I don't understand the huge difference between using +a0.0 and any value
above 0.0.
  Consider this simple scene:

camera { location -z*10 look_at 0 angle 35 }
light_source { <1000,2000,0>, 1 }
plane
{ y,-1 pigment { checker rgb <1,1,.5>, rgb <.8,.4,.2> }
  rotate <-10,30,-30>
}

  When I rendered it with +a0.01 +am2 +r4, the resulting image is the
following:
http://www.cs.tut.fi/~warp/aa1.jpg

  There are big visible artifacts.
  However, when I rendered it with +a0.0 +am2 +r4, the result is almost
perfect:
http://www.cs.tut.fi/~warp/aa2.jpg

  It doesn't seem to matter how small the threshold value is. If it's larger
than 0, it will cause the artifacts.
  I don't understand why.

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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