Volumes: support selection and outlines in viewport

Previously, one could only select a volume object in the outliner
or by clicking on the object origin. This patch allows you to click
on the actual volume.

Furthermore, the generated (invisible) mesh that is used for
selection is also used to draw an outline for the volume object now.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D9022

11 files changed, 455 insertions, 158 deletions

diff --git a/source/blender/blenkernel/BKE_volume_render.h b/source/blender/blenkernel/BKE_volume_render.h
index a42f24a5312..593f296135c 100644
--- a/source/blender/blenkernel/BKE_volume_render.h
+++ b/source/blender/blenkernel/BKE_volume_render.h
@@ -58,6 +58,16 @@ void BKE_volume_grid_wireframe(const struct Volume *volume,
                                BKE_volume_wireframe_cb cb,
                                void *cb_userdata);
 
+/* Selection Surface */
+
+typedef void (*BKE_volume_selection_surface_cb)(
+    void *userdata, float (*verts)[3], int (*tris)[3], int totvert, int tottris);
+
+void BKE_volume_grid_selection_surface(const struct Volume *volume,
+                                       struct VolumeGrid *volume_grid,
+                                       BKE_volume_selection_surface_cb cb,
+                                       void *cb_userdata);
+
 /* Render */
 
 float BKE_volume_density_scale(const struct Volume *volume, const float matrix[4][4]);
diff --git a/source/blender/blenkernel/intern/volume_render.cc b/source/blender/blenkernel/intern/volume_render.cc
index 98d3617c822..b773452b6a8 100644
--- a/source/blender/blenkernel/intern/volume_render.cc
+++ b/source/blender/blenkernel/intern/volume_render.cc
@@ -20,8 +20,11 @@
 
 #include "MEM_guardedalloc.h"
 
+#include "BLI_array.hh"
+#include "BLI_float3.hh"
 #include "BLI_math_matrix.h"
 #include "BLI_math_vector.h"
+#include "BLI_vector.hh"
 
 #include "DNA_volume_types.h"
 
@@ -166,107 +169,223 @@ void BKE_volume_grid_dense_voxels(const Volume *volume,
 /* Wireframe */
 
