Geometry Nodes: Add Mesh to Curve Node

This node creates poly curve splines from mesh edges. A selection
attribute input allows only using some of the edges from the mesh.
The node builds cyclic splines from branchless groups of edges where
possible, but when there is a three-way intersection, the spline stops.

The node also transfers all attributes from the mesh to the resulting
control points. In the future we could add a way to limit that to a
subset of the attributes to improve performance.

The algorithm is from Animation Nodes, written by @OmarSquircleArt.
I added the ability to use a selection, attribute transferring, and
used different variable names, etc, but other than that the algorithm
is the same.

Differential Revision: https://developer.blender.org/D11265

source/blender/nodes/CMakeLists.txt

@@ -178,6 +178,7 @@ set(SRC
   geometry/nodes/node_geo_mesh_primitive_ico_sphere.cc
   geometry/nodes/node_geo_mesh_primitive_line.cc
   geometry/nodes/node_geo_mesh_primitive_uv_sphere.cc
+  geometry/nodes/node_geo_mesh_to_curve.cc
   geometry/nodes/node_geo_object_info.cc
   geometry/nodes/node_geo_point_distribute.cc
   geometry/nodes/node_geo_point_instance.cc

source/blender/nodes/NOD_geometry.h

@@ -66,6 +66,7 @@ void register_node_type_geo_mesh_primitive_grid(void);
 void register_node_type_geo_mesh_primitive_ico_sphere(void);
 void register_node_type_geo_mesh_primitive_line(void);
 void register_node_type_geo_mesh_primitive_uv_sphere(void);
+void register_node_type_geo_mesh_to_curve(void);
 void register_node_type_geo_object_info(void);
 void register_node_type_geo_point_distribute(void);
 void register_node_type_geo_point_instance(void);

source/blender/nodes/NOD_static_types.h

@@ -305,6 +305,7 @@ DefNode(GeometryNode, GEO_NODE_MESH_PRIMITIVE_GRID, 0, "MESH_PRIMITIVE_GRID", Me
 DefNode(GeometryNode, GEO_NODE_MESH_PRIMITIVE_ICO_SPHERE, 0, "MESH_PRIMITIVE_ICO_SPHERE", MeshIcoSphere, "Ico Sphere", "")
 DefNode(GeometryNode, GEO_NODE_MESH_PRIMITIVE_LINE, def_geo_mesh_line, "MESH_PRIMITIVE_LINE", MeshLine, "Line", "")
 DefNode(GeometryNode, GEO_NODE_MESH_PRIMITIVE_UV_SPHERE, 0, "MESH_PRIMITIVE_UV_SPHERE", MeshUVSphere, "UV Sphere", "")
+DefNode(GeometryNode, GEO_NODE_MESH_TO_CURVE, 0, "MESH_TO_CURVE", MeshToCurve, "Mesh to Curve", "")
 DefNode(GeometryNode, GEO_NODE_OBJECT_INFO, def_geo_object_info, "OBJECT_INFO", ObjectInfo, "Object Info", "")
 DefNode(GeometryNode, GEO_NODE_POINT_DISTRIBUTE, def_geo_point_distribute, "POINT_DISTRIBUTE", PointDistribute, "Point Distribute", "")
 DefNode(GeometryNode, GEO_NODE_POINT_INSTANCE, def_geo_point_instance, "POINT_INSTANCE", PointInstance, "Point Instance", "")

