Tutorial 12: Asset Optimization#

Learning Objectives#

This tutorial details how to make robot assets more performant and where to find tradeoffs to achieve a faster simulation or rendering time.

30 Minutes Tutorial

Getting Started#

Prerequisites

Complete the Quick Tutorials series to learn the basic core concepts of how to navigate inside NVIDIA Isaac Sim.
Complete the Assemble a Simple Robots tutorial to learn the concepts of rigid body API, collision API, joints, drives, and articulations.
Read Onshape importer and watch the videos on rigging the robot in Onshape.
Familiarity with Mesh Merge Tool.

Loading the Robot

This tutorial explores the NVIDIA Jetbot Robot asset which improve performance. If you import the asset from a different source, for example from custom CAD, you may end up with numerous meshes per rigid body and this can severely impact performance

From the recording of this Jetbot asset imported from CAD that on the right side we have an unoptimized asset, and it’s achieving 40 FPS, while the asset on the left was optimized, and now achieves 64 FPS.

Asset Structure Optimization#

In this activity, we will demonstrate a workflow with the multi-layered asset structure introduced in an earlier module, by creating an optimized version of an asset. We’ll use the Jetbot robot as a starting place. This model was imported from a CAD model made in Onshape. Although the physics layer is already in place, the bodies contain a significant number of meshes, which leads to suboptimal simulation performance. We’ll begin with an empty stage to familiarize ourselves with several useful tricks for asset authoring. By the end of this activity, we’ll have transformed the initial Jetbot model into a well-structured, optimized asset ready for efficient simulation.

Set up reparenting and layers#

In Isaac Sim, go to Edit > Preferences to open the Preferences panel.
Under Stage > Authoring, ensure that Inherit Parent Transform is checked.
Open the Jetbot located at Isaac Sim/Samples/Rigging/Jetbot/Jetbot_Optimized/Jetbot_optimized.usd, you should see an empty USD
Select the Layers panel, and click the Insert Sublayer button at the bottom of the tab, and select Isaac Sim/Samples/Rigging/Jetbot/Jetbot_Base/Jetbot_base.usd and click Open.

Create asset structure#

The Jetbot asset is already close to the final goal, but since we want to work on a retargeting of the structure to get the merged meshes, let’s create a new prim to be set as default.

On the right side menu of the Stage panel, select Show Root.
Create a new Xform called Jetbot_Sim and drag it onto Root.
Right click on Jetbot_Sim and choose Set as Default Prim.
Right click and choose Create > Scope, and name it Visuals.
Drag this scope onto Root so it’s unparented from Jetbot_Sim.
Select the prims under Jetbot, and drag them onto Jetbot_Sim.

Note

To select multiple prims, use shift-select or control-select standards. For example: select one prim, then hold shift and another to choose all prims listed between them.
You’ll notice that instead of being deleted from Jetbot, they were instead deactivated.
Select them all, then right-click and choose Activate.
Delete the contents inside the prims in Jetbot_Sim.

Merge Meshes#

Now you have the stage ready. Let’s begin merging the meshes.

Open the Mesh Merge Tool by going to Tools > Robotics > Asset Editors > Mesh Merge Tool.
Select Jetbot/left_wheel prim.
Check the Combine Materials box, insert Jetbot_Sim/Looks to save the material in the Jetbot Sim xform.
Click on Merge.
Select the resulting mesh on /Merged/left_wheel, and clear the transform on the properties panel.
Right-click on the Visuals scope, create an Xform called left_wheel, and drag the resulting mesh into it. Remove the /Merged xform from the stage.
We will now create an internal reference to the wheel. Create a Visuals Xform inside left_wheel, then right-click it and choose Add > Reference.
Select Isaac Sim/Samples/Rigging/Jetbot/Jetbot_Base/Jetbot_base.usd in the dialog.
For prim_path, type in /Visuals/left_wheel.
Back in the Stage panel, select the /Visuals/left_wheel prim which we just added a reference onto. Then in the Property panel, scroll down to the References section. You’ll see the prim path in red, Select the Asset Path entry and clear it.
This will make the reference point to the internal /Visuals/left_wheel prim. You’ll see the mesh for left_wheel shows as a child. Notice that a Looks scope was created in Jetbot_Sim, with the materials for this mesh.
Notice that the wheel is referenced correctly in place, along with the base mesh that is at the origin. You can hide the Visuals scope so base meshes won’t be visible.
Save the file with CTRL+S.
To complete the mesh optimization, we’ll repeat the previous steps for other bodies.

Note

The finished USD with all mesh merges is available for you at Isaac Sim/Samples/Rigging/Jetbot/Jetbot_Optimized/Jetbot_optimized_post_merge.usd.

Scenegraph Instancing#

Scenegraph Instancing enables sharable, composed representations of subgraphs of prims. It is a directive that instructs the scene composer that a certain component of the scene is a repeatable pattern. While this allows for a leaner overall scene, it does require a few rules to be followed.

Any children of an instance cannot have attributes modified, since they all inherit from the same asset in memory. Instances must be applied on Referenced assets, so the scenegraph composer knows that from the reference and downwards things are expected to remain the same, and creates a pointer to the asset data to be used anywhere it’s referenced.

Start by opening the USD file Isaac Sim/Samples/Rigging/Jetbot/Jetbot_Optimized/Jetbot_optimized_post_merge.usd if you have not merged all the meshes.
The left and right wheel meshes are identical, we can further simplify the asset by having left and right wheel reference the same mesh. Select Visuals/left_wheel, and rename it to Visuals/wheel.
Delete Visuals/right_wheel. You will notice that the Jetbot wheel will disappear.
Select Jetbot_Sim/right_wheel/Visuals.
Under the References section of the Property panel, replace the reference Prim Path from /Visuals/right_wheel to /Visuals/wheel.
At this point, all meshes are still considered unique elements since the assets are only defined as a reference.
To leverage memory savings, shift-select all Visuals prims under /Jetbot_Sim and check Instanceable in the Property panel.
On the Visuals prims, notice the reference icon now has a blue “I” on it. This indicates they are instanceable meshes, and effectively applying any memory savings.
Save the file with CTRL+S.

Note

The finished USD with all mesh merges and scenegraph instancing is available for you at Isaac Sim/Samples/Rigging/Jetbot/Jetbot_Optimized/Jetbot_optimized_final.usd.

Other Considerations#

Minimize Number of Lights: Each light negatively impacts the performance of the rendering. By default, if the scene has more than 10 lights, the rendering reverts to sample-based lighting to avoid severe slowdown in performance.
Reduce Translucent Materials: Each translucent material generates a larger performance bottleneck than the default OmniPBR material.
Optimize Physics Performance: Search for simulation aspects that you can modify to reduce computational cost. Typically, colliders have high computational costs. The more basic that you can make a collision shape, the more performant the simulation behaves. Reducing the number of contact points can also bring huge performance benefits. Tuning this can take several experiments to achieve the best precision versus performance point for your situation.
Approximate Wheel Colliders: If you have a wheel collider, consider using a simple cylinder or sphere collider instead of a mesh collider. This can significantly improve performance, and allow the robot to drive smoothly over terrains.