ClearType™ is Microsoft's subpixel rendering display technology designed to leverage the phase carrier created by the misalignment of the red, green, and blue planes of some computer display devices to draw the lines and curves of text and graphics to sub-pixel accuracy. The Apple II family had used a very similar technique with the NTSC television system's color subcarrier two decades earlier.

The following picture shows a 4× enlargement of the word Wikipedia rendered using ClearType. The word was originally rendered using a Times New Roman 12 pt font.

Background

Normally, software treats computer displays as a matrix of square pixels where each pixel is of a uniform color. However, on a liquid crystal display, red light comes out of the left third of a pixel, green light from the middle third, and blue light from the right third. A 4×3 pixel section of such a display looks somewhat like this:

Thus, if software treats a pixel as a single unit, then the actual center of the red pixels will be 1/3 of a pixel to the left of the ideal pixels' centre, and that of the blue pixels will be 1/3 of a pixel to the right.

Most displays have RGB subpixels beside each other horizontally, while some have them stacked vertically. As well, some displays use a BGR order instead of RGB.

Drawing a ClearType image

To start, draw the image three times as wide as normal, using appropriate anti-aliasing of sloped edges. Apply a digital low-pass filter with a zero at f/3 to the image in a horizontal direction to remove color fringing caused by beats with the color subcarrier. (Microsoft uses a filter equivalent to the 3-tap FIR filter [1 1 1]/3; the 5-tap filter [1 3 4 3 1]/12, with an additional double zero at f/2 that removes color fringes caused by alternate dark and light pixels, may work better for some images.) Then sample alternately red, green, and blue components of successive pixels to produce a final image at nearly triple the apparent horizontal resolution of an ordinary image. Some displays, such as the one in the original iBook computer and the Game Boy Advance handheld video game console, order pixels in BGR order rather than RGB; a subpixel rasterizer that can run on multiple displays must take this into account.

In practice

Notice that the ClearType (sub-pixel anti-aliased) image has almost no color-fringing, but no visible extra resolution at the same time. This shows that ClearType is ineffective at achieving 300% or even any amount of perceptible extra horizontal resolution.

External links

• Explanation of ClearType at Microsoft Typography
• Sub-Pixel Font Rendering Technology: An explanation of the technique and its history, including a freely downloadable binary demo for Windows computers.
• sample implementation in the C language by Damian Yerrick of Everything2
• "Displaced Filtering for Patterned Displays" by Platt et al: a research paper detailing ClearType's techniques.