Sculpt: Preserve Mesh visibility from edit mode using the Face Sets

Before this change, when users switch from edit mode to sculpt mode, the
entire mesh would be visible. Even if in the edit mesh mode part of it was
set to invisible.

With this change the visibility is preserved, by creating a separate face set
for the visible and invisible parts of the mesh and setting their initial visibility.

Implementation details: This adds a function to initialize a new Face Set
 datalayer taking the current mesh visibility into account which is stored
in the ME_HIDE flag of the vertices.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8901

source/blender/blenkernel/BKE_paint.h

@@ -583,6 +583,11 @@ void BKE_sculpt_bvh_update_from_ccg(struct PBVH *pbvh, struct SubdivCCG *subdiv_
  * updated according to the face sets. */
 void BKE_sculpt_sync_face_set_visibility(struct Mesh *mesh, struct SubdivCCG *subdiv_ccg);
 
+/* Ensures that a Face Set datalayers exists. If it does not, it creates one respecting the
+ * visibility stored in the vertices of the mesh. If it does, it copies the visibility from the
+ * mesh to the Face Sets. */
+void BKE_sculpt_face_sets_ensure_from_base_mesh_visibility(struct Mesh *mesh);
+
 bool BKE_sculptsession_use_pbvh_draw(const struct Object *ob, const struct View3D *v3d);
 
 enum {

source/blender/blenkernel/intern/paint.c

@@ -1609,14 +1609,10 @@ static void sculpt_update_object(Depsgraph *depsgraph,
   /* Sculpt Face Sets. */
   if (use_face_sets) {
     if (!CustomData_has_layer(&me->pdata, CD_SCULPT_FACE_SETS)) {
-      ss->face_sets = CustomData_add_layer(
-          &me->pdata, CD_SCULPT_FACE_SETS, CD_CALLOC, NULL, me->totpoly);
-      for (int i = 0; i < me->totpoly; i++) {
-        ss->face_sets[i] = 1;
-      }
-
-      /* Set the default face set color if the datalayer did not exist. */
-      me->face_sets_color_default = 1;
+      /* By checking here if the datalayer already exist this avoids copying the visibility from
+       * the mesh and looping over all vertices on every sculpt editing operation, using this
+       * function only the first time the Face Sets datalayer needs to be created. */
+      BKE_sculpt_face_sets_ensure_from_base_mesh_visibility(me);
     }
     ss->face_sets = CustomData_get_layer(&me->pdata, CD_SCULPT_FACE_SETS);
   }
@@ -1881,6 +1877,57 @@ static bool check_sculpt_object_deformed(Object *object, const bool for_construc
   return deformed;
 }
 
+void BKE_sculpt_face_sets_ensure_from_base_mesh_visibility(Mesh *mesh)
+{
+  const int face_sets_default_visible_id = 1;
+  const int face_sets_default_hidden_id = -(face_sets_default_visible_id + 1);
+
+  bool initialize_new_face_sets = false;
+
+  if (CustomData_has_layer(&mesh->pdata, CD_SCULPT_FACE_SETS)) {
+    /* Make everything visible. */
+    int *current_face_sets = CustomData_get_layer(&mesh->pdata, CD_SCULPT_FACE_SETS);
+    for (int i = 0; i < mesh->totpoly; i++) {
+      current_face_sets[i] = abs(current_face_sets[i]);
+    }
+  }
+  else {
+    initialize_new_face_sets = true;
+    int *new_face_sets = CustomData_add_layer(
+        &mesh->pdata, CD_SCULPT_FACE_SETS, CD_CALLOC, NULL, mesh->totpoly);
+
+    /* Initialize the new Face Set datalayer with a default valid visible ID and set the default
+     * color to render it white. */
+    for (int i = 0; i < mesh->totpoly; i++) {
+      new_face_sets[i] = face_sets_default_visible_id;
+    }
+    mesh->face_sets_color_default = face_sets_default_visible_id;
+  }
+
+  int *face_sets = CustomData_get_layer(&mesh->pdata, CD_SCULPT_FACE_SETS);
+
+  /* Show the only the face sets that have all visibile vertices. */
+  for (int i = 0; i < mesh->totpoly; i++) {
+    for (int l = 0; l < mesh->mpoly[i].totloop; l++) {
+      MLoop *loop = &mesh->mloop[mesh->mpoly[i].loopstart + l];
+      if (mesh->mvert[loop->v].flag & ME_HIDE) {
+        if (initialize_new_face_sets) {
+          /* When initializing a new Face Set datalayer, assing a new hidden Face Set ID to hidden
+           * vertices. This way, we get at initial split in two Face Sets between hidden and
+           * visible vertices based on the previous mesh visibily from other mode that can be
+           * useful in some cases. */
+          face_sets[i] = face_sets_default_hidden_id;
+        }
+        else {
+          /* Otherwise, set the already existing Face Set ID to hidden. */
+          face_sets[i] = -abs(face_sets[i]);
+        }
+        break;
+      }
+    }
+  }
+}
+
 static void sculpt_sync_face_sets_visibility_to_base_mesh(Mesh *mesh)
 {
   int *face_sets = CustomData_get_layer(&mesh->pdata, CD_SCULPT_FACE_SETS);
@@ -1948,6 +1995,7 @@ static void sculpt_sync_face_sets_visibility_to_grids(Mesh *mesh, SubdivCCG *sub
 
 void BKE_sculpt_sync_face_set_visibility(struct Mesh *mesh, struct SubdivCCG *subdiv_ccg)
 {
+  BKE_sculpt_face_sets_ensure_from_base_mesh_visibility(mesh);
   sculpt_sync_face_sets_visibility_to_base_mesh(mesh);
   sculpt_sync_face_sets_visibility_to_grids(mesh, subdiv_ccg);
 }

source/blender/editors/sculpt_paint/sculpt.c

@@ -8077,7 +8077,8 @@ static void sculpt_init_session(Depsgraph *depsgraph, Scene *scene, Object *ob)
 
   /* Update the Face Sets visibility with the vertex visibility changes that may have been done
    * outside Sculpt Mode */
-  SCULPT_visibility_sync_all_vertex_to_face_sets(ob->sculpt);
+  Mesh *mesh = ob->data;
+  BKE_sculpt_face_sets_ensure_from_base_mesh_visibility(mesh);
 }
 
 static int ed_object_sculptmode_flush_recalc_flag(Scene *scene,