1. A method for rendering and/or evaluating a quality of a workpiece surface based on CNC program data for a machining operation, the CNC program data including a set of points defining a surface in space, with the set of points including path points describing a space curve, comprising the steps of: determining and rendering associated normal vectors for a plurality of adjacent path points; indicating surface regions having a high surface quality by normal vectors pointing in a substantially identical direction; and indicating surface regions having flaws by normal vectors pointing in different directions.