Anti-Aliased Text
   
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GraphicsAcademy.com  >  Glossary  >  Anti-Aliased Text

Anti-Aliased Text

Anti-aliasing is a method of drawing text or pictures which smooths edges, and avoids jagged edges ("jaggies").

In this article we'll discuss anti-aliasing as applied to drawing text.

Imagine that we wish to place a character glyph from an outline font on a bitmap. Looking at it closely (enlarging so that you can see the individual pixels), it might look something like this example:

Overlaying a Glyph on a Pixel Grid

There is an obvious problem, if you look closely at the above diagram. Namely, when we try to convert the glyph to a bitmap ("rasterise"), we can only have a single color for each individual pixel, but some of the cells in the pixel grid contain both red and white.

So how do we address this problem?

A simple solution (not anti-aliased) would be to look at each pixel cell:
  • If the pixel cell contains nothing but white, we make the pixel white.

  • If the pixel cell contains nothing but red, we make the pixel red.

  • If the pixel cell contains both red and white, we can choose to make the pixel red or white. For example, we might choose whichever color covers a larger proportion of the cell.
Here's an example of how this might look:

The Glyph Rasterised
Not Anti-Aliased

That isn't bad, but as you can see, it does look a little bit jagged (see "Jaggies").

This problem can be addressed by modifying our method of rasterising the glyph. This new method (anti-aliasing) works:
  • As before: If the pixel cell contains nothing but white, we make the pixel white.

  • As before: If the pixel cell contains nothing but red, we make the pixel red.

  • However: If the pixel cell contains both red and white, we make the pixel an intermediate color between red or white (a lighter shade of red).

    • The more white in the pixel cell, the lighter (more whitish) we make the final pixel color.

    • The more red in the pixel cell, the redder we make the final pixel color.
Here is the result it produces:

The Glyph Rasterised
Anti-Aliased

This time, it doesn't look jagged, but it does (to some eyes at least), look a little blurred. This difference will become clearer when we compare the images at their final sizes.

Finally, here is a comparison, at actual size, of text which has been anti-aliased, and text which has not been. (I have also included a comparison between two blocks of black and white text - in this case, the anti-aliasing is done using shades of gray rather than red).

Not Anti-Aliased Anti-Aliased
 

 
 

 
 

 
 

 



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