The workflow for the Scene Explorer will be familiar if you’ve used the Katana Scene Graph. Both the USD Scene Graph and the Katana Scene Graph panels in the Scene Explorer share working set columns for essential operations, including visibility in the viewer and renderer. However, the USD Scene Graph adds two USD-specific working set columns so that you can have more control over payloads and prim activation.

As you can see in the image below, the Scene Explorer tab includes the USD Scene Graph and the Katana Scene Graph in the bottom section.

The Scene Explorer. The top section is the USD Scene Graph, and the bottom is the Katana Scene Graph.

Each scene graph is identified by the USD or Katana logo in the top-left corner of the panel next to the Name column.

Note:  To hide the Katana Scene Graph in this panel, either drag the top of the panel bar either to the top or bottom or by toggling either the USD or Katana logo located at the top of the panel.

Use the two selection buttons at the top of the Scene Explorer tab to display the USD Scene Graph or/and the Katana Scene Graph.

If a panel is hidden, its icon is greyed-out.

Data Columns

The USD Scene Graph part of the Scene Explorer tab has four columns of key information for each item:

Common Working Set Columns

The USD Scene Graph shares three working set columns with the Katana Scene Graph:

These three working set columns control visibility in the viewer and renderer. To find out how to work with these, see Working Sets.

USD-Specific Working Set Columns

The USD Scene Graph also includes two columns for payload management and prim activation/deactivation.

Katana makes use of deferred loading to improve performance. When working with large datasets, portions of the data can be kept in reserve until needed, thus freeing up computational resources for more pressing tasks. In the Katana Scene Graph this is achieved by selectively expanding branches of the tree. In the USD Scene Graph part of the Scene Explorer, it is achieved using payloads.

A payload is a reference to scene information that can be loaded into the stage when needed. Any prim can contain one or more payloads, or have none.

For example, imagine you have a tree asset that is highly detailed. You want to include multiple instances of the tree in a forest.

Your tree asset looks like this:

#usda 1.0
def Xform "Tree" {
    def Mesh "Trunk" { /*... mesh details ...*/ }
    def Mesh "Leaves" { /*... mesh details ...*/ }
    // ... Perhaps more variations or details like fruits, flowers, etc.
}

To include trees as payloads in your forest, the USD file could be written as follows:

#usda 1.0
    ()

def Xform "Forest"() {

    def Cube "Tree1"
        (

            prepend payload = @. / tree.usda @ < /Tree>
        ) {
            double3 xformOp: translate = (0, 0, 0) uniform token[] xformOpOrder = ["xformOp:translate"]
        }

    def Cone "Tree2"(
        prepend payload = @. / tree.usda @ < /Tree>
    ) {
        double3 xformOp: translate = (5, 0, 10) // Positioned for natural tree spacinguniform token[] xformOpOrder = ["xformOp:translate"]
    }
}

Using payload enables selective loading and unloading of specific portions of a USD stage to speed up your workflow. Loading any prims or layers that are not currently needed can be deferred until required.

Deferred loading should be familiar to experienced users who have worked with the Katana Scene Graph. As part of the USD workflow, we have incorporated deferred loading using payloads into the Scene Explorer.

Payloads and Recomposition

Loading and unloading payloads triggers the recomposition of the USD stage. When a payload is loaded, it brings in new data, such as prims and properties to the stage. As a result, the scene graph needs to be updated to incorporate this newfound information. Similarly, when you remove a payload from a stage, recomposition is triggered as data is removed from the scene graph.

Payload management is activated by selecting the Payload button in the Scene Explorer toolbar.

When payload management is deactivated, payloads are loaded via expansion of USD scene graph locations. Expanding a location loads any payloads on that prim, but it does not trigger the loading of any payloads on its descendants.

Prims with payloads have a down arrow icon in the  Payload column.

Payloads are managed by assigning a rule to a prim. This rule determines if payloads are to be loaded for the prim and its descendants.

There are three stage load rules available, as shown in the image below. Either right-click the dot for  the options or use the mouse-click combinations listed.

Right-click in the payload column to open the context menu.

Note:  Though named differently, the three stage load rules map directly to the stage load rules found in USD.  Load One is the OnlyRule, Load None is the NoneRule, and Load All is the AllRule.

The rules only apply if there are payloads on the stage. When there are no payload the rules have no effect.

Effects of Stage Load Rules

The table below describes how payloads are managed according to the stage load rule on the prim.

To help understand how the rules affect payload loading,  the tree diagrams in the table below represent some simple scenes. Each box is a prim and the connecting lines show the prim hierarchy. A green box indicates if there is a payload at this prim, then this payload is loaded. A grey box indicates if there is a payload at this prim, then the payload is not loaded.

Cascading Payload Rules to Refine Prim Loading

In real-world production scenarios, it may be preferable that certain parts of a scene should not be loaded to conserve memory. However, specific sections might still be required for a particular shot, sequence, or operation. With USD, child prims can override their ancestors' load policies, giving artists and developers precise control to optimize scenes for their specific requirements.

To achieve this, you can apply rules at different levels of the hierarchy to include or exclude specific prims as needed. For example, the tree below shows the effect of combining two rules.

A combination of payload rules on a simple graph. The leftmost node has a Load None rule but if present the payload would be loaded as there is a downstream Load One rule.

Prim Payload Status

The color of both the arrow and the background change to indicate the payload status on a particular prim. The table below shows the possible states.

You can enable selective pruning by activation and deactivation using the Activation button in the Scene Explorer toolbar.

Click the power style button to enable activation/deactivation.

With the Activation column toggled on, you can easily deactivate and re-activate prims using the button in the column.

Click on the button or right-click to open the context menu.

With stage activation on, prim activation can have one of four states:

Inspecting USD Location Properties Using the Attributes Tab

The Scene Explorer tab integrates seamlessly with the Attributes tab for inspecting USD properties. As with the Katana Scene Graph, click on any location in the USD Scene Graph to view the properties in the Attributes tab.

Click on a scene graph location to view the properties in the Attributes tab.

USD location properties are of two types: