ClearType

ClearType™ is Microsoft's 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.

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 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.

Drawing a ClearType image

To start, draw the image three times as wide as normal, using appropriate anti-aliasing of sloped edges. Apply a low-pass filter with a zero at f/3 to the image to avoid 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.

Additional Resources

• Sub-Pixel Font Rendering Technology ( http://www.grc.com/cleartype.htm ): An explanation of the technique and its history, including a freely downloadable binary demo for Windows computers.
• The article on Everything2 on which this article was initially based ( http://everything2.com/?node_id=1232071 ): Also includes a sample implementation in the C language by Damian Yerrick.
• "Displaced Filtering for Patterned Displays" by Platt et al. ( http://www.research.microsoft.com/scripts/pubs/view.asp?PubID=719 ): a research paper detailing ClearType's techniques.