Houdini 17.5 Dynamics

Top ten advanced things about dynamics

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Unique IDs, object ids, and object names






Rendering with motion blur can change the results

Rendering with motion blur will cause the simulation to be evaluated at shutter open (frame number - shutter time) and shutter close, for example frame 1.7 and 2. This forced sub-stepping can change the results of the simulation.

To fix this, you can cache the simulation to disk and use a Timeblend geometry node.

Data names



For example, in this network:


You can turn off Unique data name if you really want later nodes to override the data of earlier nodes with the same name. This may be useful if you animate the activation of the earlier and later nodes, for example to switch between the settings/positions of two different fan force nodes. The nodes will be evaluated left to right, top to bottom. You can also use the "set" pop-up menu (see below) for more control over how one node overrides another using the same data name.

Data sharing





No sharing


Share data across timestep



Share data across all time





You can create groups of objects in the simulation using the Group node, just like you can create groups of particles in a particle network and groups of faces in a geometry network.

You can include objects in a group by their object ID or object name. You can also include other groups by their group name.

Groups have three uses/effects in a dynamics simulation:

  • You can set up groups so you can refer to a set of objects by their group name, instead of using complicated masks.

  • Technically, affector relationships exist between groups, not individual objects.

  • Because objects in a group affect each other, everything inside a group must always be solved simultaneously.

Collision representations

The Houdini RBD solver supports two types of collision representations: volume and thin-plate. Use volume-based collisions for normal objects, and thin-plate collisions for objects without volume.

Volume-based collision detection creates a volume representation of each object. You can visualize the volume representation of an object and control how it is created with the options on the Volume tab on the Collision tab of an object’s parameters.

To detect collisions, Houdini intersects the surface representation of one object with the volume representation of the other object, and vice versa.

The default surface representation for geometry is just the object’s points. This works well with dense geometry, but doesn’t work well if an object has sharp edges or not many points. Alternatively, you can use edges to represent the surface. In an object’s parameters, click the Collisions tab, then click the Surface tab and set Surface representation to "Edges".

Edge representation is more expensive than using points, and for most complicated models it’s unnecessary overkill. However, for certain geometry types, such as a cube without points on the faces, you will need to use edges.

If your object has no volume (such as pieces of paper and ground planes), turn off Use volume based collision detection on the object’s Collision tab to have Houdini treat it as a thin surface.

If a thin plate object collides with a volume-based object, it uses volume-based collision. However, if two thin-plate objects collide, Houdini uses a per-triangle collision test.

Thin-plate objects do not stack well. If you need to stack them up, you should increase the minimum number of sub-steps on the DOP Network container node.


Learning dynamics

Colliding objects

Simulation types


Non-DOP simulations

Next steps