 #ifdef WITH_OPENVDB
-struct VolumeWireframe {
-  std::vector<openvdb::Vec3f> verts;
-  std::vector<openvdb::Vec2I> edges;
-
-  template<typename GridType>
-  void add_grid(openvdb::GridBase::ConstPtr gridbase, const bool points, const bool coarse)
-  {
-    using TreeType = typename GridType::TreeType;
-    using Depth2Type = typename TreeType::RootNodeType::ChildNodeType::ChildNodeType;
-    using NodeCIter = typename TreeType::NodeCIter;
-    using GridConstPtr = typename GridType::ConstPtr;
-
-    GridConstPtr grid = openvdb::gridConstPtrCast<GridType>(gridbase);
-    const openvdb::math::Transform &transform = grid->transform();
-    const int depth = (coarse) ? 2 : 3;
-
-    NodeCIter iter = grid->tree().cbeginNode();
-    iter.setMaxDepth(depth);
-
-    for (; iter; ++iter) {
-      if (iter.getDepth() == depth) {
-        openvdb::CoordBBox coordbbox;
-
-        if (depth == 2) {
-          /* Internal node at depth 2. */
-          const Depth2Type *node = nullptr;
-          iter.getNode(node);
-          if (node) {
-            node->evalActiveBoundingBox(coordbbox, false);
-          }
-          else {
-            continue;
-          }
-        }
-        else {
-          /* Leaf node. */
-          if (!iter.getBoundingBox(coordbbox)) {
-            continue;
-          }
-        }
-
-        /* +1 to convert from exclusive to include bounds. */
-        coordbbox.max() = coordbbox.max().offsetBy(1);
-        openvdb::BBoxd bbox = transform.indexToWorld(coordbbox);
-
-        if (points) {
-          add_point(bbox);
-        }
-        else {
-          add_box(bbox);
-        }
+
+/** Returns bounding boxes that approximate the shape of the volume stored in the grid. */
+template<typename GridType>
+static blender::Vector<openvdb::CoordBBox> get_bounding_boxes(openvdb::GridBase::ConstPtr gridbase,
+                                                              const bool coarse)
+{
+  using TreeType = typename GridType::TreeType;
+  using Depth2Type = typename TreeType::RootNodeType::ChildNodeType::ChildNodeType;
+  using NodeCIter = typename TreeType::NodeCIter;
+  using GridConstPtr = typename GridType::ConstPtr;
+
+  GridConstPtr grid = openvdb::gridConstPtrCast<GridType>(gridbase);
+  blender::Vector<openvdb::CoordBBox> boxes;
+  const int depth = coarse ? 2 : 3;
+
+  NodeCIter iter = grid->tree().cbeginNode();
+  iter.setMaxDepth(depth);
+
+  for (; iter; ++iter) {
+    if (iter.getDepth() != depth) {
+      continue;
+    }
+
+    openvdb::CoordBBox box;
+    if (depth == 2) {
+      /* Internal node at depth 2. */
+      const Depth2Type *node = nullptr;
+      iter.getNode(node);
+      if (node) {
+        node->evalActiveBoundingBox(box, false);
       }
+      else {
+        continue;
+      }
+    }
+    else {
+      /* Leaf node. */
+      if (!iter.getBoundingBox(box)) {
+        continue;
+      }
+    }
+
+    /* +1 to convert from exclusive to inclusive bounds. */
+    box.max() = box.max().offsetBy(1);
+
+    boxes.append(box);
+  }
+
+  return boxes;
+}
+
+static blender::Vector<openvdb::CoordBBox> get_bounding_boxes(VolumeGridType grid_type,
+                                                              openvdb::GridBase::ConstPtr grid,
+                                                              const bool coarse)
+{
+  switch (grid_type) {
+    case VOLUME_GRID_BOOLEAN: {
+      return get_bounding_boxes<openvdb::BoolGrid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_FLOAT: {
+      return get_bounding_boxes<openvdb::FloatGrid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_DOUBLE: {
+      return get_bounding_boxes<openvdb::DoubleGrid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_INT: {
+      return get_bounding_boxes<openvdb::Int32Grid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_INT64: {
+      return get_bounding_boxes<openvdb::Int64Grid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_MASK: {
+      return get_bounding_boxes<openvdb::MaskGrid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_VECTOR_FLOAT: {
+      return get_bounding_boxes<openvdb::Vec3fGrid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_VECTOR_DOUBLE: {
+      return get_bounding_boxes<openvdb::Vec3dGrid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_VECTOR_INT: {
+      return get_bounding_boxes<openvdb::Vec3IGrid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_STRING: {
+      return get_bounding_boxes<openvdb::StringGrid>(grid, coarse);
+      break;
+    }
+    case VOLUME_GRID_POINTS:
+    case VOLUME_GRID_UNKNOWN: {
+      break;
+    }
+  }
+  return {};
+}
+
+static void boxes_to_center_points(blender::Span<openvdb::CoordBBox> boxes,
+                                   const openvdb::math::Transform &transform,
+                                   blender::MutableSpan<blender::float3> r_verts)
+{
+  BLI_assert(boxes.size() == r_verts.size());
+  for (const int i : boxes.index_range()) {
+    openvdb::Vec3d center = transform.indexToWorld(boxes[i].getCenter());
+    r_verts[i] = blender::float3(center[0], center[1], center[2]);
+  }
+}
+
+static void boxes_to_corner_points(blender::Span<openvdb::CoordBBox> boxes,
+                                   const openvdb::math::Transform &transform,
+                                   blender::MutableSpan<blender::float3> r_verts)
+{
+  BLI_assert(boxes.size() * 8 == r_verts.size());
+  for (const int i : boxes.index_range()) {
+    const openvdb::CoordBBox &box = boxes[i];
+
+    /* The ordering of the corner points is lexicographic. */
+    std::array<openvdb::Coord, 8> corners;
+    box.getCornerPoints(corners.data());
+
+    for (int j = 0; j < 8; j++) {
+      openvdb::Coord corner_i = corners[j];
+      openvdb::Vec3d corner_d = transform.indexToWorld(corner_i);
+      r_verts[8 * i + j] = blender::float3(corner_d[0], corner_d[1], corner_d[2]);
     }
   }
+}
 
