NurbsCurve are rendered as linear curves

[cpichard]

Describe the bug
When we render the following NurbsCurve example exported from maya

#usda 1.0
(
    defaultPrim = "curve1"
    metersPerUnit = 0.01
    upAxis = "Y"
)

def NurbsCurves "curve1" (
    kind = "component"
)
{
    int[] curveVertexCounts = [4]
    float3[] extent = [(-2.5648465, -0.6877133, -0.5), (1.4385666, 7.8890786, 0.5)]
    double[] knots = [0, 0, 0, 0, 1, 1, 1, 1]
    int[] order = [4]
    point3f[] points = [(-2.0648465, 7.3890786, 0), (0.93856657, 4.8293514, 0), (0.2901024, 2.8156996, 0), (-1.7235495, -0.18771331, 0)]
    double2[] ranges = [(0, 1)]
    float[] widths = [1] (
        interpolation = "constant"
    )
}

Arnold will render linear segment instead of a nurb curve.
If you import this USD in Maya (cmds.mayaUSDImport(file="my_file.usd")), you get a nurbsCurve (see first screenshot).
If I try to render this same curve with arnold (using hydra render delagate), I get the second screenshot.

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-15620

[cpichard]

Closing as:

• The maya-usd export and import of NurbsCurves works correctly.
• Arnold does not support native nurbs curves.
• Hydra is converting the NurbsCurves to linear patches and there is no code yet to get the nurbs parameters in a render delegate from hydra. So it needs to be implemented, at first glance using the scene index system would be the way to go.
• MTOA usd export does convert the NurbsCurves to a visually correct approximation of the nurbs compatible with Arnold and Hydra.