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Quadric Objects: Rendering Spheres, Cylinders, and Disks
The base OpenGL library only provides support for modeling and rendering simple
points, lines, and convex filled polygons. Neither 3D objects, nor commonly
used 2D objects such as circles, are directly available.
The GLU (and GLUT) provide routines to model and render tessellated, polygonal
approximations for a variety of 2D and 3D shapes (spheres, cylinders, disks,
and parts of disks), which can be calculated with quadric equations:
a1x2 + a2y2 + a3z2 + a4xy + a5yx + a6xz + a7x + a8y + a9z + a10 = 0