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diff --git a/source/blender/io/alembic/exporter/abc_custom_props.h b/source/blender/io/alembic/exporter/abc_custom_props.h
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+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2020 Blender Foundation.
+ * All rights reserved.
+ */
+
+/** \file
+ * \ingroup Alembic
+ */
+
+#pragma once
+
+#include <Alembic/Abc/OArrayProperty.h>
+#include <Alembic/Abc/OCompoundProperty.h>
+
+#include "BLI_map.hh"
+
+#include <memory>
+
+struct IDProperty;
+
+namespace blender::io::alembic {
+
+class ABCAbstractWriter;
+
+/* Write values of Custom Properties (a.k.a. ID Properties) to Alembic.
+ *
+ * Each Alembic Writer instance optionally has one CustomPropertiesExporter (CPE). This CPE not
+ * only writes the custom properties to Alembic, but also keeps references in memory so that the
+ * Alembic library doesn't prematurely finalize the data. */
+class CustomPropertiesExporter {
+ private:
+  /* Owner is used to get the OCompoundProperty and time sample index. The former should only be
+   * requested from the Alembic library when it's actually going to be used to add custom
+   * properties (otherwise an invalid Alembic file is written). */
+  ABCAbstractWriter *owner_;
+
+  /* The Compound Property that will contain the exported custom properties.
+   *
+   * Typically this the return value of abc_schema.getArbGeomParams() or
+   * abc_schema.getUserProperties(). */
+  Alembic::Abc::OCompoundProperty abc_compound_prop_;
+
+  /* Mapping from property name in Blender to property in Alembic.
+   * Here Blender does the same as other software (Maya, Houdini), and writes
+   * scalar properties as single-element arrays. */
+  Map<std::string, Alembic::Abc::OArrayProperty> abc_properties_;
+
+ public:
+  CustomPropertiesExporter(ABCAbstractWriter *owner);
+  virtual ~CustomPropertiesExporter();
+
+  void write_all(const IDProperty *group);
+
+ private:
+  void write(const IDProperty *id_property);
+  void write_array(const IDProperty *id_property);
+
+  /* IDProperty arrays are used to store arrays-of-arrays or arrays-of-strings. */
+  void write_idparray(const IDProperty *idp_array);
+  void write_idparray_of_strings(const IDProperty *idp_array);
+  void write_idparray_of_numbers(const IDProperty *idp_array);
+
+  /* Flatten an array-of-arrays into one long array, then write that.
+   * It is tempting to write an array of NxM numbers as a matrix, but there is
+   * no guarantee that the data actually represents a matrix. */
+  template<typename ABCPropertyType, typename BlenderValueType>
+  void write_idparray_flattened_typed(const IDProperty *idp_array);
+
+  /* Write a single scalar (i.e. non-array) property as single-value array. */
+  template<typename ABCPropertyType, typename BlenderValueType>
+  void set_scalar_property(StringRef property_name, const BlenderValueType property_value);
+
+  template<typename ABCPropertyType, typename BlenderValueType>
+  void set_array_property(StringRef property_name,
+                          const BlenderValueType *array_values,
+                          size_t num_array_items);
+
+  template<typename ABCPropertyType>
+  Alembic::Abc::OArrayProperty create_abc_property(StringRef property_name);
+};
+
+}  // namespace blender::io::alembic