material-final-surface#

Code

VM.PBR.001

Validator

latest+

Compatibility

open-usd

Tags

✅

Summary#

A renderable GPrim’s material must provide a physically based final surface.

Description#

A renderable GPrim resolves a material binding at the full purpose. That material MUST connect an OpenPBR surface on outputs:mtlx:surface — a shader whose info:id is ND_open_pbr_surface_surfaceshader. No other MaterialX surface node is accepted on that terminal.

An MDL surface does not satisfy this. outputs:mdl:surface is VM.MDL.003, and a profile that accepts either lists both. Requiring OpenPBR here is what lets a consumer read a FET_006_OPENPBR pass as “this asset has OpenPBR” — a feature a material can satisfy without having the surface the feature is named for cannot answer that question. Where a material has both an OpenPBR and an MDL surface, the two SHOULD represent the same appearance.

The constraint is the terminal on the Material. The name of the shader output it connects to is unconstrained: outputs:out and outputs:surface are both conformant.

The UsdPreviewSurface preview is a separate feature, com.nvidia.usd.VM.PS.001. A profile that requires both lists both.

Two subjects, and why#

The requirement is written for two shapes of asset.

An asset with geometry. The subject is the geometry a final render draws: a GPrim whose computed purpose is default or render, and the material it resolves at the full purpose. Every such material MUST connect the terminal. Proxy and guide geometry, materials bound only through material:binding:preview, unused entries in /Looks, and friction-only physics materials are all outside the subject, so no exception has to be written for them.

An asset that is a material. A material library publishes Material prims and no geometry to bind them to. Against the first subject such an asset has nothing to judge, so the requirement would report nothing and the feature would state nothing a consumer could act on. Where the stage has no geometry a final render would draw, the subject becomes the Material prims, and at least one of them MUST connect the terminal.

The second branch asks for one rather than all, because an asset with no geometry gives no way to say which of its materials a consumer will use. On an asset with geometry the first branch is the stronger statement and the second does not run — so an unused material carrying the terminal can never stand in for a bound material that lacks it.

VG.MESH.001 uses the same stage-level shape to require that a stage contains at least one mesh.

Why is it required?#

Without an OpenPBR final surface an asset produces:

  • Inconsistent appearance across RTX, USDView, and other OpenPBR-capable runtimes

  • Content that resolves only under MDL, which prevents renderer-independent rendering

  • Manual material repair before the asset is usable for camera or perception simulation

Examples#

Valid: an OpenPBR (MaterialX) final surface#

def Xform "Bracket" ( kind = "component" )
{
    def Mesh "Geo" ( prepend apiSchemas = ["MaterialBindingAPI"] )
    {
        rel material:binding = </Bracket/mtl_bracket>
    }

    def Material "mtl_bracket"
    {
        token outputs:mtlx:surface.connect = </Bracket/mtl_bracket/OpenPBR.outputs:out>

        def Shader "OpenPBR"
        {
            uniform token info:id = "ND_open_pbr_surface_surfaceshader"
            color3f inputs:base_color = (0.4, 0.4, 0.42)
            float inputs:base_metalness = 1
            float inputs:specular_roughness = 0.35
            token outputs:out
        }
    }
}

Invalid: an MDL final surface alone#

def Xform "Bracket" ( kind = "component" )
{
    def Mesh "Geo" ( prepend apiSchemas = ["MaterialBindingAPI"] )
    {
        rel material:binding = </Bracket/mtl_bracket>
    }

    def Material "mtl_bracket"
    {
        token outputs:mdl:surface.connect = </Bracket/mtl_bracket/OmniPBR.outputs:out>

        def Shader "OmniPBR"
        {
            uniform token info:implementationSource = "sourceAsset"
            uniform asset info:mdl:sourceAsset = @./materials/OmniPBR/OmniPBR.mdl@
            uniform token info:mdl:sourceAsset:subIdentifier = "OmniPBR"
            token outputs:out
        }
    }
}

An MDL surface satisfies VM.MDL.003, not this requirement. A profile that accepts either lists both codes.

Invalid: the bound material has only a preview surface#

def Xform "Bracket" ( kind = "component" )
{
    def Mesh "Geo" ( prepend apiSchemas = ["MaterialBindingAPI"] )
    {
        rel material:binding = </Bracket/mtl_bracket>
    }

    def Material "mtl_bracket"
    {
        token outputs:surface.connect = </Bracket/mtl_bracket/Preview.outputs:surface>

        def Shader "Preview"
        {
            uniform token info:id = "UsdPreviewSurface"
            color3f inputs:diffuseColor = (0.4, 0.4, 0.42)
            token outputs:surface
        }
    }
}

Invalid: the mtlx surface is a MaterialX node other than OpenPBR#

def Xform "Bracket" ( kind = "component" )
{
    def Mesh "Geo" ( prepend apiSchemas = ["MaterialBindingAPI"] )
    {
        rel material:binding = </Bracket/mtl_bracket>
    }

    def Material "mtl_bracket"
    {
        token outputs:mtlx:surface.connect = </Bracket/mtl_bracket/StandardSurface.outputs:out>

        def Shader "StandardSurface"
        {
            uniform token info:id = "ND_standard_surface_surfaceshader"
            color3f inputs:base_color = (0.4, 0.4, 0.42)
            token outputs:out
        }
    }
}

standard_surface is physically based and widely implemented. It is reported because the id on outputs:mtlx:surface is compared against ND_open_pbr_surface_surfaceshader alone.

Valid: an asset that is a material, with no geometry#

#usda 1.0

def Material "Aging_Copper"
{
    token outputs:surface.connect = </Aging_Copper/Preview.outputs:surface>
    token outputs:mtlx:surface.connect = </Aging_Copper/OpenPBR.outputs:out>

    def Shader "OpenPBR"
    {
        uniform token info:id = "ND_open_pbr_surface_surfaceshader"
        token outputs:out
    }

    def Shader "Preview"
    {
        uniform token info:id = "UsdPreviewSurface"
        token outputs:surface
    }
}

A material library publishes files of this shape. There is no geometry to resolve a binding from, so the requirement is judged against the Material prims themselves.

How to comply#

  • Connect a final surface: OpenPBR on outputs:mtlx:surface, or an MDL surface on outputs:mdl:surface during migration.

  • Give a final surface to every material the asset renders with, meaning every material a renderable GPrim resolves at full. A material bound only for preview, and one nothing binds, are outside this.

  • For new assets, use OpenPBR and the SimReady reference presets (PhysicalAI-SimReady-Materials).

For More Information#