June 29, 2007

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The difference between twofeature models

Matthijs Sypkens SmitWillem F. Bronsvoort

CAD ’07 Conference, Honolulu, Hawaii

Faculty of Electrical Engineering, Mathematics and Computer Science

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Outline

• Research motivation• Feature modelling• The feature difference• Modelling the feature difference• Application: efficient remeshing• Conclusion

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Research motivation 1

Efficient repeated processing of large models

In particular: remeshing for FEA after model modification

CAD model FEA mesh

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Research motivation 2

Model modification:

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Research motivation 3

Meshes for variants of model:

25000 points; 128,521 tets 25000 points; 128,751 tets

~1000 tets infeature

~1900 tets infeature

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Research motivation 4

1. Common practice:

Full mesh generation each time

2. Our goal:

Remeshing of previous mesh

meshing

meshing

modify model remeshing

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Feature modelling 1

• Current product modelling systems use feature models

• Products are represented with features: holes, slots, pockets, protrusions, etc.

• Features have a generic shape that is controlled through parameters

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Feature modelling 2

Modification of feature models:• Parameter values / constraints• Addition and removal of features

As a result: change in geometry

Our aim: a description of the difference that facilitates efficient remeshing

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Adapting a model: Deriving a new mesh:

Intuitive solution:Let features carry their geometry (and mesh) with them

The feature difference 1

?

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When feature geometry is preserved: mesh local to that feature can be copied

Complications for change in interaction/attachment: local changes to feature geometry:

The feature difference 2

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The feature difference 3

How to describe the geometric difference?

Look from point of view of the features

Natural choice:the variation of the model is through the features

For each feature the local change in geometry is recorded

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The feature difference 4

Copying parts of the mesh:Parts can only be copied when underlying geometry can be mapped between models

• Geometry that can be mapped is persistent• To find intuitive persistence we look at

the feature geometry ( ≠ BRep geometry )

Geometry that cannot be mapped is non-persistent

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• Features’ own geometry is persistent, unless reshaped or not present in both models

• Change in interaction non-persistent geometry

• Manifestation of persistent geometry can change

The feature difference 5

model 1

model 2

Looking from the point of view of a single feature:

persistent

non-persistentpersistence according to the baseblock:

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The feature difference 6

The difference for elements of each feature:

persistent (P) non-persistent (N)

manifestationidentical (Pi)

manifestationdifferent (Pd)

model 1 (N1)

“old”

model 2 (N2)

“new”

[geometry]

[manifestation = on bound./in volume]

( Pd1 / Pd2 )

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The feature difference -2D Example (1)

• Feature F1 has a change of interaction with attached feature F2

• For feature F2 all remains the same

Relocating a feature:

Pi identical

Pd1 bound. in 1

Pd2 bound. in 2

N1 only in 1

N2 only in 2

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The feature difference -2D Example (2)

• Feature F1 has a change of interaction due to new feature F3

• Feature F3 is completely new to the model

• For feature F2 all remains the same

Adding / removing a feature:

Pi identical

Pd1 bound. in 1

Pd2 bound. in 2

N1 only in 1

N2 only in 2

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The feature difference - reshapingHow to handle changing feature shape?

“Self-interaction”

Solution not unique!

Align on fixed reference point consistent, deterministic

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The feature difference -2D Example (3)

• Feature F1 has a change in interaction with F2

• Feature F2 has been scaled and translated

• For feature F3 the interaction with F2 changes

Combining translation, reshaping and negative nature:

Pi identical

Pd1 bound. in 1

Pd2 bound. in 2

N1 only in 1

N2 only in 2

Note:F3 is a hole

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Modelling the feature difference 1Two main steps:1. Non-regular union

merge of objects; all original entities are kept

For the complete geometry of corresponding features

Implementation on top of geometric modelling kernel (ACIS)

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Modelling the feature difference 2Two main steps:1. Non-regular union2. Categorisation of entities Pi, Pd, N1, N2

Start union: default N1/N2

On merge (Vertex-Vertex,E-E, F-F, C-C):comparison Pi /Pd

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Modelling the feature difference 3• The difference model is the set of all individual

feature differences• Complete explicit construction not necessary:

Many features will be 100% persistentNew or deleted features are 100% non-persistent a single attribute is sufficient for those featues

• Feature correspondence between models essential

difference model

input

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Application: efficient remeshing 1

Assumptions:• Model modifications influence geometry only

locally• Considerable degree of feature correspondence• Mesh generation optimisation based

time consuming construction

limited, local change

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Application: efficient remeshing 2

Original mesh:Variational tetrahedral meshing (Delaunay connectivity)

Sketch of a remeshing approach:1. Construct difference model2. Per feature, copy points based on persistent volume3. Mesh new geometry4. Mark points on/near non-persistent geometry5. Optimise marked points

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Conclusions 1

• Feature point of view leads to natural/intuitive difference

• Feature difference applies to feature aspects in general any attribute local to a feature can be compared

• Difference model and remeshing handle changes in topology!

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Conclusions 2

• Speed-up when remeshing similar models for FEA intended for quality meshes of large models

• Easier direct comparison of FEA result persistent regions with largely identical mesh

Open for investigation:• Practical investigation of remeshing (work in

progress)• Dealing with mesh sizing• Other meshing algorithms / mesh types

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Credits

Research supported by NWO(Netherlands Organisation for Scientific Research)

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