A properly authored USD file will have the extent attribute authored on all prims conforming to UsdGeomBoundable.
This cached extent information is useful because it allows the 3D range of prims to be quickly understood without reading potentially large arrays of data. Note that because the shape of prims may change over time, extent attributes are always evaluated for a given timecode.
This patch introduces support for authoring extents on meshes and volumes during export to USD.
Because extents are common to multiple kinds of geometries, the main support for authoring extents has been placed in USDAbstractWriter, whose new author_extent method can operate on any prim conforming to pxr::UsdGeomBoundable. The USD library already provides us the code necessary to compute the bounds for a given prim, in pxr::UsdGeomBBoxCache::ComputeLocalBound.
Note that not all prims that are imageable are boundable, such as transforms and cameras.
For more details on extents, see https://graphics.pixar.com/usd/release/api/class_usd_geom_boundable.html#details.
Note that when new types of geometries are introduced, such as curves in https://developer.blender.org/D16545, we will need to update the USD writer for that geometry such that it calls this->author_extent.
Update on Feb 2: This patch has been updated to include a unit test to ensure authored extents are valid. This test requires new test assets that will need to be submitted via svn. The test assets are attached in the d16837_usd_test_assets.zip file. To use, unzip and merge the contents of this zip into the lib/tests/usd folder.
This unit test also addresses #104269 by validating compliance of exported USD via UsdUtils.ComplianceChecker.
Pull Request #104676
5040c39d1a