source/blender/nodes/geometry/nodes/node_geo_mesh_to_curve.cc

@@ -0,0 +1,318 @@
+/*
+ * 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.
+ */
+
+#include "BLI_array.hh"
+#include "BLI_task.hh"
+
+#include "DNA_mesh_types.h"
+#include "DNA_meshdata_types.h"
+
+#include "BKE_attribute_math.hh"
+#include "BKE_spline.hh"
+
+#include "node_geometry_util.hh"
+
+using blender::Array;
+
+static bNodeSocketTemplate geo_node_mesh_to_curve_in[] = {
+    {SOCK_GEOMETRY, N_("Mesh")},
+    {SOCK_STRING, N_("Selection")},
+    {-1, ""},
+};
+
+static bNodeSocketTemplate geo_node_mesh_to_curve_out[] = {
+    {SOCK_GEOMETRY, N_("Curve")},
+    {-1, ""},
+};
+
+namespace blender::nodes {
+
+template<typename T>
+static void copy_attribute_to_points(const VArray<T> &source_data,
+                                     Span<int> map,
+                                     MutableSpan<T> dest_data)
+{
+  for (const int point_index : map.index_range()) {
+    const int vert_index = map[point_index];
+    dest_data[point_index] = source_data[vert_index];
+  }
+}
+
+static void copy_attributes_to_points(CurveEval &curve,
+                                      const MeshComponent &mesh_component,
+                                      Span<Vector<int>> point_to_vert_maps)
+{
+  MutableSpan<SplinePtr> splines = curve.splines();
+  Set<std::string> source_attribute_names = mesh_component.attribute_names();
+
+  /* Copy builtin control point attributes. */
+  if (source_attribute_names.contains_as("tilt")) {
+    const GVArray_Typed<float> tilt_attribute = mesh_component.attribute_get_for_read<float>(
+        "tilt", ATTR_DOMAIN_POINT, 0.0f);
+    parallel_for(splines.index_range(), 256, [&](IndexRange range) {
+      for (const int i : range) {
+        copy_attribute_to_points<float>(
+            *tilt_attribute, point_to_vert_maps[i], splines[i]->tilts());
+      }
+    });
+    source_attribute_names.remove_contained_as("tilt");
+  }
+  if (source_attribute_names.contains_as("radius")) {
+    const GVArray_Typed<float> radius_attribute = mesh_component.attribute_get_for_read<float>(
+        "radius", ATTR_DOMAIN_POINT, 1.0f);
+    parallel_for(splines.index_range(), 256, [&](IndexRange range) {
+      for (const int i : range) {
+        copy_attribute_to_points<float>(
+            *radius_attribute, point_to_vert_maps[i], splines[i]->radii());
+      }
+    });
+    source_attribute_names.remove_contained_as("radius");
+  }
+
+  /* Don't copy other builtin control point attributes. */
+  source_attribute_names.remove_as("position");
+
+  /* Copy dynamic control point attributes. */
+  for (const StringRef name : source_attribute_names) {
+    const GVArrayPtr mesh_attribute = mesh_component.attribute_try_get_for_read(name,
+                                                                                ATTR_DOMAIN_POINT);
+    /* Some attributes might not exist if they were builtin attribute on domains that don't
+     * have any elements, i.e. a face attribute on the output of the line primitive node. */
+    if (!mesh_attribute) {
+      continue;
+    }
+
+    const CustomDataType data_type = bke::cpp_type_to_custom_data_type(mesh_attribute->type());
+
+    parallel_for(splines.index_range(), 128, [&](IndexRange range) {
+      for (const int i : range) {
+        /* Create attribute on the spline points. */
+        splines[i]->attributes.create(name, data_type);
+        std::optional<GMutableSpan> spline_attribute = splines[i]->attributes.get_for_write(name);
+        BLI_assert(spline_attribute);
+
+        /* Copy attribute based on the map for this spline. */
+        attribute_math::convert_to_static_type(mesh_attribute->type(), [&](auto dummy) {
