If the equation of a plane is ax+by+cz+d=0, then a surface normal is . This can be extended to curved surfaces by consideration of the tangent plane at a point on the surface patch. If the equation of a curved surface is given by

then the surface normal is given by the vector . If the parameters p and q are defined as and then the surface normal can be written as . The quantity is called the gradient of and gradient space is the two dimensional space of all such points . Gradient space is a convenient viewer centred representation of surface orientation. Parallel planes map into a common point in gradient space. Planes perpendicular to the viewing direction map onto the origin of gradient space. Moving away from the origin in gradient space, the distance from the origin equals the tangent of the emergent angle, e, between the surface normal and the viewing point.