-  void add_point(const openvdb::BBoxd &bbox)
-  {
-    verts.push_back(bbox.getCenter());
+static void boxes_to_edge_mesh(blender::Span<openvdb::CoordBBox> boxes,
+                               const openvdb::math::Transform &transform,
+                               blender::Vector<blender::float3> &r_verts,
+                               blender::Vector<std::array<int, 2>> &r_edges)
+{
+  /* TODO: Deduplicate edges, hide flat edges? */
+
+  const int box_edges[12][2] = {
+      {0, 1},
+      {0, 2},
+      {0, 4},
+      {1, 3},
+      {1, 5},
+      {2, 3},
+      {2, 6},
+      {3, 7},
+      {4, 5},
+      {4, 6},
+      {5, 7},
+      {6, 7},
+  };
+
+  int vert_offset = r_verts.size();
+  int edge_offset = r_edges.size();
+
+  const int vert_amount = 8 * boxes.size();
+  const int edge_amount = 12 * boxes.size();
+
+  r_verts.resize(r_verts.size() + vert_amount);
+  r_edges.resize(r_edges.size() + edge_amount);
+  boxes_to_corner_points(boxes, transform, r_verts.as_mutable_span().take_back(vert_amount));
+
+  for (int i = 0; i < boxes.size(); i++) {
+    for (int j = 0; j < 12; j++) {
+      r_edges[edge_offset + j] = {vert_offset + box_edges[j][0], vert_offset + box_edges[j][1]};
+    }
+    vert_offset += 8;
+    edge_offset += 12;
   }
+}
 
-  void add_box(const openvdb::BBoxd &bbox)
-  {
-    /* TODO: deduplicate edges, hide flat edges? */
-    openvdb::Vec3f min = bbox.min();
-    openvdb::Vec3f max = bbox.max();
-
-    const int vert_offset = verts.size();
-    const int edge_offset = edges.size();
-
-    /* Create vertices. */
-    verts.resize(vert_offset + 8);
-    verts[vert_offset + 0] = openvdb::Vec3f(min[0], min[1], min[2]);
-    verts[vert_offset + 1] = openvdb::Vec3f(max[0], min[1], min[2]);
-    verts[vert_offset + 2] = openvdb::Vec3f(max[0], max[1], min[2]);
-    verts[vert_offset + 3] = openvdb::Vec3f(min[0], max[1], min[2]);
-    verts[vert_offset + 4] = openvdb::Vec3f(min[0], min[1], max[2]);
-    verts[vert_offset + 5] = openvdb::Vec3f(max[0], min[1], max[2]);
-    verts[vert_offset + 6] = openvdb::Vec3f(max[0], max[1], max[2]);
-    verts[vert_offset + 7] = openvdb::Vec3f(min[0], max[1], max[2]);
-
-    /* Create edges. */
-    const int box_edges[12][2] = {{0, 1},
-                                  {1, 2},
-                                  {2, 3},
-                                  {3, 0},
-                                  {4, 5},
-                                  {5, 6},
-                                  {6, 7},
-                                  {7, 4},
-                                  {0, 4},
-                                  {1, 5},
-                                  {2, 6},
-                                  {3, 7}};
-
-    edges.resize(edge_offset + 12);
-    for (int i = 0; i < 12; i++) {
-      edges[edge_offset + i] = openvdb::Vec2I(vert_offset + box_edges[i][0],
-                                              vert_offset + box_edges[i][1]);
+static void boxes_to_cube_mesh(blender::Span<openvdb::CoordBBox> boxes,
+                               const openvdb::math::Transform &transform,
+                               blender::Vector<blender::float3> &r_verts,
+                               blender::Vector<std::array<int, 3>> &r_tris)
+{
+  const int box_tris[12][3] = {
+      {0, 1, 4},
+      {4, 1, 5},
+      {0, 2, 1},
+      {1, 2, 3},
+      {1, 3, 5},
+      {5, 3, 7},
+      {6, 4, 5},
+      {7, 5, 6},
+      {2, 0, 4},
+      {2, 4, 6},
+      {3, 7, 2},
+      {6, 2, 7},
+  };
+
+  int vert_offset = r_verts.size();
+  int tri_offset = r_tris.size();
+
+  const int vert_amount = 8 * boxes.size();
+  const int tri_amount = 12 * boxes.size();
+
+  r_verts.resize(r_verts.size() + vert_amount);
+  r_tris.resize(r_tris.size() + tri_amount);
+  boxes_to_corner_points(boxes, transform, r_verts.as_mutable_span().take_back(vert_amount));
+
+  for (int i = 0; i < boxes.size(); i++) {
+    for (int j = 0; j < 12; j++) {
+      r_tris[tri_offset + j] = {vert_offset + box_tris[j][0],
+                                vert_offset + box_tris[j][1],
+                                vert_offset + box_tris[j][2]};
     }
+    vert_offset += 8;
+    tri_offset += 12;
   }
-};
+}
+
 #endif
 
