Editing Subpixel rendering (section)

== Implementations ==

=== Apple II ===
{{original research|section|date=August 2014}}

[[Steve Gibson (computer programmer)|Steve Gibson]] has claimed that the [[Apple II]], introduced in 1977, supports an early form of subpixel rendering in its high-resolution (280×192) graphics mode.<ref>{{cite web|url=http://grc.com/ctwho.htm|title=GRC - The Origins of Sub-Pixel Font Rendering|website=grc.com|access-date=2006-03-02|archive-url=https://web.archive.org/web/20060306043314/http://grc.com/ctwho.htm|archive-date=2006-03-06|url-status=live}}</ref> The Wozniak patent only used 2 "sub-pixels".<ref>[http://permalink.gmane.org/gmane.comp.fonts.freetype.user/1912 David Turner (24 Sep 20:00 2006) LCD Rendering Patches] {{Webarchive|url=https://web.archive.org/web/20070208200533/http://permalink.gmane.org/gmane.comp.fonts.freetype.user/1912 |date=2007-02-08 }}</ref> The [[byte]]s that comprise the Apple II high-resolution [[screen buffer]] contain seven visible bits (each corresponding directly to a pixel) and a flag bit used to select between purple/green or blue/orange color sets. Each pixel, since it is represented by a single bit, is either on or off; there are no bits within the pixel itself for specifying color or brightness. Color is instead created as an [[visual artifact|artifact]] of the [[NTSC]] color encoding scheme, determined by horizontal position: pixels with even horizontal coordinates are always purple (or blue, if the flag bit is set), and odd pixels are always green (or orange). Two lit pixels next to each other are always white, regardless of whether the pair is even/odd or odd/even, and irrespective of the value of the flag bit. This is an approximation, but it is what most programmers of the time would have in mind while working with the Apple's high-resolution mode.

Gibson's example claims that because two adjacent bits make a white block, there are in fact two bits per pixel: one which activates the purple left half of the pixel, and the other which activates the green right half of the pixel. If the programmer instead activates the green right half of a pixel and the purple left half of the next pixel, then the result is a white block that is 1/2 pixel to the right, which is indeed an instance of subpixel rendering. However, it is not clear whether any programmers of the Apple II have considered the pairs of bits as pixels—instead calling each bit a pixel.

The flag bit in each byte affects color by shifting pixels half a pixel-width to the right. This half-pixel shift was exploited by some graphics software, such as HRCG (High-Resolution Character Generator), an Apple utility that displayed text using the high-resolution graphics mode, to smooth diagonals.

=== ClearType ===
{{main|ClearType}}

[[Microsoft]] announced its subpixel rendering technology, called [[ClearType]], at [[COMDEX]] in 1998.<ref>{{Citation|title=ICT Bill Gates 1998 keynote comdex 1998|url=https://www.youtube.com/watch?v=HoVFwMnEUHA|language=en|access-date=2021-11-30|archive-date=2021-11-30|archive-url=https://web.archive.org/web/20211130191757/https://www.youtube.com/watch?v=HoVFwMnEUHA|url-status=live}}</ref> Microsoft published a paper in May 2000, ''Displaced Filtering for Patterned Displays,'' describing the filtering behind ClearType.<ref>{{Cite journal|last=Platt|first=John|last2=Keely|first2=Bert|last3=Hill|first3=Bill|last4=Dresevic|first4=Bodin|last5=Betrisey|first5=Claude|last6=Mitchell|first6=Don P.|last7=Hitchcock|first7=Greg|last8=Blinn|first8=Jim|last9=Whitted|first9=Turner|date=2000-05-01|title=Displaced Filtering for Patterned Displays|url=https://www.microsoft.com/en-us/research/publication/displaced-filtering-for-patterned-displays/|language=en-US|pages=296–299|access-date=2021-11-30|archive-date=2021-11-30|archive-url=https://web.archive.org/web/20211130191757/https://www.microsoft.com/en-us/research/publication/displaced-filtering-for-patterned-displays/|url-status=live}}</ref> It was then made available in [[Windows XP]], but it was not activated by default until [[Windows Vista]]. (Windows XP [[OEM]]s, however, could and did change the default setting.)<ref name="win7">[[Greg Hitchcock]] (with introduction by [[Steven Sinofsky]]) "[http://blogs.msdn.com/b/e7/archive/2009/06/23/engineering-changes-to-cleartype-in-windows-7.aspx Engineering Changes to ClearType in Windows 7] {{Webarchive|url=https://web.archive.org/web/20121218085312/http://blogs.msdn.com/b/e7/archive/2009/06/23/engineering-changes-to-cleartype-in-windows-7.aspx|date=2012-12-18}}", MSDN blogs, June 23, 2009</ref>

=== 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.

===CoolType===

[[Adobe Systems|Adobe]] created their own subpixel renderer called [[CoolType]], allowing them to display documents the same way across various operating systems: Windows, MacOS, Linux etc. When it was launched around the year 2001, CoolType supported a wider range of fonts than Microsoft's ClearType, which at the time was limited to [[TrueType]] fonts, whereas Adobe's CoolType also supported [[PostScript fonts]] (and their [[OpenType]] equivalent as well).<ref>Felici, James (April 2000) "[https://web.archive.org/web/20060206050218/http://www.seyboldreports.com/SRIP/free/0408/cooltype.html ClearType, CoolType: The Eyes Have It]", Seybold Report on Internet Publishing, Vol. 4 Issue 8</ref>

=== macOS ===

[[Mac OS X]] (later OS X, now macOS) used to use subpixel rendering as well, as part of [[Quartz 2D]]. However, it was removed after the introduction of Retina displays. Unlike Microsoft's implementation, which favors a tight fit to the grid ([[font hinting]]) to maximize legibility, Apple's implementation prioritizes the shape of the glyphs as set out by their designer.<ref>{{cite web|url=http://www.smashingmagazine.com/2009/11/02/the-ails-of-typographic-anti-aliasing/|title=The Ails Of Typographic Anti-Aliasing|date=November 2, 2009|access-date=2014-08-11|archive-url=https://web.archive.org/web/20140809024933/http://www.smashingmagazine.com/2009/11/02/the-ails-of-typographic-anti-aliasing/|archive-date=2014-08-09|url-status=live}}</ref>