Each vertex of a polygon, separate triangle, or separate quadrilateral specified between a glBegin / glEnd pair is marked as the start of either a boundary or nonboundary edge. If the current edge flag is true when the vertex is specified, the vertex is marked as the start of a boundary edge. Otherwise, the vertex is marked as the start of a nonboundary edge. glEdgeFlag sets the edge flag bit to GL_TRUE if flag is GL_TRUE and to GL_FALSE otherwise. 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 GL_POLYGON_MODE is set to GL_POINT or GL_LINE. See glPolygonMode.
The current edge flag can be updated at any time. In particular, glEdgeFlag can be called between a call to glBegin and the corresponding call to glEnd.
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glEdgeFlag: man2/glEdgeFlag.xml
Each vertex of a polygon, separate triangle, or separate quadrilateral specified between a glBegin / glEnd pair is marked as the start of either a boundary or nonboundary edge. If the current edge flag is true when the vertex is specified, the vertex is marked as the start of a boundary edge. Otherwise, the vertex is marked as the start of a nonboundary edge. glEdgeFlag sets the edge flag bit to GL_TRUE if flag is GL_TRUE and to GL_FALSE otherwise. 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 GL_POLYGON_MODE is set to GL_POINT or GL_LINE. See glPolygonMode.
The current edge flag can be updated at any time. In particular, glEdgeFlag can be called between a call to glBegin and the corresponding call to glEnd.