USD: Read and write custom attributes for Curves

Add support for reading and writing custom `Curves` attributes.

This allows us to roundtrip Blender's Hair grooms containing UVs and
other attribute data. Note that animated attribute values are not
supported with this change.

This will also address #120042

Pull Request: https://projects.blender.org/blender/blender/pulls/121928

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

@@ -6,11 +6,14 @@
 
 #include "usd_reader_curve.hh"
 #include "usd.hh"
+#include "usd_attribute_utils.hh"
+#include "usd_hash_types.hh"
 
 #include "BKE_attribute.hh"
 #include "BKE_curves.hh"
 #include "BKE_geometry_set.hh"
 #include "BKE_object.hh"
+#include "BKE_report.hh"
 
 #include "BLI_index_range.hh"
 #include "BLI_math_vector_types.hh"
@@ -20,8 +23,10 @@
 
 #include <pxr/base/vt/types.h>
 #include <pxr/usd/usdGeom/basisCurves.h>
+#include <pxr/usd/usdGeom/primvarsAPI.h>
 
 namespace blender::io::usd {
+
 static inline float3 to_float3(pxr::GfVec3f vec3f)
 {
   return float3(vec3f.data());
@@ -105,6 +110,27 @@ static CurveType get_curve_type(pxr::TfToken type, pxr::TfToken basis)
   return CURVE_TYPE_POLY;
 }
 
+static const std::optional<bke::AttrDomain> convert_usd_interp_to_blender(
+    const pxr::TfToken usd_domain)
+{
+  static const blender::Map<pxr::TfToken, bke::AttrDomain> domain_map = []() {
+    blender::Map<pxr::TfToken, bke::AttrDomain> map;
+    map.add_new(pxr::UsdGeomTokens->vertex, bke::AttrDomain::Point);
+    map.add_new(pxr::UsdGeomTokens->varying, bke::AttrDomain::Point);
+    map.add_new(pxr::UsdGeomTokens->constant, bke::AttrDomain::Curve);
+    map.add_new(pxr::UsdGeomTokens->uniform, bke::AttrDomain::Curve);
+    return map;
+  }();
+
+  const bke::AttrDomain *value = domain_map.lookup_ptr(usd_domain);
+
+  if (value == nullptr) {
+    return std::nullopt;
+  }
+
+  return *value;
+}
+
 void USDCurvesReader::create_object(Main *bmain, const double /*motionSampleTime*/)
 {
   curve_ = static_cast<Curves *>(BKE_curves_add(bmain, name_.c_str()));
@@ -258,6 +284,41 @@ void USDCurvesReader::read_curve_sample(Curves *curves_id, const double motionSa
 
     radii.finish();
   }
+
+  read_custom_data(curves, motionSampleTime);
+}
+
+void USDCurvesReader::read_custom_data(bke::CurvesGeometry &curves,
+                                       const double motionSampleTime) const
+{
+  pxr::UsdGeomPrimvarsAPI pv_api(curve_prim_);
+
+  std::vector<pxr::UsdGeomPrimvar> primvars = pv_api.GetPrimvarsWithValues();
+  for (const pxr::UsdGeomPrimvar &pv : primvars) {
+    if (!pv.HasValue()) {
+      continue;
+    }
+
+    const pxr::SdfValueTypeName pv_type = pv.GetTypeName();
+    const pxr::TfToken pv_interp = pv.GetInterpolation();
+
+    const std::optional<bke::AttrDomain> domain = convert_usd_interp_to_blender(pv_interp);
+    const std::optional<eCustomDataType> type = convert_usd_type_to_blender(pv_type);
+
+    if (!domain.has_value() || !type.has_value()) {
+      const pxr::TfToken pv_name = pv.StripPrimvarsName(pv.GetPrimvarName());
+      BKE_reportf(reports(),
+                  RPT_WARNING,
+                  "Primvar '%s' (interpolation %s, type %s) cannot be converted to Blender",
+                  pv_name.GetText(),
+                  pv_interp.GetText(),
+                  pv_type.GetAsToken().GetText());
+      continue;
+    }
+
+    bke::MutableAttributeAccessor attributes = curves.attributes_for_write();
+    copy_primvar_to_blender_attribute(pv, motionSampleTime, *type, *domain, {}, attributes);
+  }
 }
 
