Graphics

cgvr.korea.ac.kr Graphics Lab @ Korea University

Solid Modeling

고려대학교 컴퓨터 그래픽스 연구실

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CGVR

Graphics Lab @ Korea University

Solid Modeling

Represent Solid Interiors of Objects Surface may not be described explicitly

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Motivation

Some Acquisition Methods Generate Solids Example: CAT scan

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Motivation

Some Applications Require Solids Example: CAD/CAM

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Motivation

Some Algorithms Require Solids Example: ray tracing with refraction

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Solid Modeling Representation

Voxels Quadtrees & Octrees Binary Space Partitions Constructive Solid Geometry

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CGVR

Graphics Lab @ Korea University

Voxels

Partition Space into Uniform Grid Grid cells are called a voxels (like pixels)

Store Properties of Solid Object with Each Voxel Occupancy Color Density Temperature Etc.

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Voxel Acquisition

Scanning Devices MRI CAT

Simulation FEM

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Voxel Storage

O(n3) Storage for n x n x n Grid 1 billion voxels for 1000 x 1000 x 1000

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CGVR

Graphics Lab @ Korea University

Voxel Boolean Operations

Compare Objects Voxel by Voxel

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CGVR

Graphics Lab @ Korea University

Voxel Display

Isosurface Rendering Render surfaces bounding volumetric regions of cons

tant value (e.g., density)

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CGVR

Graphics Lab @ Korea University

Voxel Display

Slicing Draw 2D image resulting from intersecting voxels wit

h a plane

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CGVR

Graphics Lab @ Korea University

Voxel Display

Ray Casting Integrate density along rays through pixels

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Voxels

Advantages Simple, intuitive, unambiguous Same complexity for all objects Natural acquisition for some applications Trivial boolean operations

Disadvantages Approximate Large storage requirements Expensive display

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Solid Modeling Representation

Voxels Quadtrees & Octrees Binary Space Partitions Constructive Solid Geometry

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Quadtrees & Octrees

Refine Resolution of Voxels Hierarchically More concise and efficient for non-uniform objects

Uniform Voxel Quadtree

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CGVR

Graphics Lab @ Korea University

Quadtree Boolean Operations

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CGVR

Graphics Lab @ Korea University

Solid Modeling Representation

Voxels Quadtrees & Octrees Binary Space Partitions Constructive Solid Geometry

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Binary Space Partitions (BSPs)

Recursive Partition of Space by Planes Mark leaf cells as inside or outside object

a

bcd

efg

Object

a

bcd

efg

Binary Spatial Partition

1

2

3

45

6

7

12

a

3

bc

4

d

56

e

7

f

BSP Tree

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CGVR

Graphics Lab @ Korea University

BSP Fundamentals

Single Geometric Operation Partition a convex region by a hyperplane

Single Combinatorial Operation Two child nodes added as leaf nodes

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CGVR

Graphics Lab @ Korea University

BSP Display

Visibility Ordering Determine on which side of plane the viewer lies

Near-subtree -> polygons on split -> far-subtree

A

B

C

o2

o1

o3

o4

Viewer

Partitioning TreeA

B C

o1 o2 o3 o43rd 4th 1st 2nd

Viewer

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Solid Modeling Representation

Voxels Quadtrees & Octrees Binary Space Partitions Constructive Solid Geometry

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Constructive Solid Geometry (CSG) Represent Solid Object as Hierarchy of Boolean

Operations Union Intersection Difference

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

CSG Acquisition

Interactive Modeling Programs CAD/CAM

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

CSG Display & Analysis

Ray Casting

Union

Circle Box

cgvr.korea.ac.kr

CGVR

Graphics Lab @ Korea University

Summary

Voxels Octree BSP CSGNo No Some SomeNo No No YesNo No Yes Yes

Some Some No SomeYes Yes Yes No

Yes Yes Yes Yes

No No Yes No

AccurateConciseAffine InvariantEasy AcquisitionGuaranteed ValidityEfficient Boolean OperationsEfficient Display