Lecture Autodesk inventor Solid modeling data structures. The main contents of the chapter consist of the following Constructive solid geometry (CSG), boundary representation (Brep), hybrid of CSG and Brep.
Solid Modeling Data Structures Types of Solid Modelers Constructive solid geometry (CSG) Boundary representation (B-rep) Hybrid of CSG and B-rep Categorization based upon way data is stored in database Constructive Solid Modeling (CSG) Uses Boolean operations to create complex solids from primitives Primitives, Boolean operations stored in database Can make changes to primitives Tree structure Range of shapes limited (only primitives and Booleans) Since primitives stored (not surface boundaries), must recalculate model each time it is shaded Calculation of NC tool paths for machining is a problem (surface info required) CSG Tree Structure Tree structure represents model history Shows which primitives (leaves) and operations (nodes) were used, and their order Boundary Representation Modeling Stores boundaries of solid in database (e.g., vertices, edges, faces) Similar to surface modeling More difficult to edit once stored Edge information stored in a systematic way (CCW from outside) Shading, hidden line removal can be displayed at will B-rep Data Structure Euler’s Formula: V – E + F = V = # of vertices E = # of edges F = # of faces CSG and B-rep Comparison Union of two box primitives CSG information B-rep information Hybrid Systems Most commercial solid modelers employ a combination of both CSG and B-rep Store model using both CSG and B-rep methods Uses CSG as primary, B-rep as secondary for display Large file sizes Modeling Kernels Engine used by software to perform basic functions (Boolean operations, blending, mass properties, exporting geometry, chamfer/fillet) Common kernels: ACIS Parasolid Easier to translate between modelers that use same kernel 3D Modeling Software Faceted versus Exact Representation Faceted Exact Uses of Solid Models Accurate representation of 3D objects Aid to visualization, communication Can extract 2D drawing information Used to control CAM methods Interference identification Mass property determination Analysis and simulation (finite element, kinematics, ergonomics, etc) Rapid Prototyping Shortcomings of Solid Models Difficult to modify a model’s features Does not capture the design intent Design Intent is the purpose or function of a feature in a part (or of a part in an assembly) Solid Modeling Data Structures Khallas! ...Types of Solid Modelers Constructive solid geometry (CSG) Boundary representation (B-rep) Hybrid of CSG and B-rep Categorization based upon way data is stored in database Constructive Solid Modeling. .. used, and their order Boundary Representation Modeling Stores boundaries of solid in database (e.g., vertices, edges, faces) Similar to surface modeling More difficult to edit once stored Edge... Common kernels: ACIS Parasolid Easier to translate between modelers that use same kernel 3D Modeling Software Faceted versus Exact Representation Faceted Exact Uses of Solid Models Accurate representation