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The difference between two feature models

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. Outline. Research motivation Feature modelling The feature difference

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The difference between two feature models

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  1. The difference between twofeature models • Matthijs Sypkens Smit • Willem F. Bronsvoort • CAD ’07 Conference, Honolulu, Hawaii Faculty of Electrical Engineering, Mathematics and Computer Science

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

  3. Research motivation 1 Efficient repeated processing of large models In particular: remeshing for FEA after model modification CAD model FEA mesh

  4. Research motivation 2 Model modification:

  5. Research motivation 3 Meshes for variants of model: ~1000 tets in feature ~1900 tets in feature 25000 points; 128,521 tets 25000 points; 128,751 tets

  6. Research motivation 4 meshing • Common practice: Full mesh generation each time 2. Our goal: Remeshing of previous mesh modify model remeshing meshing

  7. 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

  8. 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

  9. The feature difference 1 Adapting a model: Deriving a new mesh: Intuitive solution: Let features carry their geometry (and mesh) with them ?

  10. The feature difference 2 When feature geometry is preserved:  mesh local to that feature can be copied Complications for change in interaction/attachment:  local changes to feature geometry:

  11. The feature difference 3 How to describe the geometric difference? Look from point of viewof the features Natural choice: the variation of the model is through the features For each feature the local change in geometry is recorded

  12. 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

  13. persistent non-persistent The feature difference 5 • 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 Looking from the point of view of a single feature: model 1 model 2 persistence according to the baseblock:

  14. The feature difference 6 The difference for elements of each feature: [geometry] persistent (P) non-persistent (N) manifestation identical (Pi) manifestation different (Pd) model 1 (N1) “old” model 2 (N2) “new” ( Pd1 / Pd2 ) [manifestation = on bound./in volume]

  15. Pi identical Pd1 bound. in 1 Pd2 bound. in 2 N1 only in 1 N2 only in 2 The feature difference -2D Example (1) Relocating a feature: • Feature F1 has a change of interaction with attached feature F2 • For feature F2 all remains the same

  16. Pi identical Pd1 bound. in 1 Pd2 bound. in 2 N1 only in 1 N2 only in 2 The feature difference -2D Example (2) Adding / removing a feature: • 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

  17. The feature difference - reshaping How to handle changing feature shape? “Self-interaction” Solution not unique! Align on fixed reference point  consistent, deterministic

  18. Pi identical Pd1 bound. in 1 Pd2 bound. in 2 N1 only in 1 N2 only in 2 The feature difference -2D Example (3) Combining translation, reshaping and negative nature: Note: F3 is a hole • Feature F1 has a change in interaction with F2 • Feature F2 has been scaled and translated • For feature F3 the interaction with F2 changes

  19. 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)

  20. Modelling the feature difference 2 Two main steps: 1. Non-regular union • 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

  21. 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 input difference model

  22. 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

  23. Application: efficient remeshing 2 Original mesh: Variational tetrahedral meshing (Delaunay connectivity) Sketch of a remeshing approach: • Construct difference model • Per feature, copy points based on persistent volume • Mesh new geometry • Mark points on/near non-persistent geometry • Optimise marked points

  24. 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!

  25. 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

  26. Credits Research supported by NWO (Netherlands Organisation for Scientific Research)

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