updated coordinate system documentation

git-svn-id: svn://anubis/anaglym/trunk@276 99a6e188-d820-4881-8870-2d33a10e2619

doc/index.html

@@ -81,7 +81,12 @@ The Z axis extends positively into the "air" above the XY plane, and the
 default gravity vector pulls along the negative Z axis.
 </p>
 <p>
-FIXME: 2D coordinates
+When 2D coordinates are used (with mouse cursor positions and
+overlay drawing routines), the origin is in the lower-left corner of
+the screen.
+The X axis increases positively toward the right edge of the screen.
+The Y axis increases positively toward the top of the screen.
+The upper-right pixel on the screen has coordinates (width-1, height-1).
 </p>