FreeType & Patents

The TrueType Bytecode Patents Have Expired!

Since May 2010, all patents related to bytecode hinting have expired worldwide. It it thus no longer necessary to disable the bytecode interpreter, and starting with FreeType version 2.4, it is enabled by default.

The affected patents were

Patent US5155805:
Method and apparatus for moving control points in displaying digital typeface on raster output devices

Patent US5159668:
Method and apparatus for manipulating outlines in improving digital typeface on raster output devices

Patent US5325479:
Method and apparatus for moving control points in displaying digital typeface on raster output devices

The ClearType Color Filtering Patents Have Expired!

Since August 2019, all patents related to ClearType color filtering have expired worldwide.

The affected patents were

Patent US6219025:
Mapping image data samples to pixel sub-components on a striped display device

Patent US6225973:
Mapping samples of foreground/background color image data to pixel sub-components

Patent US6239783:
Weighted mapping of image data samples to pixel sub-components on a display device

Patent US6243070:
Method and apparatus for detecting and reducing color artifacts in images

Patent US6282327:
Maintaining advance widths of existing characters that have been resolution enhanced

Patent US6307566:
Methods and apparatus for performing image rendering and rasterization operations

Patent US6393145:
Methods apparatus and data structures for enhancing the resolution of images to be rendered on patterned display devices

Patent US6421054:
Methods and apparatus for performing grid fitting and hinting operations

Patent US6624828:
Method and apparatus for improving the quality of displayed images through the use of user reference information

Patent US6973210:
Filtering image data to obtain samples mapped to pixel sub-components of a display device

Essentially, these patents covered several different things, which can be grossly sub-divided into

  • the process of displaying images where data is ‘mapped’ to individual (e.g., LCD) sub-pixels, instead of whole pixels,
  • the process of displaying said images after filtering them for various purposes, e.g., reducing color fringes, and
  • the process of dealing with metrics and spacing rounding in a sub-pixel world.

It is possible that Microsoft acquired other patents related to the field in the previous years.

Please note that above claims do not cover the enhanced interpretation of TrueType bytecode instructions as discussed in Greg Hitchcock's whitepaper.

Last update: 17-Aug-2020