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Houdini’s FEM Solver uses a “soft” collision handling approach: The strength of the collision response is controlled by the Repulsion parameter that can be found on the Solid Object and the Hybrid Object. The FEM Solver intentionally limits the strength of the collision force this way to help ensure the stability of simulations with impossible configurations (pinching) and to help guarantee predictable performance. In many situations, the Repulsion parameter will need to be adjusted to prevent visible penetrations. As a rule of thumb, try your simulation with the default Repulsion parameter. If there are penetrations, keep adding a zero to the Repulsion value (making it 10× larger) until it succeeds.
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The
Finite element solver supports both volume-based (SDF) collisions and surface collisions.-
Surface-based collisions are collisions against the boundary surfaces of tet meshes.
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Surface-based collisions are the most accurate option. They give you continuous collision detection. This means that the solver examines the trajectory of vertices between frames to see if at any time in between these frames objects intersected. This helps to ensure that collisions between fast moving objects are processed by the solver.
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Surface-based collisions are the only choice for geometry that is deforming over time.
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If you use surface-based collisions, it is important that the collision objects are tetrahedralized: Collisions of FEM objects against polygonal meshes won’t work with soft collisions.
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Volume-based collisions are collisions between primitive vertices and signed distance fields (SDFs).
Volume-based collisions are a faster, but less accurate option. They are an efficient choice for large objects that are part of the scenery.
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Does not support continuous collision detection.
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Only detects collision of vertices with SDFs (not edges or faces).
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Do not use with deforming or fast moving objects.
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You can choose whether collisions with static objects use volume collisions by turning on the Use volume based collision detection parameter on a
Static object. -
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Solid objects can be affected by other dynamics objects such as RBDs, but can only have two-way interactions (where the solid both affects and is affected by the other object) with other finite element objects (solids and cloth).
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Finite element (solid and cloth) objects can collide with themselves, if you enable Collide within object on the object’s Collisions tab. For quicker simulations turn self-collisions off.
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The
Surface Collider and 
Volume Collider tools on the FEM shelf set up Static objects with the correct parameters for colliding with finite element objects. -
There are no convexity restrictions on collision detection. Collisions work with convex and non-convex shapes.