GPv3: Select Alternate

Adds a "Select Alternate" operator for the new Grease Pencil data type.
This replaces the previous `gpencil.select_alternate`.

Resolves #109204.

Pull Request: https://projects.blender.org/blender/blender/pulls/109240

scripts/startup/bl_ui/space_view3d.py

@@ -1991,6 +1991,7 @@ class VIEW3D_MT_select_edit_grease_pencil(Menu):
         layout.separator()
 
         layout.operator("grease_pencil.select_linked", text="Linked")
+        layout.operator("grease_pencil.select_alternate", text="Alternated")
         layout.operator("grease_pencil.select_random", text="Random")
 
         layout.separator()

source/blender/editors/curves/intern/curves_selection.cc

@@ -272,6 +272,52 @@ void select_linked(bke::CurvesGeometry &curves)
   selection.finish();
 }
 
+void select_alternate(bke::CurvesGeometry &curves, const bool deselect_ends)
+{
+  if (!has_anything_selected(curves)) {
+    return;
+  }
+
+  const OffsetIndices points_by_curve = curves.points_by_curve();
+  bke::GSpanAttributeWriter selection = ensure_selection_attribute(
+      curves, ATTR_DOMAIN_POINT, CD_PROP_BOOL);
+  const VArray<bool> cyclic = curves.cyclic();
+
+  MutableSpan<bool> selection_typed = selection.span.typed<bool>();
+  threading::parallel_for(curves.curves_range(), 256, [&](const IndexRange range) {
+    for (const int curve_i : range) {
+      const IndexRange points = points_by_curve[curve_i];
+
+      if (!has_anything_selected(selection.span.slice(points))) {
+        continue;
+      }
+
+      for (const int index : points.index_range()) {
+        selection_typed[points[index]] = deselect_ends ? index % 2 : !(index % 2);
+      }
+
+      if (cyclic[curve_i]) {
+        if (deselect_ends) {
+          selection_typed[points.last()] = false;
+        }
+        else {
+          selection_typed[points.last()] = true;
+          if (points.size() > 2) {
+            selection_typed[points.last() - 1] = false;
+          }
+        }
+      }
+      else {
+        if (deselect_ends) {
+          selection_typed[points.last()] = false;
+        }
+      }
+    }
+  });
+
+  selection.finish();
+}
+
 void select_adjacent(bke::CurvesGeometry &curves, const bool deselect)
 {
   const OffsetIndices points_by_curve = curves.points_by_curve();

source/blender/editors/grease_pencil/intern/grease_pencil_select.cc

@@ -195,6 +195,45 @@ static void GREASE_PENCIL_OT_select_random(wmOperatorType *ot)
   WM_operator_properties_select_random(ot);
 }
 
+static int select_alternate_exec(bContext *C, wmOperator *op)
+{
+  const bool deselect_ends = RNA_boolean_get(op->ptr, "deselect_ends");
+  Scene *scene = CTX_data_scene(C);
+  Object *object = CTX_data_active_object(C);
+  GreasePencil &grease_pencil = *static_cast<GreasePencil *>(object->data);
+  eAttrDomain selection_domain = ED_grease_pencil_selection_domain_get(C);
+
+  grease_pencil.foreach_editable_drawing(
+      scene->r.cfra, [&](int /*drawing_index*/, GreasePencilDrawing &drawing) {
+        blender::ed::curves::select_alternate(drawing.geometry.wrap(), deselect_ends);
+      });
+
+  /* Use #ID_RECALC_GEOMETRY instead of #ID_RECALC_SELECT because it is handled as a generic
+   * attribute for now. */
+  DEG_id_tag_update(&grease_pencil.id, ID_RECALC_GEOMETRY);
+  WM_event_add_notifier(C, NC_GEOM | ND_DATA, &grease_pencil);
+
+  return OPERATOR_FINISHED;
+}
+
+static void GREASE_PENCIL_OT_select_alternate(wmOperatorType *ot)
+{
+  ot->name = "Select Alternate";
+  ot->idname = "GREASE_PENCIL_OT_select_alternate";
+  ot->description = "Select alternated points in strokes with already selected points";
+
+  ot->exec = select_alternate_exec;
+  ot->poll = editable_grease_pencil_poll;
+
+  ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
+
+  RNA_def_boolean(ot->srna,
+                  "deselect_ends",
+                  false,
+                  "Deselect Ends",
+                  "(De)select the first and last point of each stroke");
+}
+
 static int select_ends_exec(bContext *C, wmOperator *op)
 {
   const int amount_start = RNA_int_get(op->ptr, "amount_start");
@@ -275,5 +314,6 @@ void ED_operatortypes_grease_pencil_select(void)
   WM_operatortype_append(GREASE_PENCIL_OT_select_less);
   WM_operatortype_append(GREASE_PENCIL_OT_select_linked);
   WM_operatortype_append(GREASE_PENCIL_OT_select_random);
+  WM_operatortype_append(GREASE_PENCIL_OT_select_alternate);
   WM_operatortype_append(GREASE_PENCIL_OT_select_ends);
 }

source/blender/editors/include/ED_curves.h

@@ -160,6 +160,11 @@ void select_ends(bke::CurvesGeometry &curves, int amount_start, int amount_end);
  */
 void select_linked(bke::CurvesGeometry &curves);
 
+/**
+ * Select alternated points in strokes with already selected points
+ */
+void select_alternate(bke::CurvesGeometry &curves, const bool deselect_ends);
+
 /**
  * (De)select all the adjacent points of the current selected points.
  */