 void BKE_volume_grid_wireframe(const Volume *volume,
@@ -274,79 +393,96 @@ void BKE_volume_grid_wireframe(const Volume *volume,
                                BKE_volume_wireframe_cb cb,
                                void *cb_userdata)
 {
-#ifdef WITH_OPENVDB
-  VolumeWireframe wireframe;
-
   if (volume->display.wireframe_type == VOLUME_WIREFRAME_NONE) {
-    /* Nothing. */
+    cb(cb_userdata, NULL, NULL, 0, 0);
+    return;
   }
-  else if (volume->display.wireframe_type == VOLUME_WIREFRAME_BOUNDS) {
-    /* Bounding box. */
-    float min[3], max[3];
-    BKE_volume_grid_bounds(volume_grid, min, max);
 
-    openvdb::BBoxd bbox(min, max);
-    wireframe.add_box(bbox);
+#ifdef WITH_OPENVDB
+  openvdb::GridBase::ConstPtr grid = BKE_volume_grid_openvdb_for_read(volume, volume_grid);
+
+  if (volume->display.wireframe_type == VOLUME_WIREFRAME_BOUNDS) {
+    /* Bounding box. */
+    openvdb::CoordBBox box;
+    blender::Vector<blender::float3> verts;
+    blender::Vector<std::array<int, 2>> edges;
+    if (grid->baseTree().evalLeafBoundingBox(box)) {
+      boxes_to_edge_mesh({box}, grid->transform(), verts, edges);
+    }
+    cb(cb_userdata,
+       (float(*)[3])verts.data(),
+       (int(*)[2])edges.data(),
+       verts.size(),
+       edges.size());
   }
   else {
-    /* Tree nodes. */
-    openvdb::GridBase::ConstPtr grid = BKE_volume_grid_openvdb_for_read(volume, volume_grid);
-    const bool points = (volume->display.wireframe_type == VOLUME_WIREFRAME_POINTS);
-    const bool coarse = (volume->display.wireframe_detail == VOLUME_WIREFRAME_COARSE);
-
-    switch (BKE_volume_grid_type(volume_grid)) {
-      case VOLUME_GRID_BOOLEAN: {
-        wireframe.add_grid<openvdb::BoolGrid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_FLOAT: {
-        wireframe.add_grid<openvdb::FloatGrid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_DOUBLE: {
-        wireframe.add_grid<openvdb::DoubleGrid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_INT: {
-        wireframe.add_grid<openvdb::Int32Grid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_INT64: {
-        wireframe.add_grid<openvdb::Int64Grid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_MASK: {
-        wireframe.add_grid<openvdb::MaskGrid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_VECTOR_FLOAT: {
-        wireframe.add_grid<openvdb::Vec3fGrid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_VECTOR_DOUBLE: {
-        wireframe.add_grid<openvdb::Vec3dGrid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_VECTOR_INT: {
-        wireframe.add_grid<openvdb::Vec3IGrid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_STRING: {
-        wireframe.add_grid<openvdb::StringGrid>(grid, points, coarse);
-        break;
-      }
-      case VOLUME_GRID_POINTS:
-      case VOLUME_GRID_UNKNOWN: {
-        break;
-      }
+    blender::Vector<openvdb::CoordBBox> boxes = get_bounding_boxes(
+        BKE_volume_grid_type(volume_grid),
+        grid,
+        volume->display.wireframe_detail == VOLUME_WIREFRAME_COARSE);
+
+    blender::Vector<blender::float3> verts;
+    blender::Vector<std::array<int, 2>> edges;
+
+    if (volume->display.wireframe_type == VOLUME_WIREFRAME_POINTS) {
+      verts.resize(boxes.size());
+      boxes_to_center_points(boxes, grid->transform(), verts);
     }
+    else {
+      boxes_to_edge_mesh(boxes, grid->transform(), verts, edges);
+    }
+
+    cb(cb_userdata,
+       (float(*)[3])verts.data(),
+       (int(*)[2])edges.data(),
+       verts.size(),
+       edges.size());
+  }
+
+#else
+  UNUSED_VARS(volume, volume_grid);
+  cb(cb_userdata, NULL, NULL, 0, 0);
+#endif
+}
+
+static void grow_triangles(blender::MutableSpan<blender::float3> verts,
+                           blender::Span<std::array<int, 3>> tris,
+                           const float factor)
+{
+  /* Compute the offset for every vertex based on the connected edges.
+   * This formula simply tries increases the length of all edges. */
+  blender::Array<blender::float3> offsets(verts.size(), {0, 0, 0});
+  for (const std::array<int, 3> &tri : tris) {
+    offsets[tri[0]] += factor * (2 * verts[tri[0]] - verts[tri[1]] - verts[tri[2]]);
+    offsets[tri[1]] += factor * (2 * verts[tri[1]] - verts[tri[0]] - verts[tri[2]]);
+    offsets[tri[2]] += factor * (2 * verts[tri[2]] - verts[tri[0]] - verts[tri[1]]);
+  }
+  /* Apply the computed offsets. */
+  for (const int i : verts.index_range()) {
+    verts[i] += offsets[i];
   }
+}
+
+void BKE_volume_grid_selection_surface(const Volume *volume,
+                                       VolumeGrid *volume_grid,
+                                       BKE_volume_selection_surface_cb cb,
+                                       void *cb_userdata)
+{
+#ifdef WITH_OPENVDB
+  openvdb::GridBase::ConstPtr grid = BKE_volume_grid_openvdb_for_read(volume, volume_grid);
+  blender::Vector<openvdb::CoordBBox> boxes = get_bounding_boxes(
+      BKE_volume_grid_type(volume_grid), grid, true);
+
+  blender::Vector<blender::float3> verts;
+  blender::Vector<std::array<int, 3>> tris;
+  boxes_to_cube_mesh(boxes, grid->transform(), verts, tris);
+
+  /* By slightly scaling the individual boxes up, we can avoid some artifacts when drawing the
+   * selection outline. */
+  const float offset_factor = 0.01f;
+  grow_triangles(verts, tris, offset_factor);
 
