The input to the SOP crowds motion path tools is a crowd of agents.

Below is one example of a basic setup for creating a crowd of agents (see setup for more information on how these nodes are used in crowds):

The motion paths generated by the Crowd MotionPath SOP are represented by curves that describe the agents' positions at each point in time within a specified frame range. The paths have the whole time range laid out in front of them. By default, the agents' paths are generated from the animation clip assigned to the agent. The motion data is taken from the animation clip and cycled forward to generate the agents' positions at each point in time. The Crowd MotionPath tools are similar to the MotionClip tools, where the poses of the skeleton are laid out across time.

To change the animation clip(s) used to generate the agent paths:

1. On the Crowd MotionPath SOP, Clip Assignment section, set Clip Names to the name of the clip. If you have a library of animation clips, you can specify different clips to be randomly selected for agents to play. You can specify multiple clips in each Clip Names parameter.

   Note

   If you select a clip from the Clip Names drop-down menu, the clip name will be appended to the existing set of entries in Clip Names.

2. Set the Weight value for the group of clips. A higher weight makes it more likely for agents to play those clips.

3. Click beside Clip Groups to add clips with different weights.

Parent-child relationships between agents can be set up using the Agent Relationship SOP. When edits are made to the motion paths of the parent agents, the child motion paths are deformed to keep the child agents attached to their parents.

To generate motion paths for both the parent and child agents, connect the Agent Relationship SOP to the Crowd MotionPath SOP. In this example, we attach rider agents (child) to horse agents (parent):

See parent-child agent relationships for more information on the Agent Relationship SOP’s parameters and attributes, and using the SOP’s viewer state to adjust the transforms of the child agents.

To set the animation clips for the parent and child agents:

1. On the Crowd MotionPath SOP, click beside Number of Assignments to add a set of animation clip parameters for each parent and child.

2. On each set of parameters, set the Group parameter to the group of parent or child agents, for example, @agentname=horse.

   Tip

   The agentname attribute is created by upstream nodes such as the Agent Definition Cache SOP or Agent from Rig SOP. To view the agentname attribute of each agent:

   1. Select the Agent Relationship SOP.

   2. Open the geometry spreadsheet - at the top of a pane, click the New Tab icon and select New Pane Tab Type ▸ Inspectors ▸ Geometry Spreadsheet.

   3. Select Points on the top toolbar.

3. Set the Clip Names parameter to the animation clips you want to use for the agent. See generating crowd motion paths for more information on specifying animation clips for agents.

By default, the motion paths for the child agents are hidden (on the Crowd MotionPath SOP, Hide Child Agent Paths is turned on):

You can quickly draw guiding curves for agents to follow, which is useful, for example, if you want agents to follow along a path on a road. The Crowd MotionPath Follow SOP deforms the original agent paths to follow the curves drawn. It changes the position and orientation of the points along the path, but not the animation clip.

Connect the Crowd MotionPath Follow SOP between the Crowd MotionPath and Crowd MotionPath Evaluate SOPs:

Draw a curve (or multiple curves) in the viewport using the Curve SOP. By default, agent paths follow the closest curve:

1. Set the display flag on the Crowd MotionPath Follow SOP so that you can see how the paths are being deformed as you draw the curve.

2. Select the Curve SOP and click on the left toolbar to enter the viewer state.

3. Draw the curve(s).

   Tip

   Display the construction plane so that the curves automatically snap onto the construction plane. Click Display Construction Plane on the right viewport toolbar.

4. Select and display the Crowd MotionPath Evaluate SOP.

5. Click in the playbar to play the animation of the agents moving along the updated paths.

With the Crowd MotionPath Edit SOP, you can directly manipulate the paths to determine exactly where the agents should go. This node offers a finer level of control for manipulating paths than the Crowd MotionPath Follow SOP.

1. Connect the Crowd MotionPath Edit SOP between the Crowd MotionPath and Crowd MotionPath Evaluate SOPs.

2. Select and display the Crowd MotionPath Edit SOP.

3. In the viewport, drag points along the curve to deform the path. These points are called pins.

   Note

   If there is a parent-child relationship between agents, only the points on the parent path can be selected.

