# Crowd force weights

How the Houdini crowd solver decides which behaviors to apply to an agent at each time step.

## Overview

Crowd steering force nodes, and the default forces on the solver, each have weights. At any given moment, an agent can potentially be affected by multiple forces (for example, both "follow this path" and "avoid obstacles"). The crowd solver deals with multiple forces by normalizing the weights (scaling them so they add up to 1) before multiplying the individual forces by them.

## Example

Imagine an agent has the following potential forces that might apply to it at a given moment, with these weights:

Force type

Force amount

Weight

Avoid

10

1.0

1

0.5

Align

2

0.2

1. The solver normalizes the weights so they add up to 1, making the avoid weight 0.59, the follow weight 0.29, and the align weight 0.12.

2. The solver then multiplies the force amounts by the normalized weights, so the avoid force is `10 * 0.59 = 5.9`, the follow force is `1 * 0.29 = 0.29`, and the align force is `2 * 0.2 = 0.4`.

3. Finally, the solver adds the forces to get the overall force on the agent.

## Using weights

All forces in the agent’s current state (and any forces that apply to all agents regardless of state) are applied to the agent. It’s not possible to have a behavior that completely overrides any other behavior. However, you can use a very high weight (for example, `10`), so that when other forces are normalized, they are made unnoticeably small.

## Particle forces

You can use regular particle force nodes and other DOPs inside a state to define behavior, not just crowd-specific "steering" nodes. However, non-crowd-specific nodes don’t have Weight parameters, so they will not be subject to the normalization described above. Instead they will simply be added on to any other forces on the agents.

# Crowd simulations

## Getting started

• An overview of Houdini crowd simulation concepts.

• How to set up and edit a crowd simulation.

## The moving parts

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

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

• How to associate animation with agents in certain states.

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

## Next steps

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

• Useful attributes for reading in triggers, or that you can set to affect behavior.

• How to make agents behave differently based on their virtual senses.

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

• How the Houdini crowd solver decides which behaviors to apply to an agent at each time step.

• How to specify terrain geometry for agents to walk across.

• How to set up obstacles for agents to avoid.

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

• How to assert manual control over different aspects of the crowd simulation.