Sketching input for computer aided engineering

The design process often begins with a graphical description of the proposed device or system and sketching is the physical expression of the design engineer's thinking process. Computer Aided Design is a technique in which man and machine are blended into a problem solving team, intimately coupling the best characteristics of each. Solid modelling is developed to act as the common medium between man and the computer. At present it is achieved mainly by designing with volumes and hence does not leave much room for sketching input, the traditional physical expression of the thinking process of the design engineer. This thesis describes a method of accepting isometric free hand sketching as the input to a solid model. The design engineer is allowed to make a sketch on top of a digitizer indicating (i) visible lines, (ii) hidden lines, (iii) construction lines, (iv) centre lines, (v) erased lines, and (vi) redundant lines as the input. The computer then processes this sketch by identifying the line segments, fitting the best possible lines, removing the erased lines, ignoring the redundant lines and finally merging the hidden lines and visible lines to form the lines in the solid in an interactive manner. The program then uses these lines and the information about the three dimensional origin of the object and produces three dimensional information such as the faces, loops, holes, rings, edges and vertices which are sufficient to build a solid model. This is achieved in the following manner. The points in the sketch are first written into a file. The computer than reads this file, breaks the group of points into sub-groups belonging to individual line segments, fits the best lines and identify the vertices in two dimensions. These improved lines in two dimensions are then merged to form the lines and vertices in the solid. These lines are then used together with the three dimensional origin (or any other point) to produce the wireframe model in three dimensions. The loops in the wireframe models are then identified and surface equations are fitted to these loops. Finally all the necessary inputs to build a B-rep solid model are produced.