source/blender/io/usd/intern/usd_writer_abstract.cc

@@ -5,12 +5,15 @@
 #include "usd_writer_material.h"
 
 #include <pxr/base/tf/stringUtils.h>
+#include <pxr/usd/usdGeom/bboxCache.h>
 
 #include "BKE_customdata.h"
 #include "BLI_assert.h"
 
 #include "DNA_mesh_types.h"
 
+#include "WM_api.h"
+
 /* TfToken objects are not cheap to construct, so we do it once. */
 namespace usdtokens {
 /* Materials */
@@ -149,4 +152,37 @@ bool USDAbstractWriter::mark_as_instance(const HierarchyContext &context, const
   return true;
 }
 
+void USDAbstractWriter::author_extent(const pxr::UsdTimeCode timecode, pxr::UsdGeomBoundable &prim)
+{
+  /* Compute the bounds for a boundable prim, and author the result as the extent attribute.
+   *
+   * Although this method works for any boundable prim, it is preferred to use Blender's own
+   * cached bounds when possible.
+   *
+   * This method does not author the extentsHint attribute, which is also important to provide.
+   * Whereas the extent attribute can only be authored on prims inheriting from UsdGeomBoundable,
+   * an extentsHint can be provided on any prim, including scopes.  This extentsHint should be
+   * authored on every prim in a hierarchy being exported.
+   *
+   * Note that this hint is only useful when importing or inspecting layers, and should not be
+   * taken into account when computing extents during export.
+   *
+   * TODO: also provide method for authoring extentsHint on every prim in a hierarchy.
+   */
+  
+  /* do not use any existing extentsHint that may authored, instead recompute the extent when authoring it */
+  const bool useExtentsHint = false;
+  const pxr::TfTokenVector includedPurposes{pxr::UsdGeomTokens->default_};
+  pxr::UsdGeomBBoxCache bboxCache(timecode, includedPurposes, useExtentsHint);
+  pxr::GfBBox3d bounds = bboxCache.ComputeLocalBound(prim.GetPrim());
+  if (pxr::GfBBox3d() == bounds) {
+    /* This will occur, for example, if a mesh does not have any vertices. */
+    WM_reportf(RPT_ERROR, "USD Export: no bounds could be computed for %s", prim.GetPrim().GetName().GetText());
+    return;
+  }
+  
+  pxr::VtArray<pxr::GfVec3f> extent{ (pxr::GfVec3f)bounds.GetRange().GetMin(), (pxr::GfVec3f)bounds.GetRange().GetMax() };
+  prim.CreateExtentAttr().Set(extent);
+}
+
 }  // namespace blender::io::usd

source/blender/io/usd/intern/usd_writer_abstract.h

@@ -9,6 +9,7 @@
 #include <pxr/usd/usd/stage.h>
 #include <pxr/usd/usdShade/material.h>
 #include <pxr/usd/usdUtils/sparseValueWriter.h>
+#include <pxr/usd/usdGeom/boundable.h>
 
 #include <vector>
 
@@ -67,6 +68,8 @@ class USDAbstractWriter : public AbstractHierarchyWriter {
    * Reference the original data instead of writing a copy.
    */
   virtual bool mark_as_instance(const HierarchyContext &context, const pxr::UsdPrim &prim);
+  
+  virtual void author_extent(const pxr::UsdTimeCode timecode, pxr::UsdGeomBoundable &prim);
 };
 
 }  // namespace blender::io::usd

source/blender/io/usd/intern/usd_writer_hair.cc

@@ -62,6 +62,8 @@ void USDHairWriter::do_write(HierarchyContext &context)
     colors.push_back(pxr::GfVec3f(cache[0]->col));
     curves.CreateDisplayColorAttr(pxr::VtValue(colors));
   }
+  
+  this->author_extent(timecode, curves);
 }
 
 bool USDHairWriter::check_is_animated(const HierarchyContext & /*context*/) const

source/blender/io/usd/intern/usd_writer_mesh.cc

@@ -5,6 +5,7 @@
 
 #include <pxr/usd/usdGeom/mesh.h>
 #include <pxr/usd/usdGeom/primvarsAPI.h>
+#include <pxr/usd/usdGeom/bboxCache.h>
 #include <pxr/usd/usdShade/material.h>
 #include <pxr/usd/usdShade/materialBindingAPI.h>
 
@@ -247,6 +248,15 @@ void USDGenericMeshWriter::write_mesh(HierarchyContext &context, Mesh *mesh)
   if (usd_export_context_.export_params.export_materials) {
     assign_materials(context, usd_mesh, usd_mesh_data.face_groups);
   }
+  
+  /* Blender grows its bounds cache to cover animated meshes, so only author once. */
+  float bound_min[3];
+  float bound_max[3];
+  INIT_MINMAX(bound_min, bound_max);
+  BKE_mesh_minmax(mesh, bound_min, bound_max);
+  pxr::VtArray<pxr::GfVec3f> extent{ pxr::GfVec3f{ bound_min[0], bound_min[1], bound_min[2]},
+                                     pxr::GfVec3f{ bound_max[0], bound_max[1], bound_max[2]} };
+  usd_mesh.CreateExtentAttr().Set(extent);
 }
 
 static void get_vertices(const Mesh *mesh, USDMeshData &usd_mesh_data)