specializes_a_practical_example

Explanation

The specializes Composition Arc is a bit weird. The USD glossary does a decent job of explaining how it works but I was hoping to put it in simpler terms. Both for my understanding and others.

It may be an over-simplification but specializes is a way of preventing and "protecting" changes that are made to a specialized Prim.

This folder shows a scenario where specializes is used to prevent edits from other, stronger layers.

There's 3 layers:

settings.usda modelling.usda layout.usda

settings.usda has some camera settings that layout.usda is expected to use. They do this by referencing the camera and changing its settings, directly. The layout.usda layer also brings in modelling.usda so that it can reference a model and place it into their scene.

The problem is, a modeller makes a mistake and authors an opinion onto the cameras in settings.usda. If you look at the code in layout.usda and then open it as a stage, you'll see the results of this change.

The </_class_CameraSettings> class gets overridden by modelling and its value make it into the final USD stage. The </_class_CameraSettingsSpecialized> class is also overridden by modelling but the value that modelling authored doesn't make it onto the final USD stage. Why? Because </_class_CameraSettingsSpecialized> is specialized and its authored focalLength property is defined in the specialized Prim. So the value for focalLength that was authored in settings.usda makes it all the way into layout.usda even though the modelling.usda Layer tried to override focalLength in between.

If the same situation happened and </_class_CameraSettingsSpecialized> used inherits instead of specializes, the value for focaLength would have been overridden in modelling.usda.

Final Note

Asute users of USD will point out that the above scenario can also be solved even if </_class_CameraSettingsSpecialized> used inherits if you include @./settings.usda in the subLayers of layout.usda like so:

(
    subLayers = [
        @./settings.usda@,
        @./modelling.usda@,
    ]
)

This also works and should probably be preferred whenever possible. That said, just consider the specializes solution when there's no other option.