Structure definition for direction on surface data. It includes all the
input elements to create a smart direction on surface object.

The direction will be evaluated along isoparametric, normal or a specified
section direction at a pair of (u,v) parameters on the underlying surface of
a given face. Two scalar objects need to be created for the uv parameters.

If the section option is chosen, a direction object needs to be created for
specification. The direction will be projected to be perpendicular to the
surface normal at the given uv parameters. The projected direction will
underlie the output direction on surface object.