Editing Subpixel rendering (section)

=== FreeType ===

[[FreeType]], the library used by most current software on the [[X Window System]], contains two [[Open-source license|open source]] implementations. The original implementation uses the ClearType antialiasing filters and carries the following notice: "The colour filtering algorithm of Microsoft's ClearType technology for subpixel rendering is covered by patents; for this reason the corresponding code in FreeType is disabled by default. Note that subpixel rendering per se is prior art; using a different colour filter thus easily circumvents Microsoft's patent claims."<ref name="patents">{{cite web |url=https://www.freetype.org/patents.html |title=FreeType and Patents |date=February 13, 2018 |publisher=FreeType.org |access-date=November 29, 2018 |archive-url=https://web.archive.org/web/20181110205046/https://www.freetype.org/patents.html |archive-date=2018-11-10 |url-status=live }}</ref><ref name="cleartype_expl">{{cite web|url=http://david.freetype.org/cleartype-patents.html|date=June 1, 2007|access-date=April 9, 2009|title=ClearType Patents, FreeType and the Unix Desktop: an explanation|author=David Turner|archive-url=https://web.archive.org/web/20090331224109/http://david.freetype.org/cleartype-patents.html|archive-date=2009-03-31|url-status=live}}</ref>

FreeType offers a variety of color filters. Since version 2.6.2, the default filter is ''light'', a filter that is both normalized (value sums up to 1) and color-balanced (eliminate color fringes at the cost of resolution).<ref>{{cite web |title=On slight hinting, proper text rendering, stem darkening and LCD filters |url=https://www.freetype.org/freetype2/docs/text-rendering-general.html#the-default-lcd-filter-for-subpixel-rendering-has-been-changed |website=freetype.org |access-date=2020-11-14 |archive-date=2020-11-12 |archive-url=https://web.archive.org/web/20201112001611/https://www.freetype.org/freetype2/docs/text-rendering-general.html#the-default-lcd-filter-for-subpixel-rendering-has-been-changed |url-status=dead }}</ref>

Since version 2.8.1, a second implementation exists, called ''Harmony'', that "offers high quality LCD-optimized output without resorting to ClearType techniques of resolution tripling and filtering". This is the method enabled by default. When using this method, "each color channel is generated separately after shifting the glyph outline, capitalizing on the fact that the color grids on LCD panels are shifted by a third of a pixel. This output is indistinguishable from ClearType with a light 3-tap filter."<ref>{{cite web |url=https://lists.gnu.org/archive/html/freetype-announce/2017-09/msg00002.html |title=Announcing FreeType 2.8.1 |date=2017-09-16 |last=Lemberg |first=Werner |access-date=2019-04-30 |archive-date=2019-11-16 |archive-url=https://web.archive.org/web/20191116013132/https://lists.gnu.org/archive/html/freetype-announce/2017-09/msg00002.html |url-status=live }}</ref> Since the Harmony method does not require additional filtering, it is not covered by the ClearType patents.