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USD assets with MaterialX

Enabling MaterialX in USD

To enable materialx, build USD with --materialx option

python build_scripts\build_usd.py --materialx

Sample USD and MaterialX files

The MaterialX files are TM & (c) 2017 Lucasfilm Entertainment Company Ltd. and Lucasfilm Ltd.

All rights reserved, and covered by the Apache 2.0 license.

Example 1: Basic

• basic.usda - Sphere referring a MaterialX usd_preview_surface_plastic.mtlx

usd_preview_surface_plastic.mtlx defines a material called USD_Plastic using the UsdPreviewSurface MaterialX node.

• basic_flatten.usda - flattened version of basic.usda

Flattening can be performed by using usdcat

usdcat --flatten basic.usda -o basic_flatten.usda

Example 2: BasicTextured with variants

• basicTextured.usda - Teapot referring two MaterialX files

  1. usd_preview_surface_plastic.mtlx defines a material called USD_Plastic using the UsdPreviewSurface MaterialX node.
  2. standard_surface_brass_tiled.mtlx defines a material called Tiled_Brass using the standard_surface MaterialX node. This MaterialX file also uses a texture for coat_color and coat_rougness

  Both of these materials are accessible as materialvariants. To switch between variants in USDView, open the Interpreter and run the following commands:

       sbPrim = usdviewApi.stage.GetPrimAtPath("/Teapot")
       vs = sbPrim.GetVariantSet("materialvariants")
       vs.SetVariantSelection("Plastic")
       vs.SetVariantSelection("BrassTiled")
  

- basicTextured_flatten.usda - flattened version of basicTextured.usda 

![screenshot](screenshots/basicTextured_brasstiled.png)  
![screenshot](screenshots/basicTextured_plastic.png)  

 *Known limitations and MaterialX notes*: 
 - variants are lost when Usd is flattened 
 - Since the `fileprefix="./textures/"` is ignored by usdMtlx, our mtlx duplicates the fileprefix in the value input `value="./textures/`

### References
- MaterialX in USD:  ASWF Open Source 2021 [slides](https://materialx.org/assets/ASWF_OSD2021_MaterialX_slides_final.pdf).
- MaterialX examples: [MaterialX ASWF repository](https://github.com/AcademySoftwareFoundation/MaterialX/tree/main/resources/Materials/Examples).

basic.usda

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

def Scope "MaterialX" (
    references = @./usd_preview_surface_plastic.mtlx@</MaterialX>
)
{
}

def Sphere "sphere" (
    prepend apiSchemas = ["MaterialBindingAPI"]
)
{
    rel material:binding = </MaterialX/Materials/USD_Plastic>
}