PostScript fonts are font files encoded in outline font specifications developed by Adobe Systems for desktop publishing and digital typesetting. This system uses Adobe PostScript to encode font information.
Type 1 and Type 3 fonts, though introduced by Adobe in 1984 as part of the PostScript page description language, did not see widespread use until March 1985 when the LaserWriter was introduced by Apple Computer as the first laser printer to use the PostScript language. At the time, the outline fonts were resident only in the printer, and the screen used bitmap fonts as substitutes for outline fonts.
Although originally part of PostScript, Type 1 fonts used a simplified set of drawing operations compared to ordinary PostScript (programmatic elements such as loops and variables were removed, much like PDF), but Type 1 fonts added "hints" to help low-resolution rendering. Originally, Adobe kept the details of their hinting scheme undisclosed and used a (simple) encryption scheme to protect Type 1 outlines and hints, which still persists today (although the encryption scheme and key has since been published by Adobe). Despite these measures, Adobe's scheme was quickly reverse-engineered by other players in the industry. Adobe nevertheless required anyone working with Type 1 fonts to license their technology.
Type 3 fonts allowed for all the sophistication of the PostScript language, but without the standardized approach to hinting (though some companies such as ATF implemented their own proprietary schemes) or an encryption scheme. Other differences further added to the confusion.
The cost of the licensing was considered very high at this time, and Adobe continued to stonewall on more attractive rates. It was this issue that led Apple to design their own system, TrueType, around 1991. Immediately following the announcement of TrueType, Adobe published "Adobe type 1 font format", a detailed specification for the format. Font development tools such as Fontographer added the ability to create Type 1 fonts. The Type 2 format was used as a basis for the modern OpenType format, which has since superseded all legacy PostScript fonts.
By using PostScript (PS) language, the glyphs are described with cubic Bézier curves (as opposed to the quadratic curves of TrueType), and thus a single set of glyphs can be resized through simple mathematical transformations, which can then be sent to a PostScript-ready printer. Because the data of Type 1 is a description of the outline of a glyph and not a raster image (i.e. a bitmap), Type 1 fonts are commonly referred to as "outline fonts," as opposed to bitmap fonts. For users wanting to preview these typefaces on an electronic display, small versions of a font need extra hints and anti-aliasing to look legible and attractive on screen. This often came in the form of an additional bitmap font of the same typeface, optimized for screen display. Otherwise, in order to preview the Type 1 fonts in typesetting applications, the Adobe Type Manager utility was required.
Type 0 is a "composite" font format - as described in the PostScript Language Reference Manual, 2nd Edition. A composite font is composed of a high-level font that references multiple descendent fonts.
Type 1 (also known as PostScript, PostScript Type 1, PS1, T1 or Adobe Type 1) is the font format for single-byte digital fonts for use with Adobe Type Manager software and with PostScript printers. It can support font hinting.
It was originally a proprietary specification, but Adobe released the specification to third-party font manufacturers provided that all Type 1 fonts adhere to it.
Type 1 fonts had been natively supported in Mac OS X (up to macOS 10.14), and Windows 2000 and later via the GDI API. However, they are not supported in the Windows GDI+, WPF or DirectWrite APIs. Support for authoring Type 1 fonts will be officially retired by Adobe in January 2023.
Type 2 is a character string format that offers a compact representation of the character description procedures in an outline font file. The format is designed to be used with the Compact Font Format (CFF). The CFF/Type2 format is the basis for Type 1 OpenType fonts, and is used for embedding fonts in Acrobat 3.0 PDF files (PDF format version 1.2).
Type 3 font (also known as PostScript Type 3 or PS3, T3 or Adobe Type 3) consists of glyphs defined using the full PostScript language, rather than just a subset. Because of this, a Type 3 font can do some things that Type 1 fonts cannot do, such as specify shading, color, and fill patterns. However, it does not support hinting. Adobe Type Manager does not support Type 3 fonts, and they are not supported as native WYSIWYG fonts on any version of Mac OS or Windows.
Type 4 is a format that was used to make fonts for printer font cartridges and for permanent storage on a printer's hard disk. The character descriptions are expressed in the Type 1 format. Adobe does not document this proprietary format.
Type 5 is similar to the Type 4 format but is compressed for storage in the ROMs of a PostScript printer. It is also known as a CROM font (Compressed ROM font).
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