Exercise: Experimenting With Inherits#

First, we will look at a simple example of inherits using cube.usd and cube_pyramid.usd. Both can be found in composition_arcs/inherits/simple_example.

Our cube.usd is the same cube we have used in previous examples. The main difference is that there is a class called _cube_asset which the “World” prim is inheriting from. There are no authored opinions in _cube_asset.

Note

If you are trying to view the file in usdview and cannot see _cube_asset in the tree view, go to Show > Abstract Prims. This will show classes in the tree view.

The next file, cube_pyramid.usd references cube.usd three times to create a pyramid. Each cube is also a different color. Let’s get started with the exercise.

  1. In the Visual Studio Code terminal, run the code below to open the file in usdview:

Windows:

.\scripts\usdview.bat .\composition_arcs\inherits\simple_example\inherits_simple.usd

Linux:

./scripts/usdview.sh ./composition_arcs/inherits/simple_example/inherits_simple.usd

In this layer we are referencing cube_pyramid.usd, however each cube has the same color instead of different colors. Why is this?

We can see that there is an opinion on _cube_asset that has an authored value on the inputs:diffuseColor to be (1,1,0). This is being broadcasted to any prim that inherits _cube_asset. In this case, each one of our cubes in cube_pyramid.usd inherits from _cube_asset.

How does this affect prims in cube_pyramid.usd or other scenarios referencing cube_pyramid.usd? Let’s open another layer that also is referencing cube_pyramid.usd.

  1. In the Visual Studio Code terminal, run the code below to open the file in usdview:

Windows:

.\scripts\usdview.bat .\composition_arcs\inherits\simple_example\unaffected_scenario.usd

Linux:

./scripts/usdview.sh ./composition_arcs/inherits/simple_example/unaffected_scenario.usd

As we can see, this one is unaffected. However, if we were to make all the cubes to be yellow in cube_pyramid.usd then both unaffected_scenario.usd and inherits_simple.usd would show yellow cubes. You might not want that and it may be a one-off, so that’s where you would want to leverage an inherits arc to limit your change to just the context where the inherited prim is overridden.

So why create an inherits arc? Let’s look at composition_arcs/inherits/practical_example/night_scene_01.usd, where the city scene is in a night environment. The director comes in and says, “I want all of the street lights to be yellow just for this scene. The other city night scenes should preserve the white lighting.”

We could directly modify the street lamp asset that is referenced, but since the director only wants to affect the context that we were looking at and not any others this isn’t the best option. A change to the referenced asset would ripple to every scene. The next obvious choice would be to just author overrides for all the referenced lights in night_scene_01.usd. We could do it manually or scripted. This can be labor intensive, prone to error, and potentially adds a lot of extra authored data that doesn’t need to be there.

Instead, let’s look at how the inherits arc can help in this scenario. If the street lamp asset has an inherits arc, we can author a new opinion on the inherits source prim in night_scene_01.usd. The opinion is then broadcast to all prims that inherit from the source prim, but just for that context. The other scenes will be unaffected. Let’s use an inherits arc on the street lamp to change the color of the lights.

  1. Open the following file in Visual Studio Code: composition_arcs/inherits/exercise/inherits_exercise_1.py

We need to create the class, _street_lamp_dbl in which our street lamps will inherit from. Then we’ll have the default prim in our stage inherit from our newly created class.

  1. Add the following code to the script:

1class_prim = asset_stage.CreateClassPrim("/_street_lamp_dbl")
2root = asset_stage.GetDefaultPrim()
3root.GetInherits().AddInherit(class_prim.GetPath())

  1. Save the file.

  2. In the Visual Studio Code terminal, run the following code:

Windows:

python .\composition_arcs\inherits\exercise\inherits_exercise_1.py

Linux:

python ./composition_arcs/inherits/exercise/inherits_exercise_1.py

  1. After running the script, return to the terminal and run the following code to open usdview:

Windows:

.\scripts\usdview.bat .\composition_arcs\inherits\exercise\street_lamp_dbl.usd

Linux:

./scripts/usdview.sh ./composition_arcs/inherits/exercise/street_lamp_dbl.usd

As we can see, we’ve created an abstract prim and set the street light’s default prim to inherit it. The inherited abstract prim in this case is not encapsulated within the default prim and will be used as a global refinement. More on this later.

  1. Now with the inherits arc established, we can use it to give the street lights a yellow tint. In Visual Studio Code open the following file: composition_arcs/inherits/exercise/inherits_exercise_2.py

The stage, composition_arcs/inherits/exercise/scenario_02.usd, will represent the scene that the director asked to change. composition_arcs/inherits/exercise/scenario_01.usd will represent a scene that we don’t want to affect. In scenario_02.usd, we will override the same class from the street lamp asset and author our overrides inside the class which will then be broadcasted to all those inheriting from _street_lamp_dbl.

  1. Add the following code to the Python file:

 1class_prim = scenario2.OverridePrim("/_street_lamp_dbl")
 2light_prim = scenario2.OverridePrim(class_prim.GetPath().AppendPath("Lights/sphere_light_01"))
 3light = UsdLux.LightAPI(light_prim)
 4light.CreateColorAttr((0.5, 0.4, 0.1))
 5light_prim = scenario2.OverridePrim(class_prim.GetPath().AppendPath("Lights/sphere_light_02"))
 6light = UsdLux.LightAPI(light_prim)
 7light.CreateColorAttr((0.5, 0.4, 0.1))
 8shader_prim = scenario2.OverridePrim(class_prim.GetPath().AppendPath("Looks/light/light"))
 9shader = UsdShade.Shader(shader_prim)
10shader.CreateInput("emissiveColor", Sdf.ValueTypeNames.Color3f).Set((0.5, 0.4, 0.1))

  1. Save the file and run it by running the following code in the terminal:

Windows:

python .\composition_arcs\inherits\exercise\inherits_exercise_2.py

Linux:

python ./composition_arcs/inherits/exercise/inherits_exercise_2.py

Now let’s see how it affected our scenario_02.usd stage.

  1. In the terminal, run the following code to open the file in usdview.

Windows:

.\scripts\usdview.bat .\composition_arcs\inherits\exercise\scenario_02.usd

Linux:

./scripts/usdview.sh ./composition_arcs/inherits/exercise/scenario_02.usd

  1. In the terminal, run the following code to open scenario_01.usd in usdview and verify that it was unaffected.

Windows:

.\scripts\usdview.bat .\composition_arcs\inherits\exercise\scenario_01.usd

Linux:

./scripts/usdview.sh ./composition_arcs/inherits/exercise/scenario_02.usd

Now we can manipulate all of the lights in any stage with a simple override without affecting other stages.