Geometry is always clipped against the boundaries of a six-plane frustum in x, y, and z. glClipPlane allows the specification of additional planes, not necessarily perpendicular to the x, y, or z axis, against which all geometry is clipped. To determine the maximum number of additional clipping planes, call glGetIntegerv with argument GL_MAX_CLIP_PLANES. All implementations support at least six such clipping planes. Because the resulting clipping region is the intersection of the defined half-spaces, it is always convex.
glClipPlane specifies a half-space using a four-component plane equation. When glClipPlane is called, equation is transformed by the inverse of the modelview matrix and stored in the resulting eye coordinates. Subsequent changes to the modelview matrix have no effect on the stored plane-equation components. If the dot product of the eye coordinates of a vertex with the stored plane equation components is positive or zero, the vertex is in with respect to that clipping plane. Otherwise, it is out.
To enable and disable clipping planes, call glEnable and glDisable with the argument GL_CLIP_PLANE i, where i is the plane number.
All clipping planes are initially defined as (0, 0, 0, 0) in eye coordinates and are disabled.