-  cb(cb_userdata,
-     (float(*)[3])wireframe.verts.data(),
-     (int(*)[2])wireframe.edges.data(),
-     wireframe.verts.size(),
-     wireframe.edges.size());
+  cb(cb_userdata, (float(*)[3])verts.data(), (int(*)[3])tris.data(), verts.size(), tris.size());
 #else
   UNUSED_VARS(volume, volume_grid);
   cb(cb_userdata, NULL, NULL, 0, 0);
diff --git a/source/blender/draw/CMakeLists.txt b/source/blender/draw/CMakeLists.txt
index 11aea544c65..4e72e89ae99 100644
--- a/source/blender/draw/CMakeLists.txt
+++ b/source/blender/draw/CMakeLists.txt
@@ -146,6 +146,7 @@ set(SRC
   engines/overlay/overlay_particle.c
   engines/overlay/overlay_sculpt.c
   engines/overlay/overlay_shader.c
+  engines/overlay/overlay_volume.c
   engines/overlay/overlay_wireframe.c
 
   DRW_engine.h
diff --git a/source/blender/draw/engines/overlay/overlay_engine.c b/source/blender/draw/engines/overlay/overlay_engine.c
index fb235a1ff57..5a87af7238e 100644
--- a/source/blender/draw/engines/overlay/overlay_engine.c
+++ b/source/blender/draw/engines/overlay/overlay_engine.c
@@ -207,6 +207,7 @@ static void OVERLAY_cache_init(void *vedata)
   OVERLAY_outline_cache_init(vedata);
   OVERLAY_particle_cache_init(vedata);
   OVERLAY_wireframe_cache_init(vedata);
+  OVERLAY_volume_cache_init(vedata);
 }
 
