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Currently, Houdini only supports one-way interaction from agents to other solvers. For example, an agent can knock over an RBD object or splash through water, but an RBD object cannot knock over an agent.
On the Agent ROP or Agent SOP that is used to create the agents, ensure that the Generate Collision Layer parameter is turned on. This will create a layer that contains a low-resolution static shape for each bone in the rig, which can be used for efficient collision detection.
Use a Delete SOP to remove distant agents that will not collide with any objects in the simulation.
Use the RBD Fractured Object shelf tool on the Rigid Bodies tab to create RBD objects for the shapes from the agents' collision layers.
On the newly-created RBD Packed Object DOP, set the Initial Object Type parameter to Create Animated Static Objects. This will cause the RBD objects' transforms to be updated each frame to match the motion of the agents.
Consider changing the Geometry Representation parameter to a primitive shape (such as Capsule or Box), which can improve performance.
FLIP solver and POP solver
Advanced: simulation on top of agents
If you want to create secondary simulation on top of agent geometry (for example, simulated clothing), you must unpack the agent geometry.
To calculate the positions of an agent’s joints (for example, if you want to generate particles flowing from the top of an agent’s head), you can use the Agent Unpack SOP with the Output parameter set to Joints.