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README.md

Quick Explanation

The function for finding bound materials on a USD prim is unfortunately not concise enough to just paste into this file. So instead, here's some links

C++

Coming soon

Python

material_binding_api.py has a get_bound_material function. Use that to query the bound material of a USD Prim. It takes into account the new material collections API.

Longer Explanation (optional reading)

The USD documentation has a few issues when it comes to the new USD Collections API. It's partly out of date and also is missing key information for people to make use of it. This section is has one purpose - provide an up-to-date reference for discovering material bindings.

Note: In the future, new information / reference material may be found that will make this project redundant. If that's the case, please delete this page.

Summary

Do not use UsdShadeMaterial for querying information about materials. It's deprecated. Instead, prefer UsdShadeMaterialBindingAPI. [Pixar also recommends to use UsdShadeMaterialBindingAPI, too](https://graphics.pixar.com/usd/docs/UsdShade-Material-Assignment.ht ml#UsdShadeMaterialAssignment-UsdShadeAPI) but they don't go into detail on how to use it.

If you check out materials.py, you'll see the old, deprecated method for querying material bindings.

Now check out material_binding_api.py and you'll see the new-and-improved method which takes into account the new material collections API.

Details (if you're curious. Not required reading)

Justification for this project

As far as I could find, there's only two canonical pages that show source code for getting/authoring material assignment in USD. The Simple Shading in USD tutorial (and I guess, by extension, also the End to End Example). But that is direct material bindings. It's "old-hat", by USD's standards. Direct bindings are still good as long as:

  • You don't have to care about overriding shading on instanced Prims
  • You're fine with having less control over who is allowed to override material assignments

If either point is problematic for you (which is should, because most pipelines of any complexity need per-instance material assignment), the USD documentation suggests using USDCollectionAPI. From what I could see, this is the only page where USD files and API are explained in the same place, the [UsdShade Material Assignment page](https://graphics.pixar.com/usd/docs/UsdShade-Material-Assignment.h tml).

But this page has several problems.

The provided material USDA example file doesn't work

  • The materials example in this page doesn't work.
    • e.g. If you copy/paste the "Resolving Hierarchically-bound Materials" into a USD file, it'll syntax error because token is missing from outputs.

The provided Set-Shaded Office_set has multiple syntax errors

The lines that define collection:*:includes = need to end in [ or USD 19.07 will syntax error. Also every expansionRule needs to be a token / uniform token, not a Relationship.

uniform token collection:Erasers:expansionRule = "expandPrims"
rel collection:Erasers:includes = [
	</Office_set/Desk_Assembly/Cup_grp/Pencil_1/Geom/EraserHead>,
	...

Someone may dislike these nitpicks since they're simple fixes but it's the most documentation that we have currently and it doesn't work. Anyone looking to get stuff done can't use this to work.

The Pseudo-code "GetBoundMaterial" matches the name of a method that is deprecated

The [GetBoundMaterial](https://graphics.pixar.com/usd/docs/UsdShade-Material -Assignment.html#UsdShadeMaterialAssignment-MaterialResolve:Determiningt heBoundMaterialforanyGeometryPrim) function listed in the "UsdShade Material Assignment" page isn't immediately copy-and-pasteable because it's pseudo-code for the proposal. So the reader's next immediate thought is "I bet if I check the USD API documentation, I'll find the equivalent function".

[Found it](https://graphics.pixar.com/usd/docs/api/class_usd_shade_material.htm l#ac8a3ccb7c9859aabc5ba699b4addb834). Except surprise, the method is deprecated. And [so is GetBindingRel](https://graphics.pixar.com/usd/docs/api/class_usd_shade_m aterial.html#a7d254deef591583ada16825840dd49a9).

The good news though is that the pseudo-code itself is fine, as long as you're willing to write it out. Which is what this project is for.

Quick last note: it looks like pxr::UsdShadeMaterial::AllPurpose was moved to pxr::UsdShadeTokens->allPurpose? Not totally sure if that's correct.

See Also

USD ships with a command-line python module called "complianceChecker" which has an interesting way of querying material relationships on prims. It's great but it doesn't answer the question of "which material is bound to this Prim". Instead, it answers "if this Prim has material bindings, are they broken?". The use-case is different but it is still very useful.

[UsdUtils complianceChecker script](https://github.com/PixarAnimationStudios/USD/blob/32ca7df94c83ae 19e6fd38f7928d07f0e4cf5040/pxr/usd/lib/usdUtils/complianceChecker.py#L39 6-L421).

https://graphics.pixar.com/usd/docs/api/class_usd_shade_material_binding_a_p_i.html#a08ca12ce6b9929448e74e5a0ee310c05 https://graphics.pixar.com/usd/docs/UsdShade-Material-Assignment.html#UsdShadeMaterialAssignment-MaterialResolve:DeterminingtheBoundMaterialforanyGeometryPrim https://graphics.pixar.com/usd/docs/api/class_usd_shade_material.html#a52c7fe053819a1b71103f7a2b6a730e8 https://graphics.pixar.com/usd/docs/api/class_usd_collection_a_p_i_1_1_membership_query.html https://graphics.pixar.com/usd/docs/api/class_usd_shade_material_binding_a_p_i.html