We can't snap coordinates to the top and bottom of the trapezoid -- we
could snap them to the top and bottom of the pixel. Here's why:
E
\
+---------J----+
| \ |
| A \ B|
==================I |
| \|
| C K
| |\
+--------------+ \
I'm tesselating a figure and I've got three trapezoids meeting here in
this pixel. The sum of A, B and C must be exactly 1. The area to the
left of 'E' is going to be compute three times, the whole area when
drawing 'B', and part of the area when drawing 'A' and 'C'. The sub-area
computations needed for 'A' and 'C' must sum to the same value as the area
computation needed for 'B'.
Hence the x coordinate at the intersection 'I' cannot be snapped for the
computation of either A or C -- the location of 'I' is unknown during the
computation of 'B'. If snapping 'J' and 'K' would be useful, we can do
that -- those locations are known during all of the computations.
-keith
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