A vertex can begin an edge which lies in the interior of its polygon or on the polygon's boundary.
Each vertex of a polygon, separate triangle, or separate quadrilateral specified during renderPrimitive is marked as the start of either a boundary or nonboundary (interior) edge.
The vertices of connected triangles and connected quadrilaterals are always marked as boundary, regardless of the value of the edge flag.
Boundary and nonboundary edge flags on vertices are significant only if polygonMode is set to Point or Line.
Note that the current edge flag can be updated at any time, in particular during renderPrimitive.