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June 29, 2007 1 The difference between two feature models Matthijs Sypkens Smit Willem F. Bronsvoort CAD ’07 Conference, Honolulu, Hawaii Faculty of Electrical Engineering, Mathematics and Computer Science
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CAD’072 Outline Research motivation Feature modelling The feature difference Modelling the feature difference Application: efficient remeshing Conclusion
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CAD’073 Research motivation 1 Efficient repeated processing of large models In particular: remeshing for FEA after model modification CAD modelFEA mesh
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CAD’074 Research motivation 2 Model modification:
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CAD’075 Research motivation 3 Meshes for variants of model: 25000 points; 128,521 tets25000 points; 128,751 tets ~1000 tets in feature ~1900 tets in feature
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CAD’076 Research motivation 4 1.Common practice: Full mesh generation each time 2.Our goal: Remeshing of previous mesh meshing modify modelremeshing
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CAD’077 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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CAD’078 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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CAD’079 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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CAD’0710 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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CAD’0711 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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CAD’0712 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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CAD’0713 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 1model 2 Looking from the point of view of a single feature: persistent non-persistent persistence according to the baseblock:
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CAD’0714 The feature difference 6 The difference for elements of each feature: persistent (P)non-persistent (N) manifestation identical (P i ) manifestation different (P d ) model 1 (N 1 ) “old” model 2 (N 2 ) “new” [geometry] [manifestation = on bound./in volume] ( P d1 / P d2 )
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CAD’0715 The feature difference -2D Example (1) Feature F 1 has a change of interaction with attached feature F 2 For feature F 2 all remains the same Relocating a feature: P i identical P d1 bound. in 1 P d2 bound. in 2 N 1 only in 1 N 2 only in 2
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CAD’0716 The feature difference -2D Example (2) Feature F 1 has a change of interaction due to new feature F 3 Feature F 3 is completely new to the model For feature F 2 all remains the same Adding / removing a feature: P i identical P d1 bound. in 1 P d2 bound. in 2 N 1 only in 1 N 2 only in 2
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CAD’0717 The feature difference - reshaping How to handle changing feature shape? “Self-interaction” Solution not unique! Align on fixed reference point consistent, deterministic
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CAD’0718 The feature difference -2D Example (3) Feature F 1 has a change in interaction with F 2 Feature F 2 has been scaled and translated For feature F 3 the interaction with F 2 changes Combining translation, reshaping and negative nature: P i identical P d1 bound. in 1 P d2 bound. in 2 N 1 only in 1 N 2 only in 2 Note: F 3 is a hole
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CAD’0719 Modelling the feature difference 1 Two 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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CAD’0720 Modelling the feature difference 2 Two main steps: 1.Non-regular union 2.Categorisation of entities P i, P d, N 1, N 2 Start union: default N 1 /N 2 On merge (V ertex -V ertex, E-E, F-F, C-C): comparison P i /P d
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CAD’0721 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% persistent New or deleted features are 100% non-persistent a single attribute is sufficient for those featues Feature correspondence between models essential difference modelinput
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CAD’0722 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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CAD’0723 Application: efficient remeshing 2 Original mesh: Variational tetrahedral meshing (Delaunay connectivity) Sketch of a remeshing approach: 1.Construct difference model 2.Per feature, copy points based on persistent volume 3.Mesh new geometry 4.Mark points on/near non-persistent geometry 5.Optimise marked points
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CAD’0724 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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CAD’0725 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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CAD’0726 Credits Research supported by NWO (Netherlands Organisation for Scientific Research)
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CAD’0727
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