   Tip

   Turn on the construction plane so that when you drag points on a flat terrain, they will be dragged along the construction plane.

A few notes:

• Each pinned point on the motion path has a weight, which determines how strongly the path is affected by the pin. You can change the weight of each pin with the Pin Weight parameter in the HUD slider.

• When the deformed curve does not pass through the pinned point, a dotted line is displayed to show the relationship between the pin and its original point on the curve.

• By default, the length of the curve is generally preserved as the motion path is changed.

The Crowd MotionPath Avoid SOP performs collision detection and avoidance between agents.

1. Connect the Crowd MotionPath Avoid SOP between the Crowd MotionPath and Crowd MotionPath Evaluate SOPs.

2. Select and display the Crowd MotionPath Avoid SOP.

   By default, Visualize Collisions is turned on, which highlights the parts of the motion paths that have collisions. The sphere-shaped guide geometry shows the agent’s size that is used for collision detection:

   

3. Adjust the parameters in the Steering tab and the number of Iterations to try to resolve the collisions. Iterations is the number of avoidance passes that are made on the curves to try to resolve collisions. Later iterations may have little effect if the Steering parameters are too restrictive.

To...	Do this
View potential collisions	Turn on Visualize Collisions. The parts of the motion paths with collisions are highlighted. Visualize Collisions is useful for determining where collisions occur even if Avoidance is turned off.
Perform collision avoidance on specific paths	Click beside the Group parameter. In the viewport, select the paths to deform in order to avoid other agents and obstacles. Click Enter. The agents specified in Group can still collide with agents that are not listed in Group.
Specify how far in advance for agents to start avoiding potential collisions	Adjust the Max Collision Time. With a higher Max Collision Time, agents start trying to avoid potential collisions earlier.
Change the agent’s size for collision detection	Adjust the Particle Scale Multiplier to change the collision size for all agents. Each agent can also inherit the attribute pscalemultiplier that is configured upstream, for example, using the Attribute Adjust Float SOP. The pscalemultiplier attribute is scaled by the Particle Scale Multiplier.
Set the frame range to apply collision avoidance	Turn on and specify the Start Frame and End Frame.
Set the initial rotation angle at the start of the path	Turn on and set Max Initial Rotation. This is useful if the agents are very close together at the start of the motion path and you want them to move away from each other. Max Initial Rotation allows you to configure the agents to immediately turn and face the other way only in the beginning. This avoids having to adjust the Max Turn Rate, which affects the turn rate for the entire time range.
Preserve the original end position of the paths	Increase the Goal Position Weight.
Avoid neighboring agents	Turn on Neighbors. Increase Neighbor Distance to start avoiding agents earlier.
Avoid obstacles	Connect the obstacle geometry to the 3rd input of the Crowd MotionPath Avoid SOP. Turn on Obstacles. Increase Obstacle Distance to start avoiding the obstacle earlier. Increase Collision Padding to make agents stay further away from the obstacle. Adjust the Horizontal FOV to change the viewing angle at which agents can see obstacles. To perform obstacle avoidance for only the agents who are on a collision path with the obstacle, decrease the Horizontal FOV. This example demonstrates how the Crowd MotionPath Avoid SOP can be used to avoid obstacles. Load Launch MotionPathAvoidanceFiltering
Specify certain groups of agents to ignore collisions with other groups	This example demonstrates how the Crowd MotionPath Avoid SOP can be used to apply avoidance between neighboring agents. The collisionignore and collisiongroup string attributes are used to build more complex avoidance rules and collision filtering between different groups of agents. Load Launch MotionPathAvoidanceFiltering If an agent’s collisionignore string pattern matches the collisiongroup that a neighboring agent belongs to, the agent ignores the collision with the neighboring agent - it does not attempt to avoid the neighboring agent. If an agent’s collisionignore pattern = *, the agent does not attempt to avoid any collisions. In this case, the Crowd MotionPath Avoid SOP does not have any effect. If an agent’s collisionignore string is empty, the agent does not ignore any collisions - it can collide with everything.
Set a path for agents to follow	The Crowd MotionPath Avoid SOP can be used to make agents follow a path. Input a line geometry as an obstacle to the Crowd MotionPath Avoid SOP, which will be treated as a tube shape that the agent tries to avoid.
Output a point group on geometry that contains the points with collisions	Turn on Collision Point Group and specify a name for the point group.

