Workaround T81065: Merge UV's when applying modifiers

Support merging UV's that share the same vertex and are very close when
applying modifiers.

This is needed to prevent UV's becoming "detached" which can happen when
applying the subdivision surface modifier.

This regression was caused by [0] which removed selection threshold for
nearby coordinates. While restoring the UV selection threshold could be
done - some selection operations that walk around connected UV fans
wouldn't behave in a deterministic way (such as select shortest path).
There are also other cases where UV's may be compared without a
threshold such as tangent calculation and exporters which have their own
logic to handling UV's.

Also resolves T86896, T89903.

[0]: b88dd3b8e7b9c02ae08d4679bb427963c5d21250

Reviewed By: sergey

Ref D14841

source/blender/blenkernel/BKE_mesh.h

@@ -893,6 +893,14 @@ struct Mesh *BKE_mesh_merge_verts(struct Mesh *mesh,
                                   int tot_vtargetmap,
                                   int merge_mode);
 
+/**
+ * Account for custom-data such as UV's becoming detached because of of imprecision
+ * in custom-data interpolation.
+ * Without running this operation subdivision surface can cause UV's to be disconnected,
+ * see: T81065.
+ */
+void BKE_mesh_merge_customdata_for_apply_modifier(struct Mesh *me);
+
 /* Flush flags. */
 
 /**

source/blender/blenkernel/CMakeLists.txt

@@ -199,6 +199,7 @@ set(SRC
   intern/mesh_iterators.c
   intern/mesh_mapping.c
   intern/mesh_merge.c
+  intern/mesh_merge_customdata.cc
   intern/mesh_mirror.c
   intern/mesh_normals.cc
   intern/mesh_remap.c

source/blender/editors/object/object_add.cc

@@ -2894,6 +2894,7 @@ static int object_convert_exec(bContext *C, wmOperator *op)
   Object *ob1, *obact = CTX_data_active_object(C);
   const short target = RNA_enum_get(op->ptr, "target");
   bool keep_original = RNA_boolean_get(op->ptr, "keep_original");
+  const bool do_merge_customdata = RNA_boolean_get(op->ptr, "merge_customdata");
 
   const float angle = RNA_float_get(op->ptr, "angle");
   const int thickness = RNA_int_get(op->ptr, "thickness");
@@ -3122,6 +3123,10 @@ static int object_convert_exec(bContext *C, wmOperator *op)
       Mesh *new_mesh = (Mesh *)newob->data;
       BKE_mesh_nomain_to_mesh(me_eval, new_mesh, newob, &CD_MASK_MESH, true);
 
+      if (do_merge_customdata) {
+        BKE_mesh_merge_customdata_for_apply_modifier(new_mesh);
+      }
+
       /* Anonymous attributes shouldn't be available on the applied geometry. */
       MeshComponent component;
       component.replace(new_mesh, GeometryOwnershipType::Editable);
@@ -3441,7 +3446,11 @@ static void object_convert_ui(bContext *UNUSED(C), wmOperator *op)
   uiItemR(layout, op->ptr, "target", 0, nullptr, ICON_NONE);
   uiItemR(layout, op->ptr, "keep_original", 0, nullptr, ICON_NONE);
 
-  if (RNA_enum_get(op->ptr, "target") == OB_GPENCIL) {
+  const int target = RNA_enum_get(op->ptr, "target");
+  if (target == OB_MESH) {
+    uiItemR(layout, op->ptr, "merge_customdata", 0, nullptr, ICON_NONE);
+  }
+  else if (target == OB_GPENCIL) {
     uiItemR(layout, op->ptr, "thickness", 0, nullptr, ICON_NONE);
     uiItemR(layout, op->ptr, "angle", 0, nullptr, ICON_NONE);
     uiItemR(layout, op->ptr, "offset", 0, nullptr, ICON_NONE);
@@ -3477,6 +3486,13 @@ void OBJECT_OT_convert(wmOperatorType *ot)
                   "Keep Original",
                   "Keep original objects instead of replacing them");
 
