View source for Subpixel rendering

{{Short description|Technique for increasing apparent display resolution}}
{{essay|date=February 2024}}
[[File:Subpixel-rendering-RGB.png|thumb|A simulation of subpixel rendering]]
{{ multiple image | width = 128
| image1   = SubPixel.png
| caption1 = "Aa" rendered in subpixel
| image2   = SubPixel.gif
| caption2 = The previous image, with the {{fontcolor|red|R}}, {{fontcolor||lime|G}} and {{fontcolor|blue|B}} channel separated and animated
}}
[[File:Pixel geometry 01 Pengo.jpg|thumb|Examples of [[pixel geometry]], showing various arrangements of pixels and subpixels, which must be considered for subpixel rendering. LCD displays consisting of red, green, and blue subpixels (bottom right is the most typical example) are best suited to subpixel rendering.]]

'''Subpixel rendering''' is a method used to increase the effective resolution of a color [[display device]]. It utilizes the composition of each [[pixel]], which consists of three subpixels of which are red, green, and blue that can each be individually addressable on the display matrix.

Subpixel rendering is primarily used for [[Font_rasterization|text rendering]] on [[dots per inch#Computer monitor DPI standards|standard DPI]] displays.

Despite the inherent color anomalies, it can also be used to render general graphics.

== History ==

The origin of subpixel rendering as used today remains controversial. [[Apple Inc.]], [[IBM]], and [[Microsoft]] patented various implementations with certain technical differences owing to the different purposes their technologies were intended for.<ref>John Markoff, "[https://www.nytimes.com/library/tech/98/12/biztech/articles/07microsoft-flat.html Microsoft's Cleartype Sets Off Debate on Originality] {{Webarchive|url=https://web.archive.org/web/20170421095144/http://www.nytimes.com/library/tech/98/12/biztech/articles/07microsoft-flat.html |date=2017-04-21 }}", ''New York Times'', December 7, 1998</ref>

Microsoft had several [[Software patent|patents]] in the United States on subpixel rendering technology for text rendering on RGB Stripe layouts. The patents 6,219,025, 6,239,783, 6,307,566, 6,225,973, 6,243,070, 6,393,145, 6,421,054, 6,282,327, 6,624,828 were filed between October 7, 1998, and October 7, 1999, and expired on July 30, 2019.<ref name="cleartype_expl" /> Analysis of the patent by FreeType<ref name="patents" /> indicates that the idea of subpixel rendering is not covered by the patent, but by the actual filter used as a last step to balance the color. Microsoft's patent describes the smallest filter possible that distributes each subpixel value to an equal amount of R,G, and B pixels. Any other filter will either be blurrier or will introduce color artifacts.

Apple was able to use it in [[MacOS|Mac OS X]] due to a patent [[cross-licensing]] agreement.<ref>{{cite web|url=http://lists.gnu.org/archive/html/freetype/2006-09/msg00064.html|date=September 24, 2006|access-date=April 9, 2009|title=LCD Rendering Patches|archive-url=https://web.archive.org/web/20110603232951/http://lists.gnu.org/archive/html/freetype/2006-09/msg00064.html|archive-date=2011-06-03|url-status=live}}</ref>

== Characteristics ==

A single pixel on a color display is made of several subpixels, typically three arranged left-to-right as red, green, and blue (RGB). The components are easily visible when viewed with a small magnifying glass, such as a [[loupe]]. These pixel components appear as a single color to the human eye because of blurring by optics and spatial integration by nerve cells in the eye. However, the eye is much more sensitive to the location.{{clarify|date=March 2024}} Therefore, turning on the G and B of one pixel and the R of the next pixel to the right will produce a white dot, but it will appear to be 1/3 of a pixel to the right of the white dot that would be seen from the RGB of only the first pixel. Subpixel rendering takes advantage of this to provide three times the horizontal resolution of the rendered image, though it has to blur this image to produce the correct color by ensuring the same amount of red, green, and blue are turned on as when no subpixel rendering is being done.

Subpixel rendering does not necessitate the use of [[Spatial_anti-aliasing|antialiasing]], and gives a smoother result regardless of whether antialiasing is used or not <ref>{{Cite web|url=https://freddie.witherden.org/pages/font-rasterisation/|title=A Treatise on Font Rasterisation With an Emphasis on Free Software|date=2010-09-15|access-date=2023-11-11}}</ref> since it artificially increases the resolution. However, it introduces color aliasing since subpixels are colored. Subsequent filtering applied to remove the color artifacts is a form of antialiasing, although its purpose is not smoothing jagged shapes as in conventional antialiasing.

Subpixel rendering requires the software to know the layout of the subpixels. The most common reason it is wrong is monitors that can be rotated 90 (or 180) degrees, though monitors are manufactured with other arrangements of the subpixels, such as BGR or in triangles, or with 4 colors like RGBW squares. On any such display the result of incorrect subpixel rendering will be worse than if no subpixel rendering was done at all (it will not produce color artifacts, but it will produce noisy edges).

== 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>

==See also==

* {{anl|Font rasterization}}
* {{anl|Kell factor}}
* {{anl|PenTile matrix family}}
* {{anl|Sub-pixel resolution}}

== References ==

{{refs}}

== External links ==

* [https://web.archive.org/web/20091026135732/http://www.geocities.com/cuncie/ClearType.html Ron Feigenblatt's remarks on Microsoft ClearType]
* {{webarchive |url=https://web.archive.org/web/20071012031647/http://www.ductus.com/cleartype/cleartype.html |date=October 12, 2007 |title=Pixel Borrowing, ClearType and Antialiasing }}
* [https://web.archive.org/web/20140706204154/https://people.mozilla.org/~jdaggett/tests/subpixelexplorer.html Subpixel Explorer]
* [https://web.archive.org/web/20120710140914/http://www.antigrain.com/research/font_rasterization/index.html Texts Rasterization Exposures] Article from the Anti-Grain Geometry Project.
* {{cite journal | doi = 10.1111/cgf.12267 | title=Low-Cost Subpixel Rendering for Diverse Displays | journal=Computer Graphics Forum | date=2013 | volume=33 | issue=1 | pages=199–209 | first=Thomas | last=Engelhardt| s2cid=9851327 }} http://jankautz.com/publications/SubpixelCGF13.pdf

{{Computer graphics}}

[[Category:Computer graphics]]
[[Category:Vector graphics]]

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