+          using T = decltype(dummy);
+          copy_attribute_to_points<T>(
+              mesh_attribute->typed<T>(), point_to_vert_maps[i], spline_attribute->typed<T>());
+        });
+      }
+    });
+  }
+
+  curve.assert_valid_point_attributes();
+}
+
+struct CurveFromEdgesOutput {
+  std::unique_ptr<CurveEval> curve;
+  Vector<Vector<int>> point_to_vert_maps;
+};
+
+static CurveFromEdgesOutput mesh_to_curve(Span<MVert> verts, Span<std::pair<int, int>> edges)
+{
+  std::unique_ptr<CurveEval> curve = std::make_unique<CurveEval>();
+  Vector<Vector<int>> point_to_vert_maps;
+
+  /* Compute the number of edges connecting to each vertex. */
+  Array<int> neighbor_count(verts.size(), 0);
+  for (const std::pair<int, int> &edge : edges) {
+    neighbor_count[edge.first]++;
+    neighbor_count[edge.second]++;
+  }
+
+  /* Compute an offset into the array of neighbor edges based on the counts. */
+  Array<int> neighbor_offsets(verts.size());
+  int start = 0;
+  for (const int i : verts.index_range()) {
+    neighbor_offsets[i] = start;
+    start += neighbor_count[i];
+  }
+
+  /* Use as an index into the "neighbor group" for each vertex. */
+  Array<int> used_slots(verts.size(), 0);
+  /* Calculate the indices of each vertex's neighboring edges. */
+  Array<int> neighbors(edges.size() * 2);
+  for (const int i : edges.index_range()) {
+    const int v1 = edges[i].first;
+    const int v2 = edges[i].second;
+    neighbors[neighbor_offsets[v1] + used_slots[v1]] = v2;
+    neighbors[neighbor_offsets[v2] + used_slots[v2]] = v1;
+    used_slots[v1]++;
+    used_slots[v2]++;
+  }
+
+  /* Now use the neighbor group offsets calculated above as a count used edges at each vertex. */
+  Array<int> unused_edges = std::move(used_slots);
+
+  for (const int start_vert : verts.index_range()) {
+    /* The vertex will be part of a cyclic spline. */
+    if (neighbor_count[start_vert] == 2) {
+      continue;
+    }
+
+    /* The vertex has no connected edges, or they were already used. */
+    if (unused_edges[start_vert] == 0) {
+      continue;
+    }
+
+    for (const int i : IndexRange(neighbor_count[start_vert])) {
+      int current_vert = start_vert;
+      int next_vert = neighbors[neighbor_offsets[current_vert] + i];
+
+      if (unused_edges[next_vert] == 0) {
+        continue;
+      }
+
+      std::unique_ptr<PolySpline> spline = std::make_unique<PolySpline>();
+      Vector<int> point_to_vert_map;
+
+      spline->add_point(verts[current_vert].co, 1.0f, 0.0f);
+      point_to_vert_map.append(current_vert);
+
+      /* Follow connected edges until we read a vertex with more than two connected edges. */
+      while (true) {
+        int last_vert = current_vert;
+        current_vert = next_vert;
+
+        spline->add_point(verts[current_vert].co, 1.0f, 0.0f);
+        point_to_vert_map.append(current_vert);
+        unused_edges[current_vert]--;
+        unused_edges[last_vert]--;
+
+        if (neighbor_count[current_vert] != 2) {
+          break;
+        }
+
+        const int offset = neighbor_offsets[current_vert];
+        const int next_a = neighbors[offset];
+        const int next_b = neighbors[offset + 1];
+        next_vert = (last_vert == next_a) ? next_b : next_a;
+      }
+
+      spline->attributes.reallocate(spline->size());
+      curve->add_spline(std::move(spline));
+      point_to_vert_maps.append(std::move(point_to_vert_map));
+    }
+  }
+
+  /* All remaining edges are part of cyclic splines (we skipped vertices with two edges before). */
+  for (const int start_vert : verts.index_range()) {
+    if (unused_edges[start_vert] != 2) {
+      continue;
+    }
+
+    int current_vert = start_vert;
