Dealing with FEM in Houdini might me one of the coolest and most challenging things to do. What is FEM? Finite element methods (FEM) let you simulate solid objects, that is, objects with “stuff inside”. Here are some awesome examples from Erik Ferguson for you to better understand the magic here.
You can find more of his crazy Houdini stuff here.
Interested in mastering FEM? To do that you need to understand how finite element solver works in Houdini:
Before each solve, the finite element solver reads the simulation state of each object from the attributes on the simulation geometry Geometry to get the previous state. After the solve, the new state is written to attributes the Geometry. In addition, the solver may maintain an EmbeddedGeometry. This would typically be a more higher-resolution geometry that moves and fractures along with Geometry. The embedded geometry can consists of polygons or tetrahedrons (or a mix of these two).
The finite element solver approximates the physics of continuous materials by splitting them up into a finite number of elements. In the case of the Solid object, the elements are determined by the tetrahedrons. In the case of the Cloth Object, the elements are determined by triangles and quadrangles. The resolution of the tetrahedrons and the orientations of the individual tetrahedrons have little influence over the overall movement; as long as the overall solid shape is the same the behavior is roughly the same (except for very coarse meshes). The finite element method (FEM) treats the elements an approximation of a continuous material. This property makes the results very predictable when you simulate the same shape using a lower-res and a higher-res mesh.