Can one USD material refer to two MDL files?

[xiaobai9007]

Hi experts, I’m learning about MDL and USD. USD file is able to refer to MDL file as assert. For example:

def Material "sample"
{
    color3f inputs:base_color          = (0.5, 0.5, 0.5)
    float   inputs:anisotropy          = 0
    float   inputs:anisotropy_rotation = 0

    token outputs:mdl:surface.connect = </sample/flex_material.outputs:out>
    token outputs:mdl:volume.connect = </sample/flex_material.outputs:out>
    token outputs:mdl:displacement.connect = </sample/flex_material.outputs:out>

    def Shader "flex_material"
    {
        uniform token info:implementationSource = "sourceAsset"
        uniform asset info:mdl:sourceAsset = @nvidia/core_definitions.mdl@
        uniform token info:mdl:sourceAsset:subIdentifier = "flex_material"

        color3f inputs:base_color.connect = </sample.inputs:base_color>
        float   inputs:anisotropy.connect = </sample.inputs:anisotropy>
        float   inputs:anisotropy_rotation.connect = </sample.inputs:anisotropy_rotation>
    }
}

But if there was another shader in the Material “sample”, and the shader refer to another MDL file, could the material work? And Is this case meaningful?

[mati-nvidia]

Hi @xiaobai9007. Welcome to the forums! Please consider also joining us on Discord: NVIDIA Omniverse.

Yes, a material can be built up of more than one MDL. Here’s an example created using the material graph in USD Composer:

def Xform "World"
{
    def Scope "Looks"
    {
        def Material "BlendedMaterial"
        {
            token outputs:mdl:displacement.connect = </World/Looks/BlendedMaterial/blend.outputs:out>
            token outputs:mdl:surface.connect = </World/Looks/BlendedMaterial/blend.outputs:out>
            token outputs:mdl:volume.connect = </World/Looks/BlendedMaterial/blend.outputs:out>

            def Shader "blend" (
                prepend apiSchemas = ["NodeGraphNodeAPI"]
            )
            {
                uniform token info:implementationSource = "sourceAsset"
                uniform asset info:mdl:sourceAsset = @nvidia/core_definitions.mdl@
                uniform token info:mdl:sourceAsset:subIdentifier = "blend"
                token inputs:base (
                    displayName = "Base material"
                    doc = "The material the blend is based on"
                    hidden = true
                    renderType = "material"
                )
                token inputs:base.connect = </World/Looks/BlendedMaterial/PBRShader.outputs:out>
                token inputs:blend (
                    displayName = "Blend Material"
                    doc = "Surface properties to use for the blend"
                    hidden = true
                    renderType = "material"
                )
                token inputs:blend.connect = </World/Looks/BlendedMaterial/OmniSurfaceBase.outputs:out>
                float inputs:weight (
                    customData = {
                        float default = 0
                        dictionary range = {
                            float max = 1
                            float min = 0
                        }
                    }
                    displayName = "Blend weight"
                    doc = "Blend weight or mask texture"
                    hidden = false
                    renderType = "float"
                )
                float inputs:weight.connect = </World/Looks/BlendedMaterial/lookup_float.outputs:out>
                token outputs:out (
                    renderType = "material"
                )
                uniform token ui:nodegraph:node:expansionState = "open"
                uniform float2 ui:nodegraph:node:pos = (-413.5083, 30.022224)
            }

            def Shader "OmniSurfaceBase" (
                prepend apiSchemas = ["NodeGraphNodeAPI"]
            )
            {
                uniform token info:implementationSource = "sourceAsset"
                uniform asset info:mdl:sourceAsset = @OmniSurface/OmniSurfaceBase.mdl@
                uniform token info:mdl:sourceAsset:subIdentifier = "OmniSurfaceBase"
                
                float outputs:out (
                    renderType = "float"
                )
                uniform token ui:nodegraph:node:expansionState = "closed"
                uniform float2 ui:nodegraph:node:pos = (-767.252, 289.14746)
            }

            def Shader "PBRShader" (
                prepend apiSchemas = ["NodeGraphNodeAPI"]
            )
            {
                uniform token info:implementationSource = "sourceAsset"
                uniform asset info:mdl:sourceAsset = @OmniPBR.mdl@
                uniform token info:mdl:sourceAsset:subIdentifier = "OmniPBR"
                token outputs:out (
                    renderType = "material"
                )
                uniform token ui:nodegraph:node:expansionState = "minimized"
                uniform float2 ui:nodegraph:node:pos = (-782.1521, 19.353767)
            }
        }
    }
}

[xiaobai9007]

Thank you very much for the example! @mati-nvidia
And I come up with another question, can a USD shader reference multiple MDL files?

[mati-nvidia]

Hi @xiaobai9007. I’m pretty sure that’s not possible, but I can ask to confirm. It would help if you describe what you are trying to do and why you would want one USD shader to reference multiple MDL files.