Exercise: Lofting Primvars#

The accentColor primvar that we added in the “Asset Parameterization” exercise is now hidden behind the payload. When the payload is unloaded, consumers can’t easily discover this asset parameter. In this exercise, we will loft the accentColor primvar above the payload so that it is still discoverable even with the payload unloaded.

  1. In Visual Studio Code, open the following file:
    asset_structure/exercise_07/lrg_bldgF.usd.

Note that the accentColor primvar is not present in this file.

 1#usda 1.0
 2(
 3	defaultPrim = "World"
 4	metersPerUnit = 0.01
 5	upAxis = "Y"
 6)
 7
 8def Xform "World" (
 9	prepend payload = @./contents.usd@
10)
11{
12}

  1. In Visual Studio Code, open the following file: asset_structure/exercise_07/loft_parameters.py

  2. Let’s add the code to loft our properties. Add the code below into the block:

1prim_spec = contents_layer.GetPrimAtPath("/World")
2prop_spec: Sdf.PropertySpec
3for prop_spec in prim_spec.properties:
4    Sdf.CopySpec(contents_layer, prop_spec.path, asset_layer, prop_spec.path)
5    prim_spec.RemoveProperty(prop_spec)

This code uses the Sdf API, which is a bit lower-level than the Usd API and operates on individual layers without considering composition. Here, we’re copying all the properties found on the default prim in contents.usd and moving them to the same prim in lrg_bldgF.usd. Specifically, we’re interested in the primvars:accentColor attribute so that it’s discoverable even when the payload is unloaded.

  1. Save the file and then execute the script using the following command:

Windows:

python .\asset_structure\exercise_07\loft_parameters.py

Linux:

python ./asset_structure/exercise_07/loft_parameters.py

  1. Open the output lrg_bldgF in usdview by running the following command in Visual Studio Code:

Windows:

.\scripts\usdview.bat .\asset_structure\exercise_07\lrg_bldgF.usd

Linux:

./scripts/usdview.sh ./asset_structure/exercise_07/lrg_bldgF.usd

  1. In usdview, click on the “World” (default) prim. Note the “primvars:accentColor” attribute in the Properties panel.

  1. In usdview, right-click on the “World” (default) prim and select “Unload”. Note that the “primvars:accentColor” attribute is still listed in the Properties panel.