 void USDCurvesReader::read_geometry(bke::GeometrySet &geometry_set,

source/blender/io/usd/intern/usd_reader_curve.hh

@@ -13,7 +13,8 @@
 struct Curves;
 namespace blender::bke {
 struct GeometrySet;
-}
+class CurvesGeometry;
+}  // namespace blender::bke
 
 namespace blender::io::usd {
 
@@ -43,6 +44,8 @@ class USDCurvesReader : public USDGeomReader {
   void read_geometry(bke::GeometrySet &geometry_set,
                      USDMeshReadParams params,
                      const char **err_str) override;
+
+  void read_custom_data(bke::CurvesGeometry &curves, const double motionSampleTime) const;
 };
 
 }  // namespace blender::io::usd

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

@@ -3,18 +3,25 @@
  * SPDX-License-Identifier: GPL-2.0-or-later */
 
 #include <numeric>
+#include <string>
 
 #include <pxr/usd/usdGeom/basisCurves.h>
 #include <pxr/usd/usdGeom/curves.h>
 #include <pxr/usd/usdGeom/nurbsCurves.h>
+#include <pxr/usd/usdGeom/primvar.h>
+#include <pxr/usd/usdGeom/primvarsAPI.h>
 #include <pxr/usd/usdGeom/tokens.h>
 #include <pxr/usd/usdShade/material.h>
 #include <pxr/usd/usdShade/materialBindingAPI.h>
 
+#include "usd_attribute_utils.hh"
 #include "usd_hierarchy_iterator.hh"
 #include "usd_writer_curves.hh"
 
 #include "BLI_array_utils.hh"
+#include "BLI_generic_virtual_array.hh"
+#include "BLI_span.hh"
+#include "BLI_virtual_array.hh"
 
 #include "BKE_attribute.hh"
 #include "BKE_curve_legacy_convert.hh"
@@ -354,6 +361,119 @@ void USDCurvesWriter::set_writer_attributes(pxr::UsdGeomCurves &usd_curves,
   }
 }
 
