Houdini 20.0 Crowd simulations

Interaction with other solvers

Tips on setting up interaction between agents and other types of dynamics.

On this page


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.

General tips

  • 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.


Bullet solver

  1. Use the Agent Edit SOP to switch the agents' current layers to their collision layer, and use an Agent Unpack SOP to unpack the agents.

  2. Use the RBD Fractured Object shelf tool on the Rigid Bodies tab to create RBD objects for the shapes from the agents' collision layers.

  3. 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

  1. Use the Agent Unpack SOP to unpack the agents.

  2. Use the Deforming Object shelf tool on the Collisions tab to bring the collision geometry into the fluid or particle simulation.

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.

Crowd simulations

Getting started

  • Basics

    An overview of Houdini crowd simulation concepts.

  • Setup

    How to set up and edit a crowd simulation.

The moving parts

  • Agents

    About agents, the moving actors that make up a crowd simulation.

  • States

    About agent states, the virtual mood of each agent that controls the agent’s animation and the behaviors it runs.

  • Triggers

    How to specify conditions that cause agents to change from one state to another.

  • Caches

    Tips for efficiently caching and loading crowd sims.

SOP crowds



  • Diversity

    How to create a more realistic crowd by making agents look and act differently.

  • Attaching cloth

    You can add and constrain vellum cloth as part of agent shape geometry, and then simulate the cloth based on agent movements.


  • Foot planting

    How to set up agents to adapt their animation to terrain and prevent skating.

  • Terrain

    How to specify terrain geometry for agents to walk across.

  • Obstacles

    How to set up obstacles for agents to avoid.