the reason that the wrong characters may be printed is that the printer may be storing the rendered glyphs in its font cache, addressed by UniqueID. So, if two fonts, /Foo and /Bar, both have UniqueID=5 and /Foo's 10pt ``A'' is currently in the cache, a request for /Bar's 10pt ``A'' will cause the wrong character to be printed. Rather than rendering /Bar's ``A'' from its (correct and unambiguous) outline, the printer will note that the cache contains a 10pt ``A'' for font 5 and will copy it from the cache (resulting in /Foo's ``A'' printing for /Bar).
Adobe's ``Red Book'' contains a detailed discussion of this topic.
David Lemon contributes:
There are three ways to get grey into a font. The first is to make a series of Type 1 fonts, each of which will be used for a single shade of grey (or other color). The user then sets copies of the characters on top of each other, selecting each and setting it to the shade desired. It's a bit inconvenient (and won't work in a word processor) but it gets full resolution, good hinting and gives the user lots of control. This is the approach Adobe has used in its "chromatic" fonts (as in Adobe Wood Type 3 and Copal) and is viable for both Type 1 and TrueType formats.
As an alternative, the designer can approximate shades of grey in the characters by using many little dots (a sort of halftone effect) or lines (as in cross-hatching). This leads to pretty complex characters, which may choke some rasterizers, and won't hint well. As with the first method, this is viable (more or less) for both Type 1 and TrueType.
The third method is more direct but limited. In this approach, the designer/producer creates the shades of grey in a font-editing program. The limitation is that such a font must be written in Type 3, which is a generalized PostScript format (Type 1 and TrueType recognize only solid shapes). Such a font won't be supported by ATM, so your screen display will suffer and you'll be restricted to PostScript printers. On the plus side, your greys will be rendered at the full resolution of the printer you use.