Abstract: A process for representing on a picture screen of a numerical control system a simulated workpiece. The workpiece is represented in several viewing planes with shaded surfaces. The viewing planes and the associated shadings are hierarchically ordered, which is achieved by synchronized shading of different density. A tool is simulated by a silhouette, which presents the shading of the momentary viewing or processing plane which is being worked. In the movement of the shaded silhouette along with the programmed tool path, higher lying viewing planes are erased (or etched) and equally high or lower lying viewing planes are not affected.