1. 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

2. CAD’072 Outline Research motivation Feature modelling The feature difference Modelling the feature difference Application: efficient remeshing Conclusion

3. CAD’073 Research motivation 1 Efficient repeated processing of large models In particular: remeshing for FEA after model modification CAD modelFEA mesh

4. CAD’074 Research motivation 2 Model modification:

5. 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

6. CAD’076 Research motivation 4 1.Common practice: Full mesh generation each time 2.Our goal: Remeshing of previous mesh meshing modify modelremeshing

7. 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

8. 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

9. CAD’079 Adapting a model:Deriving a new mesh: Intuitive solution: Let features carry their geometry (and mesh) with them The feature difference 1 ?

10. 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

11. 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

12. 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

13. 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:

14. 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 )

15. 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

16. 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

17. CAD’0717 The feature difference - reshaping How to handle changing feature shape? “Self-interaction” Solution not unique! Align on fixed reference point  consistent, deterministic

18. 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

19. 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)

20. 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

21. 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

22. 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

23. 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

24. 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!

25. 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

26. CAD’0726 Credits Research supported by NWO (Netherlands Organisation for Scientific Research)

27. CAD’0727