Thiết kế flash - Solid modeling data structures

Solid Modeling Data StructuresTypes of Solid ModelersConstructive solid geometry (CSG)Boundary representation (B-rep)Hybrid of CSG and B-repCategorization based upon way data is stored in database Constructive Solid Modeling (CSG)Uses Boolean operations to create complex solids from primitivesPrimitives, Boolean operations stored in databaseCan make changes to primitivesTree structure Range of shapes limited (only primitives and Booleans)Since primitives stored (not surface boundaries), must recalculate model each time it is shadedCalculation of NC tool paths for machining is a problem (surface info required)CSG Tree StructureTree structure represents model historyShows which primitives (leaves) and operations (nodes) were used, and their orderBoundary Representation Modeling Stores boundaries of solid in database (e.g., vertices, edges, faces)Similar to surface modelingMore difficult to edit once storedEdge information stored in a systematic way (CCW from outside)Shading, hidden line removal can be displayed at will B-rep Data StructureEuler’s Formula: V – E + F = 2V = # of verticesE = # of edgesF = # of facesCSG and B-rep ComparisonUnion of two box primitives CSG information B-rep informationHybrid SystemsStore model using both CSG and B-rep methodsUses CSG as primary, B-rep as secondary for displayLarge file sizesMost commercial solid modelers employ a combination of both CSG and B-repModeling KernelsEngine used by software to perform basic functions (Boolean operations, blending, mass properties, exporting geometry, chamfer/fillet) Common kernels: ACISParasolid Easier to translate between modelers that use same kernel 3D Modeling SoftwareFaceted versus Exact RepresentationFacetedExactUses of Solid ModelsAccurate representation of 3D objectsAid to visualization, communicationCan extract 2D drawing informationUsed to control CAM methodsInterference identificationMass property determinationAnalysis 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 StructuresKhallas!