Fix: USD: Particle hair has wrong/missing materials assignments

The particle material slot was not used by Hydra, and USD export did not bind a material at all. Pull Request: https://projects.blender.org/blender/blender/pulls/160433
2026-09-29 04:37:17 +03:00 · 2026-05-28 05:19:31 +02:00 · 2026-05-28 05:19:31 +02:00 · 3edc7cc0ea
commit 3edc7cc0ea
parent 1cdf15ae4f
46 changed files with 78 additions and 110 deletions
--- a/source/blender/io/usd/hydra/curves.cc
+++ b/source/blender/io/usd/hydra/curves.cc
@ -318,8 +318,7 @@ void emit_hair_for_object(PopulateContext &ctx, Object *object, EmittedObject &e
    if (hair.curve_vertex_counts.empty()) {
      continue;
    }
-    /* TODO: Using only first material. Add support for multi-material. */
-    const CurvesMaterial mat = resolve_curves_material(ctx, object, 1);
+    const CurvesMaterial mat = resolve_curves_material(ctx, object, psys.part->omat);
    if (mat.material) {
      emitted.materials.append_non_duplicates(mat.material);
    }
@ -351,7 +350,7 @@ void emit_hair_proto(PopulateContext &ctx, Object *source)
    if (hair.curve_vertex_counts.empty()) {
      continue;
    }
-    const CurvesMaterial mat = resolve_curves_material(ctx, source, 1);
+    const CurvesMaterial mat = resolve_curves_material(ctx, source, psys.part->omat);

    char ps_name[32];
    SNPRINTF(ps_name, "PS_%p", &psys);
--- a/source/blender/io/usd/intern/usd_writer_hair.cc
+++ b/source/blender/io/usd/intern/usd_writer_hair.cc
@ -6,12 +6,15 @@

 #include <pxr/usd/usdGeom/basisCurves.h>
 #include <pxr/usd/usdGeom/tokens.h>
+#include <pxr/usd/usdShade/materialBindingAPI.h>

+#include "BKE_material.hh"
 #include "BKE_particle.h"

 #include "BLI_math_matrix.hh"
 #include "BLI_math_vector_types.hh"

+#include "DNA_material_types.h"
 #include "DNA_object_types.h"
 #include "DNA_particle_types.h"

@ -83,9 +86,35 @@ void USDHairWriter::do_write(HierarchyContext &context)
    write_id_properties(prim, psys->part->id, time);
  }

+  assign_material(context, curves, psys->part->omat);
+
  this->author_extent(curves, time);
 }

+void USDHairWriter::assign_material(const HierarchyContext &context,
+                                    const pxr::UsdGeomBasisCurves &curves,
+                                    const int material_slot)
+{
+  if (!usd_export_context_.export_params.export_materials) {
+    return;
+  }
+
+  Material *material = BKE_object_material_get_eval(context.object, material_slot);
+  if (material == nullptr) {
+    return;
+  }
+
+  pxr::UsdShadeMaterial usd_material = ensure_usd_material(context, material);
+  if (!usd_material) {
+    return;
+  }
+
+  auto curves_prim = curves.GetPrim();
+  pxr::UsdShadeMaterialBindingAPI binding(curves_prim);
+  binding.Bind(usd_material);
+  pxr::UsdShadeMaterialBindingAPI::Apply(curves_prim);
+}
+
 bool USDHairWriter::check_is_animated(const HierarchyContext & /*context*/) const
 {
  return true;
--- a/source/blender/io/usd/intern/usd_writer_hair.hh
+++ b/source/blender/io/usd/intern/usd_writer_hair.hh
@ -14,6 +14,9 @@ class USDHairWriter : public USDAbstractWriter {

 protected:
  void do_write(HierarchyContext &context) override;
+  void assign_material(const HierarchyContext &context,
+                       const pxr::UsdGeomBasisCurves &curves,
+                       const int material_slot);
  bool check_is_animated(const HierarchyContext &context) const override;
 };

--- a/tests/files/render/hair/storm_hydra_renders/hair_basemesh_intercept.png
+++ b/tests/files/render/hair/storm_hydra_renders/hair_basemesh_intercept.png
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--- a/tests/files/render/hair/storm_hydra_renders/hair_close_up.png
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