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 d

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/basic_renderers.agdoc.html http://agg.sourceforge.net/antigrain.com/doc/basic_renderers...