Key functions of TrueType rasterization

1. Run the 'fpgm' program, initializing all the subroutines to be called later by the 'prep' program and the glyph programs.

1. Scale the 'cvt' reference values to the relevant pixel distance for this ppem;
2. Run the 'prep' program to adjust these values, and set other information for all glyphs at this size.

1. Decode the glyph from its compressed representation in the TrueType file;
2. Scale the glyph according to the desired point-size and output device, so that its coordinates are measured in pixels, to an accuracy of one sixty-fourth of a pixel;
3. Execute the glyph's hinting program, with the effect of distorting the glyph's control points;
4. Scan-convert (fill) the "grid-fitted" hinted outline, making a bitmap image of the glyph (scan-conversion may involve drop-out control, a mechansism to preserve pixels in very thin parts of glyphs);
5. Pass the bitmap to the system, which stores it in a cache for efficient re-use.

Implementations

1. Mainstream implementations
   • In March 1991, an extension to Macintosh System 6.0.5 became the very first publicly-available TrueType rasterizer. In May of the same year, the new System 7.0 integrated TrueType fully.
   • Windows 3.1, version 1.00 of the Microsoft TrueType rasterizer (also in Windows 3.11 and Windows NT 3.1). Because much of the code was 16-bit, it had problems where complex glyphs would not display at some sizes and revert to the "missing character".
   • Windows NT 3.5, version 1.50 of the Microsoft TrueType rasterizer.
   • Windows 95, version 1.65 of the Microsoft TrueType rasterizer, 32-bit throughout. This version corrects problems with complex glyphs previously inherent in Microsoft's rasterizer. The rasterizer is capable of grey-scale rendering on SVGA screens, which can make some fonts much more legible and aesthetic (this feature is unlocked using the Windows 95 Plus! pack).
   • The Hewlett-Packard LaserJet 4 printer, and subsequent printers, handle "PCLETTO" fonts (PCL Encapsulated TrueType Outline). TrueType fonts can be downloaded, or installed in printer EPROM, and interpreted directly.
   • The rasterizer for Type 42 fonts in some PostScript Level 2 printers. Type 42 fonts are TrueType fonts with a rather trivial PostScript "wrapper", as described in Adobe's Type 42 specification, in PDF format.
   • Apple have developed a downloadable TrueType rasterizer to work on printers having a 680x0 processor, but it is not publicly available. (Mentioned in the Type 42 spec.)

2. Rasterizers for Original Equipment Manufacturers
   • Font Fusion incorporates Sampo Kaasila's super-fast T2K with Bitstream's rasterizer for Type 1, Speedo, Intellifont, and TrueDoc PFR WebFonts. See also the Bitstream Font Fusion page.
   • Agfa UFST (Universal Font Scaling Technology) from Agfa handles PostScript Type 1, TrueType and Agfa IntelliFont. Installations include a custom version for the Wordworth word processor on the Amiga.

3. Shareware/Freeware TrueType rasterizers
   • FreeType is a free TrueType rasterizer, by David Turner, Robert Wilhelm and Werner Lemberg. Written in C, it does interpret instructions. There's a Java port of FreeType by Roland Paterson-Jones.
   • Pascal source code does not interpret TrueType instructions.
   • A Chinese rasterizer: it does not interpret TrueType instructions.

Other references

• The TrueType Rasterizer (Microsoft)

• The Font Engine (Apple)

• How the Font Manager Renders Outline Fonts (Apple)