Appendix: Cleaning and Preparing STL Surfaces for Contact Calculation

To use the AnyForceSurfaceContact, it is necessary that the STL- or AnySurf-file fulfils some requirements to obtain good results.

  • The surface should not have sharp edges in the region of contact.

    Sharp edges can cause a jump in calculation of the penetration depth of the two surfaces and thus can cause jumps in the contact force. For the FDK-solver, this is not easy to handle and can cause slow convergence or even failure.

  • The surface should not have too large curvatures.

    Large curvatures on a surface cause, similar to sharp edges, lead to jumps in the calculation depth.

    On the master surface in a contact pairing (i.e. the surface for which distances for each vertex to the slave surface are calculated) big curvatures on the surface means big variations of the vertex normal, which are used to calculate the force direction. Thus, large curvatures causes can cause large variations in the force directions.

  • The surface should not contain too small triangles.

    Small triangles, i.e. vertices which are too close to each other are collapsed into one vertex during the process of converting STL-files to AnySurf objects. Thus, too small triangles end up as degenerated triangles with edges of zero length and without area. These triangles must be ignored in the contact calculation and warnings show up.

  • The face normal for each face of the surface should have a unique orientation.

    Make sure that the surface normal and the numbering of triangles (best counter clockwise for a consistent definition of the outer face normal) are consistent. The normal is used to define inside and outside of the surface and thus, which direction is penetration.

  • To check the above mentioned requirements, you can use an external tool such as MeshLab ( ). To smooth sharp edges and large curvatures, filters, such as Laplacian filters, or others can be used, depending on the surface. Checking the face normals can be done by using the function Render->Show Face Normal.

    To delete small triangles in the surface, the following workflow can be used:

  1. Load STL-file in MeshLab

  2. Use Filters->Cleaning and Repairing->Merge Close Vertices. Use an abs tolerance bigger than 0.0001

  3. Use Filters->Cleaning and Repairing->Remove Duplicate Faces

  4. Use Filters->Cleaning and Repairing->Remove Duplicate Vertices

  5. Use Filters->Cleaning and Repairing->Remove Zero Area Faces