Subpixel rendering

Normal rendering without anti-aliasing

Subpixel rendering without anti-aliasing

Normal rendering with antialiasing

Subpixel rendering with antialiasing

Subpixel rendering (Engl.) Translates as subpixel rendering and represents algorithms , the sub pixels on a color screen in addition to increasing the resolution of text and images shown use. Since subpixels are mostly arranged horizontally, the horizontal resolution is increased.

The improvement in display is most noticeable on screens where the sub-pixels are clearly separated, as is the case with LCD or AMOLED screens. Depending on the quality of the device, the legibility of CRT monitors can also be increased.

history

The technology was developed by IBM in 1988 to improve the display of characters on color LCDs such as laptops, cellular telephones or flat screens.

The process was offered on a larger scale by Apple for Mac OS 9 and by Microsoft for Windows XP as ClearType. In both operating systems, however, it was not activated in the basic setting. This only changed with macOS and Windows Vista . XFree and its successor products, which are widespread in BSD systems and Linux , also enable subpixel rendering.

function

Nine pixels in RGB arrangement

By driving a single subpixel instead of a full (i.e. consisting of several subpixels) pixel, the usable resolution of the screen increases by a factor of three in one direction . The direction depends on both the internal structure of the screen and its orientation, for example when using the pivot function . In principle, the higher resolution allows a finer representation of details.

The use of subpixel rendering is always bought with color fringes , because with additive color mixing , three subpixels in the basic colors red , green and blue are required per pixel in order to display the color white on the screen. However, this is not always guaranteed for pixels that are in the area of the transition between text and background, since their subpixels display the color black, provided they are included in the representation of the characters. The basic colors of the other subpixels lying outside the characters then result in colors other than white when mixed.

The same text rendered differently:
Above: Without ClearType Below: With ClearType
Left: Normal size text Middle: Text enlarged 8 × on a screen without subpixels Right: Text enlarged 8 × on a screen with subpixels

If subpixel rendering with antialiasing , i. H. Text smoothing , is connected, there is a further effect of a reduction in contrast . The subpixels of the characters are surrounded with differently bright subpixels so that the pixel stairs are further resolved for the eye.

Both effects can be individually perceived as pleasant or annoying. Basically, subpixel rendering and comparable techniques are based on the fact that the human eye can perceive brightness contrasts much better than color differences.

The picture on the right shows sub-pixel rendering in comparison to conventional font display. The conventional typeface is shown above, the ClearType below. On the left you can see the normal display, in the middle and on the right the one that has been enlarged many times. Here, the screen of the two right partial images can display subpixels in contrast to that of the two middle images.

poll

Octagonal sub-pixel arrangement

Subpixel processes such as ClearType must be individually coordinated for each display device. In particular, the arrangement of the subpixels to one another is decisive for the calculation of the color deviations. Most flat screens (LCD or AMOLED) have the subpixels arranged horizontally in the order RGB (red-green-blue). In rare cases, the order BGR is used. There are also screens that use entirely different arrangements, such as B. vertical or octagonal arrangements (similar to the arrangement of pixels in most CRT screens), more green than red and blue or even additional white subpixels.

The software used for subpixel rendering must know the arrangement of the subpixels. Otherwise, the false sub-pixels will be weakened and the edge will appear blurred than before. In addition, some monitors are equipped with diffusion foils that distribute the brightness of the subpixels over the entire logical pixel. Many desktop environments therefore offer the possibility of informing the system of the actual physical arrangement of the pixels.

Voting under Windows

On Windows, Cleartype can be configured in the following ways:

It can be turned on and off completely.
Both RGB and BGR pixels are supported as the pixel arrangement. Other pixel geometries as well as vertically rotated monitors are neither recognized nor supported (applies to XP, Vista, 7, 8 / 8.1 and 10).
You can choose between subpixel rendering, pure antialiasing and a mixture of both (ClearType Level: 0… 100)
Vertical antialiasing can be activated.
The gamma factor of the display can be specified (gamma: 1800… 2200).
Font can be rendered in different thicknesses (Enhanced Contrast: 0… 100).

Otherwise, the following things must be observed:

Text rendering with ClearType is a little slower than without.
ClearType is only used for small font sizes.
Words must not be rendered as individual letters, otherwise errors will occur if the letters overlap slightly.
Writing fonts with the XOR operation, which is particularly popular in 16-color modes, leads to incorrect results.
The result of ClearType differs depending on the interface used, GDI or DirectX / Direct2D. With the latter, letters can not only be rendered subpixel, but also shifted by subpixels.

Voting using the ClearType Text Tuner

ClearType Text Tuner is a 9 screen point high font that was internally rendered with a resolution of 1/6 pixel × 1 pixel. The line width is 1 pixel horizontally, 1 1/3 pixel vertically.

The meaning of the dialogues remains somewhat in the dark, so it is explained here.

Part 1/4: Adjustment RGB vs. BGR-Pixel
In this first dialog you define the sub-pixel structure of a pixel. On the left you select the common RGB arrangement, on the right the rather uncommon BGR arrangement. The latter is obtained by turning a display upside down and at the same time rotating the image output by 180 °. Windows has not supported other subpixel arrangements or screens rotated by 90 ° or 270 ° for over 10 years.

Part 2/4: Gamma Adjustment
In this dialog you set the gamma with which the display works. The 6 settings are calculated for a gamma of 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0. The last two settings are best for normal monitors (gamma 1.8 and 2.0), but the dialog can be misused to "make" the font output thicker.

Part 3/4: Consideration of the position of the screen sub-pixels

Part 4/4: Still under investigation

Availability

Subpixel rendering is supported by Mac OS 9 and macOS as well as various X Window systems , the graphics environment for Unix-like systems . Starting with macOS 10.14 Mojave , subpixel rendering is no longer available because the operating system's rendering pipeline no longer supports it.

ClearType has been available for Microsoft Windows XP, Windows Server 2003 , Windows Vista, and Windows Mobile since 2003.

When turning screens from the usual wide format display to the portrait format display, the pixel arrangement changes. Windows XP, Windows Vista and Windows 7 do not take this into account and thus lead to a display that is difficult to read and has clear colored borders. They only master horizontal RGB and BGR arrangements. For Windows Mobile, Cleartype is disabled when the device is rotated.

criticism

The disadvantages of subpixel rendering are discussed in internet forums and blogs . Web designers find that the font can shift a few pixels in a given layout and then make a different layout necessary. This problem can also occur for other reasons - for example when using a different font, using the browser zoom function or using a significantly different screen resolution - so that the cause should not be seen as the subpixel rendering, but rather the desire of the Web designer based on pixel-perfect page design. For this purpose, additional browser-dependent CSS style descriptions are sometimes created, which one wanted to reduce with new web browsers and current W3C standards.

In the case of vertical contrast lines (e.g. "I" bar in black-on-white display), the subpixel-specific rendering of brightness values sometimes allows clear color shadows to be seen, which are noticeable in a similar way to unclean color overprints in paper printing. The problem occurs in a somewhat weaker form, even without ClearType, because colors are always resolved on liquid crystal screens by horizontally adjacent color pixels. As a result, the individual subpixels are in different locations. The blurry display can affect the text recognition of screen software ; For example, the character recognition of Babylon Translator is significantly limited. However, this is a general problem with any form of anti-aliasing.

Furthermore, the display is perceived as blurred by many users. The unconscious effort of the eye to focus the artificially blurred typeface can lead to symptoms of fatigue and headaches.

TFT screen without sub-pixel rendering
TFT screen with ClearType activated
Sub-pixel screen on a PSP-2000 and PSP-3000