[TODO] MaterialX support

[syoyo]

Working branch: mtlx https://github.com/syoyo/tinyusdz/tree/mtlx

MaterialX spec: https://materialx.org/assets/MaterialX.v1.38.Spec.pdf
(version: 1.38)

Adding TinyUSDZ to load USD model in ASF MaterialXViewer https://github.com/lighttransport/materialx
(We can use this as a reference and for verification)

• UsdMtlx Prim
• Visualization & rendering validation
  • Plug tinyusdz + MaterialX into pbrlab? https://github.com/lighttransport/pbrlab
  • Use tinyusdz support fork of ASF MaterialX https://github.com/lighttransport/materialx

Shading model support

• UsdPreviewSurface
• Autodesk Standard Surface
• OpenPBR https://academysoftwarefoundation.github.io/OpenPBR/ [TODO] Support OpenPBR #172

TODO

• MaterialX v1.39 support.

Don't support

• Shaders in OSL, GLSL, etc
  • Support hardcoded shader only

Dataset for USD + MaterialX

• https://github.com/RogerDass/Usd-Mtlx-Example/

[syoyo]

Example syntax(?) of materialx

#usda 1.0

def "bora" 
(
   # Seems we need to specify </MaterialX> prim path
   references = @usd_preview_surface_default.mtlx@</MaterialX>
) {

}

$ usdcat --flatten materialx-001.usda 
#usda 1.0
(
    doc = """Generated from Composed Stage of root layer /mnt/data/work/tinyusdz/tests/usda/materialx-001.usda
"""
)

def "bora"
{
    def "Materials"
    {
        def Material "USD_Default"
        {
        }
    }
}

• Use references to load .mtlx
• Need to add </MaterialX> prim path(otherwise flatten fails)
• It looks Mtlx support in pxrUSD is not yet well supported(material/shader parameter not expanded)

[syoyo]

https://materialx.org/assets/MaterialX.v1.38.Supplement.pdf

Texture nodes

• tiledimage(for texture mapping)
• triplanarprojection(for cube mapping?)
• ramp4

Math nodes

• place2d
  • Mapping place2d to UsdTransform2d looses some infos(e.g. pivot)
• safepower

Adjustment nodes

• contrast
• range
• hsvadjust
• saturate

Channel nodes

• extract
• separate2, separate3, separate4