return the coefficients of the specified clipping plane
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).
It is always the case that GL_CLIP_PLANE = GL_CLIP_PLANE0 + .
If an error is generated, no change is made to the contents of equation .
GL_INVALID_ENUM is generated if plane is not an accepted value.
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