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An Introduction to Mesh Generation An Introduction to Mesh Generation AlgorithmsAlgorithms
Part 2Part 2
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 1
Part 2Part 2
Fathi El-YafiProject and Software Development Manager
Engineering Simulation
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Overview
� Remeshing
� Decimation
Adaptive Meshing:
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 2
� Decimation
� Optimization
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Finite Element Context
Objective:
2D Adaptive Mesh according to the specifications sizes imposed by the user
Adaptive Meshing
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 3
Method:� Calculating the density of the mesh from a tree Quaternary� Nodal generation within each cell of the tree� Meshing by advancing front method according to a criterion of Delaunay
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Quaternary Tree
Finite Element Context
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 4
In computer science, a B+ tree (also known as a Quaternary Tree ) is a type of tree, which represents sorted data in a way that allows for efficient insertion, retrieval and removal of records, each of which is identified by a key. It is a dynamic, multilevel index, with maximum and minimum bounds on the number of keys in each index segment (usually called a block' or node'). In a B+ tree, in contrast to a B+ tree, all records are stored at the lowest level of the tree; only keys are stored in interior blocks.
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Remeshing a Polygon
n points
?
Finite Element Context
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 5
Possible Remeshing?
?
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Principle
=
Finite Element Context
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 6
=
=
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Principle
n(n − 1)(n − 2)
n nodes lead to n-2 triangles
=
Finite Element Context
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 7
Tn = n(n − 1)(n − 2)
6It shows that we can create Tn
different triangles
120845635201041
109876543
N=10, 1430 possible triangulations!
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Adaptive Mesh
State of the Art
� Methods that use the surface parameterization:
� Appropriate Plane + transport functions
� Appropriate Plane + Metric: INRIA
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 8
� Discrete Models:
Very Refined Initial Mesh + Simplifications
Reconstruction:
• Direct Method: Frontal Triangulation (R. Löhner)
• Remeshing: HDI (A. Rassineux)
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Typical Examples
Adaptive Mesh
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Adaptive Meshing using Diffuse Interpolation
Objective:
Building an adaptive mesh of a 3D surface according to size map, using the initial mesh as only data
Initial Mesh
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 10
Initial Mesh
Local geometric model
Diffuse Interpolation
Hermite type
Adaptive mesh
Remeshing Procedure
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Remeshing
Adaptive Mesh
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Some Remeshing Tools
Edge splitting
Edge collapsing
Adaptive Mesh
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Swapping
Nodal Shifting
Node removing
Edge collapsing and nodal insertion
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Remeshing
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 13
Remeshing according to a size map
Simplification of a mesh (70%)
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Remeshing
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 14
Initial Mesh Intermediate Mesh Optimized Mesh
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Surface Remeshing
Adaptive Mesh
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Initial Mesh
Optimized Mesh
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Surface Remeshing
Reduction: 60%Reduction: 75%
Initial model
Adaptive Mesh
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Adapted to curvature
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Surface Meshing
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 17
Adapted to curvature
Simplified model
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Simplified Curvature
In a plane 360iθ = °∑
ααααιιιι
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 18
Θ1
Θ2
Of all the triangles 360iα ≥ °∑default angle = f (Gaussian Curvature)( ) =∆=−∑ θθα ii
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Feature Lines Detection
Adaptive Mesh
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Feature Lines Detection
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 20
� Curvature using Diffuse Interpolation Model
(a) (b)
� Identification of geometrical parameters (radius, axis)
� Detecting neighborhood
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Principal Curvatures
� Segmentation into subsets
� Surface Identification
� Detection of Feature lines
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 21
� Meshing
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Example� Segmentation into subsets
� Surface Identification
� Detection of Feature lines
� Meshing
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 22…
� Meshing
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Main Steps
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 23…
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Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 24
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Diffuse Model
z
y
αα TPyxyxyxyxfz =>=<= 22 ,,,,,1),(Local model
MLS Technique : Diffuse approximation
Minimisation of an interpolation criterion
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 25
x
( ) ( )( )21
1 ),( iiT
ni
iix zxxpxxwJ −−×=∑
=
=αα
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Moving Least Squares
( ) ( )2 21, , , , , ,T Tx y x xy y p x y p x< >= =� Polynomial Base
( )i n=
n = Total number of points ⇒ «standard» Least Squares
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 26
( )( )2
1
( ) ( )i n
Tx i i
i
J p x x f x xα α=
=
= − − −∑
X
Z
R
� Select a neighbor V(x) around a point(x,y)
( )( )2
( )
( ) ( )Tx i i
i V x
J p x x f x xα α∈
= − − −∑
⇒ The approximation is local
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Weighted Moving Least Squares
( ) ( )( )2( ) ( )TJ w x x p x x f x xα α= − − − −∑
Weight Function
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 27
( ) ( )( )2
( )
( ) ( )Tx i i i
i V x
J w x x p x x f x xα α∈
= − − − −∑
/R d l
)l()-l(,x)w(x
w,x)w(x
ii
iii
xR
xxirefi
=+×=
=
−
avec
211 2
)(
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Moving Least Squares Interpolation
( ) ( )( )2
( )
( ) ( )Tx i i i
i V x
J w x x p x x f x xα α∈
= − − − −∑
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 28
[ ]
−→
),(1
),(),(
xxw
xxwxxw
i
ii
If w(x,xi)→→→→ ∞∞∞∞ ⇒⇒⇒⇒ interpolation at point xi
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CAD-STL Face Per Face
Adaptive Mesh
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Finite Element Context
STL/Mesh/Adaptive Mesh
STL Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 30
Mesh Face per Face Adaptive Mesh
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STL/Adaptive Mesh
Adaptive Mesh
Adaptive Mesh
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STL Mesh
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CAD/Adaptive Meshes
Adaptive Mesh
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Mesh Details
Adaptive Mesh
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Feature Lines
Adaptive Mesh
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Surface Remeshing
Adaptive Mesh
Meshing Generation TutorialInstituto Tecnológico de Veracruz 21-25 April 2008 35
Initial model Simplified model
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Surface Remeshing
Adaptive Mesh
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Surface Remeshing
Adaptive Mesh
Adaptive Mesh
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Initial Mesh
Adaptive Mesh
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Surface Remeshing
Adaptive Mesh
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Repairing Surface Mesh
Adaptive Mesh
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Demonstrations
Adaptive Mesh
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