Specifies a clipping plane. The number of clipping planes depends on the implementation, but at least six clipping planes are supported. They are identified by symbolic names of the form GL_CLIP_PLANE where i ranges from 0 to the value of GL_MAX_CLIP_PLANES - 1.
Returns four double-precision values that are the coefficients of the plane equation of plane in eye coordinates. The initial value is (0, 0, 0, 0).