Transition to a new animation clip using the Crowd MotionPath Transition SOP.

1. Connect the Crowd MotionPath Transition SOP between the Crowd MotionPath and Crowd MotionPath Evaluate SOPs.

2. On the Crowd MotionPath Transition SOP, set Clip Name to the new animation clip.

   By default, Crowd MotionPath Transition switches the agents' animation clips at the beginning of the frame range:

To transition to the new animation clip at a later point on the path:

1. On the Crowd MotionPath Transition SOP, click beside Trigger Group.

2. In the viewport, select the points on the paths at which to start the transition. Hold SHIFT to select points on different paths. If multiple points are selected along a single path, the transition will occur at the first selected point.

3. Click ENTER. The Trigger Group parameter will be populated with the selected points.

The Crowd MotionPath Trigger and Crowd MotionPath Transition SOPs work together to switch the agents' animation clips when certain conditions are met.

Crowd MotionPath Trigger sets the conditions that trigger a transition to another animation clip, and Crowd MotionPath Transition switches the agents' animation clips when the trigger conditions are met.

1. Connect the Crowd MotionPath Trigger and Crowd MotionPath Transition SOPs between the Crowd MotionPath and Crowd MotionPath Evaluate SOPs:

   

2. On the Crowd MotionPath Trigger SOP, Trigger Name is set to $OS by default, which sets the trigger name to the name of the Crowd MotionPath Trigger SOP. You can change Trigger Name to a more meaningful name, as it is used by subsequent nodes, such as the Crowd MotionPath Transition SOP, to reference the trigger.

3. On the Crowd MotionPath Trigger SOP, set the trigger type in the Type parameter. To switch the animation clip at a particular frame:

   • Set Type to Time.

   • Set Frame to the frame at which to transition to a different animation clip.

   In the example below, the Crowd MotionPath Trigger SOP is selected and displayed. The points on the path that satisfy the trigger condition are highlighted:

   

4. On the Crowd MotionPath Transition SOP:

   • Set Trigger Group to the name of the trigger (defined in the upstream Crowd MotionPath Trigger SOP, Trigger Name parameter).

   • Set Clip Name to the new animation clip.

   In the example below, the Crowd MotionPath Transition SOP is selected and displayed. The points on the path that satisfy the trigger condition are transitioned to the new animation clip:

   

To...	Do this
Switch to a new animation clip when the agents are inside an area or geometry	On the Crowd MotionPath Trigger SOP, set Type to Bounding Region. In the viewport, resize the box using the handles. Set Comparison to specify whether points that are inside, outside, entering, or leaving the box trigger a transition to the new animation clip. Select and display the Crowd MotionPath Trigger SOP to see the highlighted points that satisfy the trigger condition: Select and display the Crowd MotionPath Transition SOP to see the updated agent paths: The Comparison method of Incoming highlights the one point on each path where the transition is triggered: You can also use other geometry as the bounding region that triggers a transition. In the example below, the rubber toy test geometry is used as the bounding region: Connect the rubber toy test geometry to the Crowd MotionPath Trigger SOP (3rd input). Set Type to Bounding Region. Set Bounding Type to Bounding Object. Set Comparison to Inside. The transition to the new animation clip is triggered when the agents are inside the rubber toy geometry (the rubber toy geometry is templated below): See the Crowd MotionPath Trigger node doc for the available pre-built triggers. This example demonstrates how to layer a clip onto an agent and activate the clip when the agent is inside a bounding object. Load Launch MotionPathLayerClip
Switch a child agent’s animation when its parent’s animation changes	If the agent is attached to another agent, turn on Match Parent Agent’s Trigger on the Crowd MotionPath Trigger SOP to switch the animation of the child agent when the parent’s animation changes.