+static std::optional<pxr::TfToken> convert_blender_domain_to_usd(
+    const bke::AttrDomain blender_domain, bool is_bezier)
+{
+  switch (blender_domain) {
+    case bke::AttrDomain::Point:
+      return is_bezier ? pxr::UsdGeomTokens->varying : pxr::UsdGeomTokens->vertex;
+    case bke::AttrDomain::Curve:
+      return pxr::UsdGeomTokens->uniform;
+
+    default:
+      return std::nullopt;
+  }
+}
+
+void USDCurvesWriter::write_generic_data(const bke::CurvesGeometry &curves,
+                                         const bke::AttributeIDRef &attribute_id,
+                                         const bke::AttributeMetaData &meta_data,
+                                         const pxr::UsdGeomCurves &usd_curves)
+{
+  const CurveType curve_type = CurveType(curves.curve_types().first());
+  const bool is_bezier = curve_type == CURVE_TYPE_BEZIER;
+
+  const std::optional<pxr::TfToken> pv_interp = convert_blender_domain_to_usd(meta_data.domain,
+                                                                              is_bezier);
+  const std::optional<pxr::SdfValueTypeName> pv_type = convert_blender_type_to_usd(
+      meta_data.data_type);
+
+  if (!pv_interp || !pv_type) {
+    BKE_reportf(this->reports(),
+                RPT_WARNING,
+                "Attribute '%s' (Blender domain %d, type %d) cannot be converted to USD",
+                std::string(attribute_id.name()).c_str(),
+                meta_data.domain,
+                meta_data.data_type);
+    return;
+  }
+
+  const GVArray attribute = *curves.attributes().lookup(
+      attribute_id, meta_data.domain, meta_data.data_type);
+  if (attribute.is_empty()) {
+    return;
+  }
+
+  const pxr::UsdTimeCode timecode = get_export_time_code();
+  const pxr::TfToken pv_name(pxr::TfMakeValidIdentifier(attribute_id.name()));
+  const pxr::UsdGeomPrimvarsAPI pv_api = pxr::UsdGeomPrimvarsAPI(usd_curves);
+
+  pxr::UsdGeomPrimvar pv_attr = pv_api.CreatePrimvar(pv_name, *pv_type, *pv_interp);
+
+  copy_blender_attribute_to_primvar(
+      attribute, meta_data.data_type, timecode, pv_attr, usd_value_writer_);
+}
+
+void USDCurvesWriter::write_uv_data(const bke::CurvesGeometry &curves,
+                                    const bke::AttributeIDRef &attribute_id,
+                                    const pxr::UsdGeomCurves &usd_curves)
+{
+  const VArray<float2> buffer = *curves.attributes().lookup<float2>(attribute_id,
+                                                                    bke::AttrDomain::Curve);
+  if (buffer.is_empty()) {
+    return;
+  }
+
+  const pxr::UsdTimeCode timecode = get_export_time_code();
+  const pxr::TfToken pv_name(pxr::TfMakeValidIdentifier(attribute_id.name()));
+  const pxr::UsdGeomPrimvarsAPI pv_api = pxr::UsdGeomPrimvarsAPI(usd_curves);
+
+  pxr::UsdGeomPrimvar pv_uv = pv_api.CreatePrimvar(
+      pv_name, pxr::SdfValueTypeNames->TexCoord2fArray, pxr::UsdGeomTokens->uniform);
+
+  copy_blender_buffer_to_primvar<float2, pxr::GfVec2f>(buffer, timecode, pv_uv, usd_value_writer_);
+}
+
+void USDCurvesWriter::write_custom_data(const bke::CurvesGeometry &curves,
+                                        pxr::UsdGeomCurves &usd_curves)
+{
+  const bke::AttributeAccessor attributes = curves.attributes();
+
+  attributes.for_all(
+      [&](const bke::AttributeIDRef &attribute_id, const bke::AttributeMetaData &meta_data) {
+        /* Skip "internal" Blender properties and attributes dealt with elsewhere. */
+        const StringRef attr_name = attribute_id.name();
+        if (attr_name[0] == '.' || attribute_id.is_anonymous() ||
+            ELEM(attr_name,
+                 "position",
+                 "radius",
+                 "resolution",
+                 "id",
+                 "curve_type",
+                 "handle_left",
+                 "handle_right",
+                 "handle_type_left",
+                 "handle_type_right"))
+        {
+          return true;
+        }
+
+        /* Spline UV data */
+        if (meta_data.domain == bke::AttrDomain::Curve && meta_data.data_type == CD_PROP_FLOAT2) {
+          if (usd_export_context_.export_params.export_uvmaps) {
+            this->write_uv_data(curves, attribute_id, usd_curves);
+          }
+        }
+
+        /* Everything else. */
+        else {
+          this->write_generic_data(curves, attribute_id, meta_data, usd_curves);
+        }
+
+        return true;
+      });
+}
+
 void USDCurvesWriter::do_write(HierarchyContext &context)
 {
   Curves *curves_id;
@@ -481,6 +601,8 @@ void USDCurvesWriter::do_write(HierarchyContext &context)
 
   assign_materials(context, *usd_curves);
 
+  write_custom_data(curves, *usd_curves);
+
   auto prim = usd_curves->GetPrim();
   write_id_properties(prim, curves_id->id, timecode);
 }

source/blender/io/usd/intern/usd_writer_curves.hh

@@ -9,6 +9,12 @@
 #include <pxr/usd/usdGeom/curves.h>
 #include <pxr/usd/usdGeom/nurbsCurves.h>
 
+namespace blender::bke {
+class AttributeIDRef;
+struct AttributeMetaData;
+class CurvesGeometry;
+}  // namespace blender::bke
+
 namespace blender::io::usd {
 
 /* Writer for writing Curves data as USD curves. */
@@ -38,6 +44,18 @@ class USDCurvesWriter final : public USDAbstractWriter {
                                        const pxr::VtArray<double> &knots,
                                        const pxr::VtArray<int> &orders,
                                        const pxr::UsdTimeCode timecode);
+
+  void write_generic_data(const bke::CurvesGeometry &curves,
+                          const bke::AttributeIDRef &attribute_id,
+                          const bke::AttributeMetaData &meta_data,
+                          const pxr::UsdGeomCurves &usd_curves);
+
+  void write_uv_data(const bke::CurvesGeometry &curves,
+                     const bke::AttributeIDRef &attribute_id,
+                     const pxr::UsdGeomCurves &usd_curves);
+
+  void write_custom_data(const blender::bke::CurvesGeometry &curves,
+                         pxr::UsdGeomCurves &usd_curves);
 };
 
 }  // namespace blender::io::usd