Specializes

Specializing is taking a base definition of a prim (and its hierarchy) and further refining it into a specialized version of itself. For example taking a Metal material and specializing certain properties to make a CorrodedMetal material. This specialization could refine certain material properties, redefine or add shaders, textures, etc… It is not dissimilar from inherits, however with one key difference.

If you try to express an opinion on a specialized property, your opinion will essentially be discarded. This is not the case however with inherits.

Example → Corroded Material specialization

There are two layers, robot.usd and world.usd.

Robot.usd

robot.usd defines a Robot prim and three Material prims, Metal, CorrodedMetal and InheritedMetal. Metal defines two attributes, inputs:diffuseGain and inputs:specularRoughness.

CorrodedMetal is a specialization of Metal where inputs:specularRoughness is the specialized attribute, we say it always has a value of 0.2.

def Material "CorrodedMetal" (
    specializes = </Robot/Materials/Metal>
)
{
    # specialize roughness...
    float inputs:specularRoughness = 0.2
}

Whereas InheritedMetal is merely an inherits arc to Metal . You will notice that regardless of inherits or specializes, the result is the same. Both CorrodedMetal and InheritedMetal take over the base properties of Metal, but have an opinion on the inputs:specularRoughness attribute.

world.usd

world.usda references robot.usda and expresses and opinion on the base Metal prim. It says that inputs:diffuseGain and inputs:specularRoughness are to be 0.3 and 0.1 respectively.

#usda 1.0
 
def Xform "World"
{
    def "RosieTheRobot" (
        references = @./robot.usda@</Robot>
    )
    {
        over "Materials"
        {
            over "Metal"
            {
                    float inputs:diffuseGain = 0.3
                    float inputs:specularRoughness = 0.1
            }
        }
    }
}

Difference between Inherits and Specializes

When inspecting the resolved opinions in world.usd, notice that the overridden values for inputs:diffuseGain and inputs:specularRoughness on the base Metal propagate fully to InheritedMaterial, but not to our specialized CorrodedMaterial.

Base Metal Override	Result on InheritedMaterial	Result on CorrodedMaterial
over “Metal” {     float inputs:diffuseGain = 0.3     float inputs:specularRoughness = 0.1 }

As seen above, even when we have a stronger opinion on the base Metal definition for both properties, specializations always win.