The Crowd MotionPath Layer SOP provides an easy way to add additional motion onto an agent, like waving or turning the agent’s head, without having a whole new animation clip that performs the desired combination of different motions. Crowd MotionPath Layer is similar to Crowd MotionPath Transition, but instead of switching to a completely new animation clip, Crowd MotionPath Layer adds different clips on top of the agent’s base animation. Layered clips can also be applied to specific parts of the skeleton.

Connect the Crowd MotionPath Trigger and Crowd MotionPath Layer SOPs between the Crowd MotionPath and Crowd MotionPath Evaluate SOPs:

1. The Agent Transform Group SOP creates a named group of joints that specifies the parts of the skeleton to apply the layered clip to. See layer animation clips for more information. On the Agent Transform Group SOP, we set Name to upper_body.

2. On the Crowd MotionPath Trigger SOP, specify a name for the trigger in the Trigger Name parameter. In our example, we set Trigger Name to trigger_zombie.

3. Set up the layer using the Crowd MotionPath Layer SOP:

   • Set Trigger Name to the trigger that causes the layer clip to start playing, in our example, trigger_zombie.

   • Set Transform Group to the joint group that will play the layered clip, in our example, upper_body.

   • Set Clip Name to the layered clip.

In the example below, the agents' upper bodies start playing the layered zombie clip at a certain point on the path, while the lower bodies keep playing the original walking clip:

The Crowd MotionPath Arcing Layer SOP allows you to blend between animation clips based on an agent’s turn rate, giving you more realistic motion when the agent is turning quickly:

It’s important that the animation clips for the turning motion are synced with the base animation. The Crowd MotionPath Arcing Layer SOP blends between the different animation clips, so the turning clips need to be at the same plane as the base animation.

In this example, we deform the motion path using a Crowd MotionPath Follow SOP. We then blend between the animation clips of the horse agents along the bend of the motion path using the Crowd MotionPath Arcing Layer SOP:

The agent definition has the following animation clips (see layering animation for information on how to view an agent’s clips):

1. Connect the Crowd MotionPath Arcing Layer SOP between the Crowd MotionPath and Crowd MotionPath Evaluate SOPs.

   The Crowd MotionPath SOP stores the “rest pose” of the motion path by adding a rest_transform attribute to the motion paths. After we make edits to the motion path (for example, using the Crowd MotionPath Follow SOP), the Crowd MotionPath Arcing layer SOP compares the updated path to the original path, and this difference is used to compute the agent’s turn rate. It’s important that the turn rate along the motion path is smooth because the turn rate is used to drive the blend weight of the animation clips, so a turn rate that is changing rapidly will cause the animation to jump back and forth between the different clips.

2. On the Crowd MotionPath Arcing Layer SOP, set Group to the agents you want to apply the turning animation to.

3. Set Base Clip Name to the base animation clip. To add multiple base animation clips and their associated turning clips, click beside Clip Sets.

4. Specify the different turning clips to blend between:

   • Click beside Arcing Clips to add a different clip.

   • Set Clip Name to the name of the turning clip.

   • Set the Turn Direction and Radius. To figure out the turning radius, connect the agent clip to a Crowd MotionPath SOP, and set Clip Names to the clip you want to view:

     

     Note

     The turning radius does not need to be completely precise. A ballpark value works.

5. To visualize how sharply the agent is turning along the path, turn on Turn Rate Attribute, and click to the right of the parameter. We want the turn rate (colors) to change smoothly along the path so that the clip blending also changes smoothly.

   

   Tip

   If the animation jumps between different animation clips, it’s because the blend weights between the clips are jumping back and forth. Try increasing the value of the Crowd MotionPath Arcing Layer SOP’s Strength parameter to smooth out the turn rate of the path so that the blending between clips is also smoother.