# Normals of non planar Surfaces

## Example : parametric surface

Not all surfaces are given as planar facets. A common
example of such a surface is called a parametric surface. For a parametric surface
the three-space coordinates are determined by functions of two parameters, u
and v in our case. For parametric surfaces two vectors in the tangent plane
can be found by computing partial derivatives as follows. And the normal is
computed as before: