[Note] UsdPreviewSurface specification

[Syoyo Fujita]

https://graphics.pixar.com/usd/release/spec_usdpreviewsurface.html

Latest version

2.5

UDIM

• Up to 10 tiles in U direction
• Tile id is in range [1001, 1109]
  • https://openusd.org/release/spec_usdpreviewsurface.html#version-2-5-current-head

UsdPreviewSurface

Blender

• Do we allow specular attribute?(Used in Blender USD)
• float3 and color3f PixarAnimationStudios/OpenUSD#2729
  • Currently TinyUSDZ accepts float3 attribute

UsdUVTexture

• black border color: Not sure what actual black color should be. Possible value is transparent black (0, 0, 0, 0) or opaque black (0, 0, 0, 1)
  • Currently TinyUSDZ Tydra uses transparent black.

UDIM

• pxrUSD does not support UDIM 1100 tile PixarAnimationStudios/OpenUSD#2229 . until 23.11? tinyusdz supports it

[Syoyo Fujita]

Tangents, binormals

The spec says

    #
    # Parameters only useful for tangent space normal mapping
    # Note : Currently, we only support one tangent frame
    #
    # Details : Name of the primvar in your geom to use for the tangents.
    # Default : "tangents"
    string inputs:frame:tangentsPrimvarName = "tangents"

    # Details : Name of the primvar in your geom to use for the binormals
    # Default : "binormals"
    string inputs:frame:binormalsPrimvarName = "binormals"

    # Details : Name of the texture coordinate to be use to calculate
    #            the tangent frame. Mikktspace or similar is recommended for
    #            consistency
    # Default : "st"
    string inputs:frame:stPrimvarName = "st"

Although there is no actual example usage of inputs:frame:* in pxrUSD git repo(and some USDZ files), we are better to consider supporting it for better normal mapping(supply user-defined per-vertex tangents/binormals)