Houdini 20.0 Crowd simulations


Tips for efficiently caching and loading crowd sims.

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Reducing File Size

When saving a set of agent primitives to disk, their shared agent definition(s) are also embedded in the geometry file so that the agents can be reconstructed when the file is loaded.

Each component of the agent definition can be marked as an external reference to a file on disk. This occurs when, for example, when the Agent SOP’s Input parameter is set to Disk, or when using the Agent Clip SOP to load animation clips from disk. In such cases, only the path to that file will be stored in the geometry file when saving out the agent definition (similar to a packed disk primitive), which significantly reduces the size of the geometry files when saving a sequence of frames. Otherwise, the contents of that component (such as the clip or shape library) will be embedded in the geometry file in the same manner as a packed geometry primitive.


Set the Agent ROP’s Source parameter to Agent Primitive to write out the agent definition of an existing agent primitive to disk.

Loading File Caches

When loading agents from a geometry file, the agent definition and any of its externally referenced files will also be read from disk.

If the agent definition does not contain external references, this may reduce performance when loading a sequence of files. In addition to having to load larger files, having a separate copy of the agent definition in each file may cause slower viewport performance since the agent definition will appear to change each frame.

If the agent definition components are external references, Houdini is able to avoid reloading the agent definition files on each frame to improve performance. Houdini maintains a reference-counted cache of agent definition files, so entries will be removed when no agent primitive in memory references those files. This means that it is recommended to set the File SOP’s Cache Frames parameter to at least 1. Otherwise, there can be unnecessary flushing and reloading of the agent definition files when the File SOP’s geometry is cleared before loading the next file in the sequence.


For equivalent performance to viewing a cached sim from a DOP network, set the File SOP’s Cache Frames parameter to the number of simulation frames.

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.