Here, those cases are considered which can be expressed in mathematical equations and then be clearly solved . In practical operation, the designer sets up a model and defines several constraints (such as continuities). An evaluation of the surface quality proves unsatisfying, since for example the shadow lines on the model show too many disturbing oscillations. Consequently, he defines new lines which he considers better, whereupon the system from his definitions calculates and solves the equations belonging to the problem. As a result, the designer gets a surface with his defined shadow lines.

The direct approach has the advantage of exactly delivering the desired result, but it is not always applicable or the constraints it requires are too hard for industrial use.