Explanation of the Fill Rules
For the sake of completeness, we are describing how these fill-rules work,
but you don't need to know the details in order to use them. The nonzero rule
determines whether a point is inside or outside a polygon by drawing a line from
the point in question to infinity. It counts how many times that line crosses the
polygon's lines, adding one if the polygon line is going right to left, and
subtracting one if the polygon line is going left to right. If the total comes out to
zero, the point is outside the polygon. If the total is nonzero (hence the name)
the point is inside the polygon.
The evenodd rule also draws a line from the point in question to infinity, but it
simply counts how many times that line crosses your polygon's lines. If the total
number of crossings is odd, then the point is inside; if even, then the point is
outside.