Research On Similarity Evaluation Of Free-form Surface CAD Model

Free-form surfaces are widely used,but their modeling and processing are complex.By retrieving existing similar designs and processing examples,engineers and technicians can learn from and inspire.This re-use technology can greatly improve the efficiency of engineers and reduce Enterprise costs increase profits.Judging whether the two free-form surfaces are similar is the key technology for reuse.Therefore,this thesis studies the theory and method of the similarity evaluation of free-form surfaces.In this thesis,we study the skeleton algorithm for extracting the main normal direction monotone free-form surface(any line along the main normal direction of the surface has at most one intersection with the surface),and use the skeleton as the feature of the surface to complete the two freedoms by using the EMD algorithm.The similarity of the surface skeleton is compared,and the comparison result is taken as the similarity value of the two free-form surfaces.Finally,the rationality of the research results is verified by experiments.The research work overview and the innovative conclusions obtained are as follows:(1)Simple skeleton extraction algorithm: First calculate the total normal vector of the free-form surface,project the free-form surface to the two-dimensional plane according to the inverse of the total normal vector,and obtain the projected surface.Calculate the three main directions of the projected surface by PCA algorithm(The projection plane is represented by three-dimensional space point coordinates,and the two sets of planes are intersected with the projection surface by using the first and second main directions as normal vectors respectively,and intersection lines are generated to find the points,and these midpoints are used as skeleton points of the projection surface.According to certain rules,these skeleton points are connected to obtain the skeleton line of the projected surface.Finally,the skeleton line is taken as the busbar,and the cylinder is stretched in the direction of the total normal vector and intersects with the free surface.The resulting intersection line is a simple skeleton line of the free surface.(2)Similarity comparison algorithm: firstly extract the points on the simple skeleton line by using certain rules,and then calculate the Euclidean distance between the point taken and the centroid of the free-form surface,and the total number of points and each of the Euclidean distance and the skeleton line.The weight of the point is used as the three parameters of the EMD algorithm.The EMD algorithm is used to calculate the similarity value of the skeleton line as the similarity value of the free-form surface.(3)This thesis uses Microsofa Visual Studio 2010(VS2010)as the development environment and the open source geometry kernel Open CASCADE(OCC)as the development platform to write a free-form surface similarity prototype system.At the same time,the monotonic free-form surface is taken as an example,and the experiment is carried out to verify the effectiveness and feasibility of the proposed algorithm.