An algorithm in AutoLisp to draw an ellipse inscribed in a generic quadrangle based on the projective geometry.

This paper concerns the realization of an algorithm using AutoLisp, in order to draw by AutoCAD an ellipse whose main axis are not known, but only the quadrangle in which that ellipse has been inscribed. In the first part of this paper, we describe the geometric setting of the problem describing with consecutive passages the geometrical constructions necessary to determinate the main axis of the ellipse. The application of the geometrical theory in the development of procedures AutoLISP, described in the second part of this paper, allows to evaluate and solve some problems in the field of computer aided design (CAD) and of the drawing for the descriptive geometry. The application made considerable use CAD graphics allowing you to appreciate the primary interoperability between AutoCAD and AutoLISP.