 BLI_INLINE OVERLAY_DupliData *OVERLAY_duplidata_get(Object *ob, void *vedata, bool *do_init)
@@ -355,6 +356,10 @@ static void OVERLAY_cache_populate(void *vedata, Object *ob)
     OVERLAY_pose_cache_populate(vedata, ob);
   }
 
+  if (ob->type == OB_VOLUME) {
+    OVERLAY_volume_cache_populate(vedata, ob);
+  }
+
   if (in_edit_mode && !pd->hide_overlays) {
     switch (ob->type) {
       case OB_MESH:
@@ -551,6 +556,7 @@ static void OVERLAY_draw_scene(void *vedata)
   OVERLAY_particle_draw(vedata);
   OVERLAY_metaball_draw(vedata);
   OVERLAY_gpencil_draw(vedata);
+  OVERLAY_volume_draw(vedata);
   OVERLAY_extra_draw(vedata);
 
   if (DRW_state_is_fbo()) {
diff --git a/source/blender/draw/engines/overlay/overlay_outline.c b/source/blender/draw/engines/overlay/overlay_outline.c
index e904066248f..f1467ff9794 100644
--- a/source/blender/draw/engines/overlay/overlay_outline.c
+++ b/source/blender/draw/engines/overlay/overlay_outline.c
@@ -272,6 +272,17 @@ static void OVERLAY_outline_gpencil(OVERLAY_PrivateData *pd, Object *ob)
                                   pd->cfra);
 }
 
+static void OVERLAY_outline_volume(OVERLAY_PrivateData *pd, Object *ob)
+{
+  struct GPUBatch *geom = DRW_cache_volume_selection_surface_get(ob);
+  if (geom == NULL) {
+    return;
+  }
+
+  DRWShadingGroup *shgroup = pd->outlines_grp;
+  DRW_shgroup_call(shgroup, geom, ob);
+}
+
 void OVERLAY_outline_cache_populate(OVERLAY_Data *vedata,
                                     Object *ob,
                                     OVERLAY_DupliData *dupli,
@@ -293,6 +304,11 @@ void OVERLAY_outline_cache_populate(OVERLAY_Data *vedata,
     return;
   }
 
+  if (ob->type == OB_VOLUME) {
+    OVERLAY_outline_volume(pd, ob);
+    return;
+  }
+
   if (ob->type == OB_POINTCLOUD && pd->wireframe_mode) {
     /* Looks bad in this case. Could be relaxed if we draw a
      * wireframe of some sort in the future. */
diff --git a/source/blender/draw/engines/overlay/overlay_private.h b/source/blender/draw/engines/overlay/overlay_private.h
index 2cf2ca34801..ada2c2c8d1f 100644
--- a/source/blender/draw/engines/overlay/overlay_private.h
+++ b/source/blender/draw/engines/overlay/overlay_private.h
@@ -119,6 +119,7 @@ typedef struct OVERLAY_PassList {
   DRWPass *particle_ps;
   DRWPass *pointcloud_ps;
   DRWPass *sculpt_mask_ps;
+  DRWPass *volume_ps;
   DRWPass *wireframe_ps;
   DRWPass *wireframe_xray_ps;
   DRWPass *xray_fade_ps;
@@ -285,6 +286,7 @@ typedef struct OVERLAY_PrivateData {
   DRWShadingGroup *particle_shapes_grp;
   DRWShadingGroup *pointcloud_dots_grp;
   DRWShadingGroup *sculpt_mask_grp;
+  DRWShadingGroup *volume_selection_surface_grp;
   DRWShadingGroup *wires_grp[2][2];      /* With and without coloring. */
   DRWShadingGroup *wires_all_grp[2][2];  /* With and without coloring. */
   DRWShadingGroup *wires_hair_grp[2][2]; /* With and without coloring. */
@@ -511,6 +513,10 @@ void OVERLAY_edit_text_cache_init(OVERLAY_Data *vedata);
 void OVERLAY_edit_text_cache_populate(OVERLAY_Data *vedata, Object *ob);
 void OVERLAY_edit_text_draw(OVERLAY_Data *vedata);
 