+  RNA_def_boolean(
+      ot->srna,
+      "merge_customdata",
+      true,
+      "Merge UV's",
+      "Merge UV coordinates that share a vertex to account for imprecision in some modifiers");
+
   prop = RNA_def_float_rotation(ot->srna,
                                 "angle",
                                 0,

source/blender/editors/object/object_modifier.cc

@@ -1434,8 +1434,10 @@ static int modifier_apply_exec_ex(bContext *C, wmOperator *op, int apply_as, boo
   Scene *scene = CTX_data_scene(C);
   Object *ob = ED_object_active_context(C);
   ModifierData *md = edit_modifier_property_get(op, ob, 0);
+  const ModifierTypeInfo *mti = BKE_modifier_get_info((ModifierType)md->type);
   const bool do_report = RNA_boolean_get(op->ptr, "report");
   const bool do_single_user = RNA_boolean_get(op->ptr, "single_user");
+  const bool do_merge_customdata = RNA_boolean_get(op->ptr, "merge_customdata");
 
   if (md == nullptr) {
     return OPERATOR_CANCELLED;
@@ -1460,6 +1462,11 @@ static int modifier_apply_exec_ex(bContext *C, wmOperator *op, int apply_as, boo
     return OPERATOR_CANCELLED;
   }
 
+  if (do_merge_customdata &&
+      (mti->type & (eModifierTypeType_Constructive | eModifierTypeType_Nonconstructive))) {
+    BKE_mesh_merge_customdata_for_apply_modifier((Mesh *)ob->data);
+  }
+
   DEG_id_tag_update(&ob->id, ID_RECALC_GEOMETRY);
   DEG_relations_tag_update(bmain);
   WM_event_add_notifier(C, NC_OBJECT | ND_MODIFIER, ob);
@@ -1518,6 +1525,12 @@ void OBJECT_OT_modifier_apply(wmOperatorType *ot)
   edit_modifier_properties(ot);
   edit_modifier_report_property(ot);
 
+  RNA_def_boolean(
+      ot->srna,
+      "merge_customdata",
+      true,
+      "Merge UV's",
+      "Merge UV coordinates that share a vertex to account for imprecision in some modifiers");
   PropertyRNA *prop = RNA_def_boolean(ot->srna,
                                       "single_user",
                                       false,
@@ -1599,11 +1612,18 @@ static int modifier_convert_exec(bContext *C, wmOperator *op)
   ViewLayer *view_layer = CTX_data_view_layer(C);
   Object *ob = ED_object_active_context(C);
   ModifierData *md = edit_modifier_property_get(op, ob, 0);
+  const ModifierTypeInfo *mti = BKE_modifier_get_info((ModifierType)md->type);
+  const bool do_merge_customdata = RNA_boolean_get(op->ptr, "merge_customdata");
 
   if (!md || !ED_object_modifier_convert(op->reports, bmain, depsgraph, view_layer, ob, md)) {
     return OPERATOR_CANCELLED;
   }
 
+  if (do_merge_customdata &&
+      (mti->type & (eModifierTypeType_Constructive | eModifierTypeType_Nonconstructive))) {
+    BKE_mesh_merge_customdata_for_apply_modifier((Mesh *)ob->data);
+  }
+
   DEG_id_tag_update(&ob->id, ID_RECALC_GEOMETRY);
   WM_event_add_notifier(C, NC_OBJECT | ND_MODIFIER, ob);
 
@@ -1631,6 +1651,13 @@ void OBJECT_OT_modifier_convert(wmOperatorType *ot)
   /* flags */
   ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO | OPTYPE_INTERNAL;
   edit_modifier_properties(ot);
+
+  RNA_def_boolean(
+      ot->srna,
+      "merge_customdata",
+      true,
+      "Merge UV's",
+      "Merge UV coordinates that share a vertex to account for imprecision in some modifiers");
 }
 
 /** \} */