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For more fun, draw a filled circle in Inkscape. Then copy and paste it in the same place, and again, and again ... and watch the edges get more and more jagged. Try to explain that to an average user.

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"The pixels on the edge are supposed to be halfway between white and black to indicate being half-covered by the circle, but if you pile them up on top of each other anything with a little black adds up with its duplicates to complete black."

But if you zoom in, it looks the same.

That's because people, including me when I started programming, confuse alpha with coverage. Most people use the alpha value for coverage. Most of the time it doesn't really matter visually but there some edge cases where it does matter.

The best docs I could find in a short amount of time, are those from anti-grain[1] where the author describes it as a "second alpha channel.

[1] http://agg.sourceforge.net/antigrain.com/doc/basic_renderers...

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