Editing Subpixel rendering (section)

=== 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.