Robust interrogation of UsdAttribute via USD APIs

[HamedSabri-adsk]

Description of Issue

We would like to have a set of APIs that could reliably and efficiently answer if single attribute(s) (e.g radius) or two-tiered attribute(s) (e.g transform op/order) can be allowed to edit depending on where the stronger opinion(s) come from.

We are currently achieving this by comparing the calculated “position index” between a strongest layer containing the attribute and edit target layer across all the sites. Strongest layer always has a lower index than a weak layer(s).

https://github.com/Autodesk/maya-usd/blob/8b00e510717e9339fe174b5e07bcfd0f607becc9/lib/mayaUsd/ufe/Utils.cpp#L87

https://github.com/Autodesk/maya-usd/blob/8b00e510717e9339fe174b5e07bcfd0f607becc9/lib/mayaUsd/ufe/Utils.cpp#L378

https://github.com/Autodesk/maya-usd/blob/94a3e3e3f91835caae462c7cf89b82ac5741ffc4/lib/mayaUsd/ufe/UsdTransform3dMayaXformStack.cpp#L510

Example 1:

Simple example involving reference arc, where an attribute ( e.g “doubleSided” ) is being interrogated in "LayerA.usda"

LayerA.usda

#usda 1.0
over "refSphere" (
    prepend references = @./Sphere.usda@
)
{
    uniform token[] xformOpOrder = []
}

Sphere.usda

#usda 1.0
(
    defaultPrim = "transform"
)

def Xform "transform"
{
    def Sphere "sphereShape"
    {
        uniform bool doubleSided = 1
        float3[] extent = [(-2, -2, -2), (2, 2, 2)]
        color3f[] primvars:displayColor = [(0, 0, 1)]
        double radius = 2
    }
}

In this scenario, strongest Layer for “doubleSided” attribute is “Sphere.usda” where its value is set to true. Authoring a new opinion for “doubleSided” attribute in LayerA.usda should be possible.

Example 2:

Simple example that shows when an attribute edit is blocked in weaker layers. Consider following Sublayer hierarchy:

anonymousLayer1
    anonymousLayer6
    anonymousLayer5
    anonymousLayer4
    anonymousLayer3
    anonymousLayer2

With “anonymousLayer5” selected as an "edit target", the user authors a new height/radius opinion:

over "Capsule1"
{
    float3[] extent = [(-1, -1, -3.5), (1, 1, 3.5)]
    double height = 5
    double radius = 1
}

Any edits in weaker layers (anonymousLayer4, anonymousLayer3, anonymousLayer2) are blocked. Making edits in stronger layers (anonymousLayer1, anonymousLayer6) should be still allowed.

[jilliene]

Filed as internal issue #USD-6629