Subpixel-Rendering
Subpixel rendering ( german) means subpixel rendering and is a software technique that improves the appearance of text on color monitors. The sub-pixels are controlled individually in order to allow inclined edges appear smoother.
The improvement in the display screens most clearly, in which the sub-pixels are clearly separated, as in the case of LCD screens. Depending on the quality of the device but can also be increased readability in CRT displays.
History
The technique was developed in 1988 by IBM to improve the font display on color LCDs like laptop, cell phone or flat panel displays.
In a larger scale the process by Apple in Mac OS 9 and Microsoft Windows XP ( as ClearType ) was offered. In both operating systems it was not enabled by default. This changed only with Mac OS X and Windows Vista. Also the spread in BSD systems and Linux XFree or its derivatives enable subpixel rendering.
Function
By using only a single sub-pixel instead of a complete pixel, the usable resolution of the screen is increased in one direction by a factor of three. The direction is from the internal structure of the screen dependent as well as its orientation, such as when using the pivot function. The higher resolution in principle allows a finer representation of details.
The use of subpixel rendering but is always bought with color fringing. For in the additive color mixture per pixel ( picture element ), three sub-pixels in the primary colors red, green and blue needed to represent the color white on the screen. This is not always guaranteed, because the subpixels display the color black, they are included in the presentation of the character in pixels that lie in the region of the transition between the print and background, however. The basic colors of the remaining out-of- character sub arise in their mixture then colors other than white.
If sub-pixel rendering with anti-aliasing, that is, font smoothing is connected, occurs as an additional effect to a reduction in contrast. The sub-pixel of the characters are finely combined with different bright subpixels, so that resolve to the eye, the pixel stairs on.
Both effects can be individually perceived as pleasant or disturbing. Basically, based subpixel rendering and similar technologies on the fact that the human eye can perceive brightness contrasts much better than color differences.
The picture makes subpixel rendering in comparison to conventional font display clearly. Above the conventional typeface is shown below the one with ClearType. On the left is the normal display, in the middle and on the right the enlarged many times. Here, the two right-hand partial images sub screen can display in contrast to the average of the two.
Vote
Subpixel techniques such as ClearType have to be adjusted individually for each display device. In particular, the arrangement of each subpixel is decisive for the calculation of the color differences. In most TFT displays the subpixels horizontally in the order of RGB (Red -Green-Blue ) are arranged. In rarer cases, comes the order BGR apply. There are TFT displays, which use completely different arrangements such as vertical or octagonal arrangements ( similar to the arrangement of pixels in most CRT displays ).
The software used for the subpixel rendering is necessary to know the arrangement of the subpixels. Otherwise, the wrong sub-pixels are attenuated and the edge appears fuzzier than before. Also, some monitors are equipped with diffusion sheets, spread the brightness of the sub-pixels on the entire logical pixels. Therefore, many desktop environments have the ability to communicate the correct physical arrangement of the pixels the system.
Availability
Subpixel rendering is supported by Mac OS 9 and Mac OS X and various X Window systems, the graphics environment for unix -like systems.
ClearType is available for Microsoft Windows XP, Windows Server 2003, Windows Vista, and Windows Mobile since 2003.
When rotating screens of the usual widescreen display to portrait display, the pixel array changes. Windows XP, Windows Vista and Windows 7 do not take this into account and thus lead to a poorly readable, provided with clear color representation of edges. You master only horizontal RGB and BGR arrangements. For Windows Mobile ClearType is disabled when the device is rotated.
Criticism
In Internet forums and blogs disadvantages of subpixel rendering are discussed. Web designers are finding that to move the font in a given layout by a few pixels and then you can make another layout necessary. This problem can also be other reasons occur - for example, when using a different font, the use of the browser zoom function or the use of an essentially different screen resolution - so that as the cause of the subpixel rendering should not be considered, but the desire of the Web designers by pixel-accurate page design. For some additional browser- dependent CSS style descriptions are created, which were intended to reduce current with new web browsers and W3C standards.
In contrast vertical lines (eg "I" line in black-on - white illustration ) are sometimes identified by the subpixelspezifische rendering of brightness values distinct color shadows that make up similar to impure color overprints in paper printing noticeable. The problem occurs but in somewhat weaker form without ClearType on, because colors are always resolved on liquid crystal displays by horizontally adjacent color pixels. Consequently, the individual subpixels located in different places. The fuzzy representation can affect the text recognition software of screen; as the character recognition of Babylon Translator for example, is significantly restricted. This is a general issue with any form of anti-aliasing.
Furthermore, the representation is perceived by many users as a focus. The unconscious effort of the eye to focus the artificially blurred typeface can lead to fatigue and headaches.
TFT display with ClearType enabled
Display with subpixels in a PSP -2000 and PSP -3000