Solid Modeling Data Structures
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
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
Brep Data Structure
Euler’s Formula: V – E + F = 2
V = # of vertices
E = # of edges
F = # of faces
CSG and Brep Comparison
CSG
information
Union of two
box primitives
Brep
information
Hybrid Systems
Most commercial solid modelers employ
a combination of both CSG and Brep
Store model using both CSG and Brep
methods
Uses CSG as primary, Brep 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
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