Using Event Source Nodes#

Note

This document contains features from OmniGraph version 2023.1.0.

In this tutorial, we explore the use of Event Nodes in Action Graph using various approaches.

Note

While you use Omniverse USD Composer in this tutorial, you can follow similar steps to achieve the same results in other Omniverse Apps.

Before You Begin#

While this is an introductory-level tutorial on OmniGraph, we recommend you complete Introduction to OmniGraph first. That tutorial provides more detailed information about some of the same steps that you complete here.

What Are Event Source Nodes?#

Action graphs are event driven, which means that each chain of nodes must start with an Event Source Node. Each Event Source Node can be thought of as an entry point of the Action Graph.

Event Source nodes are named with an On prefix; they never have an input Execution Attribute, and always have at least one output Execution Attribute.

Event Source Nodes

On Closing

On Gamepad Input

On Impulse Event

On Keyboard Input

On Loaded

On Message Bus Event

On Mouse Event

On Stage Event

On Tick

On Object Change

Note

Read more about Event Source Nodes in the developer documentation.

Prepare the Stage#

All of our examples start with the same setup:

First, add a torus to the stage:

Numbered steps for creating a torus.

Next add a new Action Graph:

Numbered steps for creating a new Action Graph.

Drag your torus to the graph editor and create a WritePrimAttribute node:

Numbered steps for creating a WritePrimAttribute node.

Set the WritePrimAttribute node’s name attribute to xformOp:translate and the value attribute to (0.0, 20.0, 0.0):

Numbered steps for setting the translate on a WritePrimAttribute node.

Duplicate that node with Ctrl+D so that there are two WritePrimAttribute nodes. Set the new node’s value attribute to (0.0, 0.0, 0.0):

Note

You may need to reset the name attribute to xformOp:translate after you duplicate.

Numbered steps for duplicating a WritePrimAttribute node.

Example 1: On Stage Event#

For this example we are going change the value of the torus when the graph starts playing, then reset it when it stops.

The On Stage Event node allows you to send an execution signal based on certain stage events:

  • Saved: USD file saved.

  • Selection Changed: USD Prim selection has changed.

  • Hierarchy Changed: USD stage hierarchy has changed, e.g. a prim is added, deleted or moved.

  • OmniGraph Start Play: OmniGraph started

  • OmniGraph Stop Play: OmniGraph stopped

  • Simulation Start Play: Simulation started

  • Simulation Stop Play: Simulation stopped

  • Animation Start Play: Animation playback has started

  • Animation Stop Play: Animation playback has stopped

First, create two OnStageEvent nodes:

Numbered steps for creating OnStageEvent nodes.

Set one OnStageEvent nodes eventName attribute to OmniGraph Start Play, and the other to OmniGraph Stop Play:

Numbered steps for setting the event name to stop on an Event Node.

Connect each WritePrimAttribute node to the OnStageEvent nodes by dragging the Exec Out pin to the Exec In pin:

Numbered steps for connecting OnStageEvent nodes to WritePrimAttribute nodes.

When you press the play button, the torus translates to (0.0, 20.0, 0.0), and returns to (0.0, 0.0, 0.0) when stopped.

Result animation of setting the OnStageEvent nodes.

This is a very useful setup for ensuring an attribute is set to a specific value when the graph starts or stops executing.

Example 2: On Keyboard Input#

In this example, we move a mesh in response to a key press.

The On Keyboard Input event node fires when a keyboard event occurs.

First, create an OnKeyboardInput node, set the keyIn attribute to J:

Numbered steps for creating an OnKeyboardInput node.

Connect the first WritePrimAttribute node’s Exec In pin to the OnKeyboardInput Pressed pin, and the second WritePrimAttribute node’s Exec In to the OnKeyboardInput Released pin:

Numbered steps for connecting an OnKeyboardInput node.

Now, press Play. When the J key is pressed, the torus moves to (0.0, 20.0, 0.0), and returns to the origin when released.

Result animation of setting the OnStageEvent nodes.

Example 3: On Message Bus Event#

In this example, we move a mesh in response to a signal sent from a script.

The On Message Bus Event is an event node which fires when the specified event is popped from the Application Message Bus, either from a script or from a SendMessageBusEvent node.

First, create a OnMessageBusEvent node and set the eventName attribute to StartEvent:

Numbered steps for creating OnStageEvent nodes.

Next, create a new pointd[3] attribute called position using the Add + button:

Numbered steps for creating OnStageEvent nodes.

Now, duplicate the OnMessageBusEvent node using Ctrl+D and change the eventName attribute to EndEvent:

Numbered steps for creating OnStageEvent nodes.

Connect the respective WritePrimAttribute node’s Exec In pin to the OnMessageBusEvent Received pin, and the OnMessageBusEvent position output to the WritePrimAttribute value input:

Numbered steps for connecting an OnKeyboardInput node.

Press play to begin executing your graph.

Then, in a Script Editor, and copy and run the following code:

import carb.events
import omni.kit.app

msg = carb.events.type_from_string("StartEvent")
omni.kit.app.get_app().get_message_bus_event_stream().push(msg, payload={ "position": (0.0, 20.0, 0.0) })

The torus now moves to (0.0, 20.0, 0.0)! To return to the origin, you can run:

import carb.events
import omni.kit.app

msg = carb.events.type_from_string("EndEvent")
omni.kit.app.get_app().get_message_bus_event_stream().push(msg, payload={ "position": (0.0, 0.0, 0.0) })
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