OpenGL Programming Guide

Appendix AOrder of Operations

Geometric Operations

Primitive Assembly

Primitive assembly differs, depending on whether the primitive is a point, a line, or a polygon. If flat shading is enabled, the colors or indices of all the vertices in a line or polygon are set to the same value. If special clipping planes are defined and enabled, they're used to clip primitives of all three types. (The clipping-plane equations are transformed by the inverse of the modelview matrix when they're specified.) Point clipping simply passes or rejects vertices; line or polygon clipping can add additional vertices depending on how the line or polygon is clipped. After this clipping, the spatial coordinates of each vertex are transformed by the projection matrix, and the results are clipped against the standard viewing planes x = ± w, y = ± w, and z = ± w.

If selection is enabled, any primitive not eliminated by clipping generates a selection-hit report, and no further processing is performed. Without selection, perspective division by w occurs and the viewport and depth-range operations are applied. Also, if the primitive is a polygon, it's then subjected to a culling test (if culling is enabled). A polygon might convert to vertices or lines, depending on the polygon mode.

Finally, points, lines, and polygons are rasterized to fragments, taking into account polygon or line stipples, line width, and point size. Rasterization involves determining which squares of an integer grid in window coordinates are occupied by the primitive. Color and depth values are also assigned to each such square.

OpenGL Programming Guide