Core_Computer Studies : What is Anti-aliasing?

Anti-aliasing is a technique employed by many graphics programs to make graphics and particularly type easy to read on screen as anti-aliasing makes raster images appear smoother. Notice I used the word appear for really it is a trick of the eye. Take a look at these images.

Anti-alias option off.

Anti-alias option on.

The first set of letters are un-anti-aliased and looks coarse compared with the second set which are anti-aliased.

If we zoom in a little we can see what is happening.

A close up of the letters with anti-alias option on.

Look at how the un-anti-aliased example steps the curves into jagged shapes. This makes the letter appear to be rough and coarse. The next example was created with anti-aliasing turned on in Photoshop. In order to make the letter appear smoother Photoshop has substituted shades around the linear edges. Anti-aliasing is a way of tricking the eye into seeing straight lines and smooth curves where there are none.

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A close up of the letters with anti-alias option off.

How anti-aliasing works:

As you know raster images are made up of pixels. Pixels that are at the edge of a un-anti-aliased shape can only be part of the shape or not part of it:it is quite literally a black and white situation. The computer draws the line as a jagged set of pixels roughly following the edge of the shape. When the computer anti-aliases the shape, what it does, is to first find all the pixels on the edge of the shape, then it calculates how much of each pixel is either side of that line. For instance it works out if the pixel in hanging over the edge by %10 or %30 and so on. It then works out a shade of %10 or %30 and so on, and draws those pixels in as a shade or shades between background and foreground.

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These notes are written for students who undertake the Core Computer course at School of Art, Australian National University. Students of the course should feel free to contact any member of staff with comments questions and general suggestions..

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