ClearType

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ja:クリアタイプ

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 10X enlargement of the word WikipediA rendered using ClearType. The word was originally rendered using a Times New Roman 12pt font.

Background

Normal software treats computer displays as containing square white pixels. On the other hand, actual pixels on a color liquid crystal display are tall rectangles of red, green, and blue, and the hardware can generally address the individual components of a pixel separately. A 4x3 pixel section of such a display looks somewhat like this:

Thus, if software treats RGB as a single unit, an image of all red pixels will be offset 1/3 pixel to the left of an image of all green pixels, and an image of all blue pixels will be offset 1/3 pixel to the right.

In theory, it's also possible for a display to stack pixels vertically, but this occurs rarely in practice.

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.

External links

• 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.