Using Flow Control Nodes#

Note

This doc contains features from OmniGraph version 2023.1.0.

In this tutorial, we explore the use of Flow Control Nodes in Action Graph.

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 and Using Event Source Nodes first. Those tutorials provide more detailed information about some of the same steps that you complete here.

What Are Flow Control Nodes?#

Many Action Graphs need to do different things depending on some state. In a python script you would use an if or while loop to accomplish this. In Action Graph there are Flow Control Nodes which provide this branching functionality. Flow Control Nodes have one or more Execution Attribute outputs which are used to branch the evaluation flow.

Flow Control Nodes

Branch

Flip Flop

For Each

For Loop

Gate

Multigate

Delay

Note

Read more about Flow Control 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 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.

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

Numbered steps for creating an OnKeyboardInput node.

Example 1: Flip Flop Node#

In this example, we toggle the transformation of a mesh based on a keyboard input.

The Flip Flop is a flow control node that alternates its outputs.

First, create a FlipFlop nodes:

Numbered steps for creating FlipFlop nodes.

Now, connect the Pressed output of the OnKeyboardInput to the Execute In input of the FlipFlop node.

Then connect the Execute A output to the Exec In input of the first WritePrimAttribute and Execute B output to the Exec In input of the second WritePrimAttribute.

Numbered steps for connecting the FlipFlop nodes.

Now press Play. Every time the J key is pressed, the torus alternates between (0.0, 20.0, 0.0) and (0.0, 0.0, 0.0).

Result animation of setting the FlipFlop nodes.

Example 2: Multi Gate Node#

In this example, we toggle between multiple different transformations based on a keyboard input.

The Multigate is a flow control node the cycles through all of its inputs.

First, duplicate a WritePrimAttribute.

Then, create a Multigate node:

Numbered steps for creating Multigate nodes.

Next, create two new outputs on the Multigate node by clicking the + button in the Property Panel:

Numbered steps for creating Multigate dynamic attributes.
Connect Pressed to Execute In, then

  • Output0 to Exec In of the first WritePrimAttribute node and set value to (0.0, -20.0, 0.0)

  • Output1 to Exec In of the second WritePrimAttribute node and set value to (0.0, 20.0, 0.0)

  • Output2 to Exec In of the third WritePrimAttribute node and set value to (0.0, 0.0, 0.0)

Numbered steps for connecting Multigate nodes.

Now press Play. Every time the J key is pressed, the torus cycles between (0.0, 20.0, 0.0), (0.0, 0.0, 0.0) and (0.0, -20.0, 0.0).

Result animation of setting the Multigate nodes.
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