+void OVERLAY_volume_cache_init(OVERLAY_Data *vedata);
+void OVERLAY_volume_cache_populate(OVERLAY_Data *vedata, Object *ob);
+void OVERLAY_volume_draw(OVERLAY_Data *vedata);
+
 void OVERLAY_edit_mesh_init(OVERLAY_Data *vedata);
 void OVERLAY_edit_mesh_cache_init(OVERLAY_Data *vedata);
 void OVERLAY_edit_mesh_cache_populate(OVERLAY_Data *vedata, Object *ob);
diff --git a/source/blender/draw/engines/overlay/overlay_volume.c b/source/blender/draw/engines/overlay/overlay_volume.c
new file mode 100644
index 00000000000..ffa664c90d5
--- /dev/null
+++ b/source/blender/draw/engines/overlay/overlay_volume.c
@@ -0,0 +1,67 @@
+/*
+ * 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.
+ */
+
+/** \file
+ * \ingroup draw_engine
+ */
+
+#include "DNA_volume_types.h"
+
+#include "DRW_render.h"
+#include "GPU_shader.h"
+
+#include "overlay_private.h"
+
+void OVERLAY_volume_cache_init(OVERLAY_Data *vedata)
+{
+  OVERLAY_PassList *psl = vedata->psl;
+  OVERLAY_PrivateData *pd = vedata->stl->pd;
+  const bool is_select = DRW_state_is_select();
+
+  if (is_select) {
+    DRWState state = DRW_STATE_WRITE_DEPTH | DRW_STATE_DEPTH_LESS_EQUAL;
+    DRW_PASS_CREATE(psl->volume_ps, state | pd->clipping_state);
+    GPUShader *sh = OVERLAY_shader_depth_only();
+    DRWShadingGroup *grp = DRW_shgroup_create(sh, psl->volume_ps);
+    pd->volume_selection_surface_grp = grp;
+    DRW_shgroup_uniform_block(grp, "globalsBlock", G_draw.block_ubo);
+  }
+}
+
+void OVERLAY_volume_cache_populate(OVERLAY_Data *vedata, Object *ob)
+{
+  OVERLAY_PrivateData *pd = vedata->stl->pd;
+  const bool is_select = DRW_state_is_select();
+
+  if (is_select) {
+    struct GPUBatch *geom = DRW_cache_volume_selection_surface_get(ob);
+    if (geom != NULL) {
+      DRW_shgroup_call(pd->volume_selection_surface_grp, geom, ob);
+    }
+  }
+}
+
+void OVERLAY_volume_draw(OVERLAY_Data *vedata)
+{
+  OVERLAY_PassList *psl = vedata->psl;
+  OVERLAY_FramebufferList *fbl = vedata->fbl;
+
+  if (DRW_state_is_fbo()) {
+    GPU_framebuffer_bind(fbl->overlay_default_fb);
+  }
+
+  DRW_draw_pass(psl->volume_ps);
+}
diff --git a/source/blender/draw/intern/draw_cache.c b/source/blender/draw/intern/draw_cache.c
index 0fbc9e2ed82..52e7e91a995 100644
--- a/source/blender/draw/intern/draw_cache.c
+++ b/source/blender/draw/intern/draw_cache.c
@@ -3279,6 +3279,12 @@ GPUBatch *DRW_cache_volume_face_wireframe_get(Object *ob)
   return DRW_volume_batch_cache_get_wireframes_face(ob->data);
 }
 
+GPUBatch *DRW_cache_volume_selection_surface_get(Object *ob)
+{
+  BLI_assert(ob->type == OB_VOLUME);
+  return DRW_volume_batch_cache_get_selection_surface(ob->data);
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */
diff --git a/source/blender/draw/intern/draw_cache.h b/source/blender/draw/intern/draw_cache.h
index 7164a477d8a..5da9f4b7964 100644
--- a/source/blender/draw/intern/draw_cache.h
+++ b/source/blender/draw/intern/draw_cache.h
@@ -236,6 +236,7 @@ typedef struct DRWVolumeGrid {
 
 DRWVolumeGrid *DRW_volume_batch_cache_get_grid(struct Volume *volume, struct VolumeGrid *grid);
 struct GPUBatch *DRW_cache_volume_face_wireframe_get(struct Object *ob);
+struct GPUBatch *DRW_cache_volume_selection_surface_get(struct Object *ob);
 
