Advertisment

Clearer Displays

author-image
PCQ Bureau
New Update

Whenever an object with a sloping diagonal edge is drawn on a computer screen, you’ll notice that the line’s edges are not smooth, but jagged. This is due to the pixel rendering technology used. In a CRT (Cathode Ray Tube) screen, electron guns shoot electron beams through a metal mask/grille to the monitor’s glass. This glass is coated with very fine phosphor dots, which glow when the electron beam hits them. These dots are arranged in sets of three (so there are three electron guns for red, green and blue respectively), and each set forms a pixel. Different colors are formed based on the intensity with which the electron beams hit the phosphorous dots. Each dot that is made on the screen is called a pixel and a collection of pixels makes the image that you see on your screen. On the other hand, on a patterned display like a LCD panel, the pixel is more of a concept rather than a physical entity. The display can be best described as an array of singly colored sub-pixels of red, green and blue colors, which can be grouped together to form one full pixel for convenience. 

Advertisment

Traditionally, font rendering on these displays was based on pixels that are rectangular in shape. However, since fonts are curved and also have diagonal lines, they appear with jagged edges. Traditional techniques for solving this problem include anti-aliasing and pixel-borrowing. While anti-aliasing addresses this problem at the whole pixel level, pixel-borrowing goes to the sub-pixel level to solve the same. Anti-aliasing uses shades of gray in the fonts where font designers wanted to show part pixels. This technique was based on the assumption that our eyes would average out the adjacent gray pixels to see the pixel in the middle. This technique was partly successful but tended to smudge the fonts out rather than smoothen them. The smudging generally occured in the smaller font sizes.

Pixel-borrowing, on the other hand, has its roots in the Apple II. The Apple II’s resolution was 280 horizontal and 192 vertical, with 280 being the sum of all the colored pixels. Like in LCDs, where each pixel is made up of RGB, the Apple II’s display generated two sub-pixels per pixel - green and purple - and both needed to be activated in order to get white. So, instead of using a whole pixel for a diagonal line, it used a half of each adjacent sub-pixels, thus creating a smoother edge.

Following the usage of sub-pixel rendering is the Cleartype font rendering by Microsoft. It was first announced in 1998 and the used on the Pocket PC, which included Microsoft Reader that also used this software. This comes as a built-in feature in Win XP. It works on the sub-pixel level and thus is more useful on LCD screens rather than CRTs which don’t work at the

sub-pixel level. 

Advertisment

How Cleartype works



ClearType uses a model of the human visual system to solve the problem jagged fonts. The human visual system is more sensitive to errors in an image that has a spatially large extent rather than fine detail. Also, the human eye is more sensitive to errors in black and white rather than in color (that is, it is more sensitive to sharpness rather than shades of gray)



Based on this human visual system of error detection, the process of finding out the best LCD pixel values involves taking a RGB input image and producing the RGB output image at sub-pixel output resolution using mathematical formulas developed by Microsoft. 

Some shortcomings of ClearType are that it can only be used on LCD panels and not on CRTs. Since most PC users are currently using CRTs, they will not notice much of a difference in their display. At times, the fonts may even appear to be smudged when ClearType is enabled on CRTs. ClearType can only increase the resolution horizontally. It means that ClearType cannot be used when panels are used in portrait mode with the current LCD technologies. ClearType originally had problems with pattern displays other than RGB. Some LCDs (in the older iBooks and Gameboy color) used BGR stripes rather than RGB stripes. ClearType originally had a problem displaying text properly on these screens, but the issue has now

been resolved. 

ClearType in Win XP



ClearType can be activated on Win XP from the display-properties dialog. In the display-properties

dialog, open the Appearance tab and click on effects. Click on the check box with the label ‘Use the following method to smooth edges of screen fonts.’ 

Advertisment

Choose ClearType from the adjacent dropdown.

Microsoft also offers a web-based interface for optimizing ClearType on your windows machine. Log on to www. microsoft. com/typography and follow the link to the ClearType website to optimize your display.

Geetaj Channana

Advertisment