Lecture Autodesk inventor: Solid modeling data structures. The main contents of the chapter consist of the following: Constructive solid geometry (CSG), boundary representation (B-rep), hybrid of CSG and B-rep.
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Solid Modeling Data Structures
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Types of Solid Modelers
Constructive solid geometry (CSG)
Boundary representation (Brep)
Hybrid of CSG and Brep
Categorization based upon way data is stored in database
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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)
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CSG Tree Structure
Tree structure
represents model
history
Shows which
primitives (leaves)
and operations
(nodes) were used,
and their order
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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
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Brep Data Structure
Euler’s Formula: V – E + F = 2
V = # of vertices
E = # of edges
F = # of faces
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CSG and Brep Comparison
Union of two
box primitives
CSG information
Brep information
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Hybrid Systems
Store model using both CSG and Brep methods
Uses CSG as primary, Brep as secondary for display
Large file sizes
Most commercial solid modelers employ
a combination of both CSG and Brep
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Modeling Kernels
Engine used by software to perform basic functions (Boolean operations, blending, mass properties, exporting geometry,
chamfer/fillet)
Common kernels:
Parasolid
Easier to translate between modelers that use same kernel
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3D Modeling Software
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Faceted versus Exact Representation
Faceted Exact
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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
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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)
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Solid Modeling Data Structures
Khallas!