 /* GPencil */
 struct GPUBatch *DRW_cache_gpencil_strokes_get(struct Object *ob, int cfra);
diff --git a/source/blender/draw/intern/draw_cache_impl.h b/source/blender/draw/intern/draw_cache_impl.h
index 9e7a2e2916c..e5cc18f6e09 100644
--- a/source/blender/draw/intern/draw_cache_impl.h
+++ b/source/blender/draw/intern/draw_cache_impl.h
@@ -152,6 +152,7 @@ struct GPUBatch **DRW_cache_pointcloud_surface_shaded_get(struct Object *ob,
 int DRW_volume_material_count_get(struct Volume *volume);
 
 struct GPUBatch *DRW_volume_batch_cache_get_wireframes_face(struct Volume *volume);
+struct GPUBatch *DRW_volume_batch_cache_get_selection_surface(struct Volume *volume);
 
 /* Mesh */
 void DRW_mesh_batch_cache_create_requested(struct TaskGraph *task_graph,
diff --git a/source/blender/draw/intern/draw_cache_impl_volume.c b/source/blender/draw/intern/draw_cache_impl_volume.c
index e39c4976e38..7b03070e32b 100644
--- a/source/blender/draw/intern/draw_cache_impl_volume.c
+++ b/source/blender/draw/intern/draw_cache_impl_volume.c
@@ -62,6 +62,9 @@ typedef struct VolumeBatchCache {
     GPUBatch *batch;
   } face_wire;
 
+  /* Surface for selection */
+  GPUBatch *selection_surface;
+
   /* settings to determine if cache is invalid */
   bool is_dirty;
 } VolumeBatchCache;
@@ -132,6 +135,7 @@ static void volume_batch_cache_clear(Volume *volume)
 
   GPU_VERTBUF_DISCARD_SAFE(cache->face_wire.pos_nor_in_order);
   GPU_BATCH_DISCARD_SAFE(cache->face_wire.batch);
+  GPU_BATCH_DISCARD_SAFE(cache->selection_surface);
 }
 
 void DRW_volume_batch_cache_free(Volume *volume)
@@ -209,6 +213,49 @@ GPUBatch *DRW_volume_batch_cache_get_wireframes_face(Volume *volume)
   return cache->face_wire.batch;
 }
 
+static void drw_volume_selection_surface_cb(
+    void *userdata, float (*verts)[3], int (*tris)[3], int totvert, int tottris)
+{
+  Volume *volume = userdata;
+  VolumeBatchCache *cache = volume->batch_cache;
+
+  static GPUVertFormat format = {0};
+  static uint pos_id;
+  if (format.attr_len == 0) {
+    pos_id = GPU_vertformat_attr_add(&format, "pos", GPU_COMP_F32, 3, GPU_FETCH_FLOAT);
+  }
+
+  /* Create vertex buffer. */
+  GPUVertBuf *vbo_surface = GPU_vertbuf_create_with_format(&format);
+  GPU_vertbuf_data_alloc(vbo_surface, totvert);
+  GPU_vertbuf_attr_fill(vbo_surface, pos_id, verts);
+
+  /* Create index buffer. */
+  GPUIndexBufBuilder elb;
+  GPU_indexbuf_init(&elb, GPU_PRIM_TRIS, tottris, totvert);
+  for (int i = 0; i < tottris; i++) {
+    GPU_indexbuf_add_tri_verts(&elb, UNPACK3(tris[i]));
+  }
+  GPUIndexBuf *ibo_surface = GPU_indexbuf_build(&elb);
+
+  cache->selection_surface = GPU_batch_create_ex(
+      GPU_PRIM_TRIS, vbo_surface, ibo_surface, GPU_BATCH_OWNS_VBO | GPU_BATCH_OWNS_INDEX);
+}
+
+GPUBatch *DRW_volume_batch_cache_get_selection_surface(Volume *volume)
+{
+  VolumeBatchCache *cache = volume_batch_cache_get(volume);
+  if (cache->selection_surface == NULL) {
+    VolumeGrid *volume_grid = BKE_volume_grid_active_get(volume);
+    if (volume_grid == NULL) {
+      return NULL;
+    }
+    BKE_volume_grid_selection_surface(
+        volume, volume_grid, drw_volume_selection_surface_cb, volume);
+  }
+  return cache->selection_surface;
+}
+
 static DRWVolumeGrid *volume_grid_cache_get(Volume *volume,
                                             VolumeGrid *grid,
                                             VolumeBatchCache *cache)