+    int next_vert = neighbors[neighbor_offsets[current_vert]];
+
+    std::unique_ptr<PolySpline> spline = std::make_unique<PolySpline>();
+    Vector<int> point_to_vert_map;
+    spline->set_cyclic(true);
+
+    spline->add_point(verts[current_vert].co, 1.0f, 0.0f);
+    point_to_vert_map.append(current_vert);
+
+    /* Follow connected edges until we loop back to the start vertex. */
+    while (next_vert != start_vert) {
+      const int last_vert = current_vert;
+      current_vert = next_vert;
+
+      spline->add_point(verts[current_vert].co, 1.0f, 0.0f);
+      point_to_vert_map.append(current_vert);
+      unused_edges[current_vert]--;
+      unused_edges[last_vert]--;
+
+      const int offset = neighbor_offsets[current_vert];
+      const int next_a = neighbors[offset];
+      const int next_b = neighbors[offset + 1];
+      next_vert = (last_vert == next_a) ? next_b : next_a;
+    }
+
+    spline->attributes.reallocate(spline->size());
+    curve->add_spline(std::move(spline));
+    point_to_vert_maps.append(std::move(point_to_vert_map));
+  }
+
+  curve->attributes.reallocate(curve->splines().size());
+  return {std::move(curve), std::move(point_to_vert_maps)};
+}
+
+/**
+ * Get a separate array of the indices for edges in a selection (a boolean attribute).
+ * This helps to make the above algorithm simpler by removing the need to check for selection
+ * in many places.
+ */
+static Vector<std::pair<int, int>> get_selected_edges(GeoNodeExecParams params,
+                                                      const MeshComponent &component)
+{
+  const Mesh &mesh = *component.get_for_read();
+  const std::string selection_name = params.extract_input<std::string>("Selection");
+  if (!selection_name.empty() && !component.attribute_exists(selection_name)) {
+    params.error_message_add(NodeWarningType::Error,
+                             TIP_("No attribute with name \"") + selection_name + "\"");
+  }
+  GVArray_Typed<bool> selection = component.attribute_get_for_read<bool>(
+      selection_name, ATTR_DOMAIN_EDGE, true);
+
+  Vector<std::pair<int, int>> selected_edges;
+  for (const int i : IndexRange(mesh.totedge)) {
+    if (selection[i]) {
+      selected_edges.append({mesh.medge[i].v1, mesh.medge[i].v2});
+    }
+  }
+
+  return selected_edges;
+}
+
+static void geo_node_mesh_to_curve_exec(GeoNodeExecParams params)
+{
+  GeometrySet geometry_set = params.extract_input<GeometrySet>("Mesh");
+
+  geometry_set = bke::geometry_set_realize_instances(geometry_set);
+
+  if (!geometry_set.has_mesh()) {
+    params.set_output("Curve", GeometrySet());
+    return;
+  }
+
+  const MeshComponent &component = *geometry_set.get_component_for_read<MeshComponent>();
+  const Mesh &mesh = *component.get_for_read();
+  Span<MVert> verts = Span{mesh.mvert, mesh.totvert};
+  Span<MEdge> edges = Span{mesh.medge, mesh.totedge};
+  if (edges.size() == 0) {
+    params.set_output("Curve", GeometrySet());
+    return;
+  }
+
+  Vector<std::pair<int, int>> selected_edges = get_selected_edges(params, component);
+
+  CurveFromEdgesOutput output = mesh_to_curve(verts, selected_edges);
+  copy_attributes_to_points(*output.curve, component, output.point_to_vert_maps);
+
+  params.set_output("Curve", GeometrySet::create_with_curve(output.curve.release()));
+}
+
+}  // namespace blender::nodes
+
+void register_node_type_geo_mesh_to_curve()
+{
+  static bNodeType ntype;
+
+  geo_node_type_base(&ntype, GEO_NODE_MESH_TO_CURVE, "Mesh to Curve", NODE_CLASS_GEOMETRY, 0);
+  node_type_socket_templates(&ntype, geo_node_mesh_to_curve_in, geo_node_mesh_to_curve_out);
+  ntype.geometry_node_execute = blender::nodes::geo_node_mesh_to_curve_exec;
+  nodeRegisterType(&ntype);
+}