mesh_bsurfaces.py

        bpy.ops.object.duplicate('INVOKE_REGION_WIN')
        curves_duplicate_2 = bpy.context.object
        objects_to_delete.append(curves_duplicate_2)

        # Duplicate the duplicate and add Shrinkwrap to it, with the grease pencil strokes curve as target
        bpy.ops.object.select_all('INVOKE_REGION_WIN', action='DESELECT')
        curves_duplicate_2.select_set(True)
        bpy.context.view_layer.objects.active = curves_duplicate_2

        bpy.ops.object.modifier_add('INVOKE_REGION_WIN', type='SHRINKWRAP')
        curves_duplicate_2.modifiers["Shrinkwrap"].wrap_method = "NEAREST_VERTEX"
        curves_duplicate_2.modifiers["Shrinkwrap"].target = GP_strokes_mesh
        bpy.ops.object.modifier_apply('INVOKE_REGION_WIN', modifier='Shrinkwrap')

        # Get the distance of each vert from its original position to its position with Shrinkwrap
        nearest_points_coords = {}
        for st_idx in range(len(curves_duplicate_1.data.splines)):
            for bp_idx in range(len(curves_duplicate_1.data.splines[st_idx].bezier_points)):
                bp_1_co = curves_duplicate_1.matrix_world @ \
                          curves_duplicate_1.data.splines[st_idx].bezier_points[bp_idx].co

                bp_2_co = curves_duplicate_2.matrix_world @ \
                          curves_duplicate_2.data.splines[st_idx].bezier_points[bp_idx].co

                if bp_idx == 0:
                    shortest_dist = (bp_1_co - bp_2_co).length
                    nearest_points_coords[st_idx] = ("%.4f" % bp_2_co[0],
                                                     "%.4f" % bp_2_co[1],
                                                     "%.4f" % bp_2_co[2])

                dist = (bp_1_co - bp_2_co).length

                if dist < shortest_dist:
                    nearest_points_coords[st_idx] = ("%.4f" % bp_2_co[0],
                                                     "%.4f" % bp_2_co[1],
                                                     "%.4f" % bp_2_co[2])
                    shortest_dist = dist

        # Get all coords of GP strokes points, for comparison
        GP_strokes_coords = []
        for st_idx in range(len(GP_strokes_curve.data.splines)):
            GP_strokes_coords.append(
                    [("%.4f" % x if "%.4f" % x != "-0.00" else "0.00",
                    "%.4f" % y if "%.4f" % y != "-0.00" else "0.00",
                    "%.4f" % z if "%.4f" % z != "-0.00" else "0.00") for
                    x, y, z in [bp.co for bp in GP_strokes_curve.data.splines[st_idx].bezier_points]]
                    )

        # Check the point of the GP strokes with the same coords as
        # the nearest points of the curves (with shrinkwrap)

        # Dictionary with GP stroke index as index, and a list as value.
        # The list has as index the point index of the GP stroke
        # nearest to the spline, and as value the spline index
        GP_connection_points = {}
        for gp_st_idx in range(len(GP_strokes_coords)):
            GPvert_spline_relationship = {}

            for splines_st_idx in range(len(nearest_points_coords)):
                if nearest_points_coords[splines_st_idx] in GP_strokes_coords[gp_st_idx]:
                    GPvert_spline_relationship[
                        GP_strokes_coords[gp_st_idx].index(nearest_points_coords[splines_st_idx])
                        ] = splines_st_idx

            GP_connection_points[gp_st_idx] = GPvert_spline_relationship

        # Get the splines new order
        splines_new_order = []
        for i in GP_connection_points:
            dict_keys = sorted(GP_connection_points[i].keys())  # Sort dictionaries by key

            for k in dict_keys:
                splines_new_order.append(GP_connection_points[i][k])

        # Reorder
        curve_original_name = self.main_curve.name

        bpy.ops.object.select_all('INVOKE_REGION_WIN', action='DESELECT')
        self.main_curve.select_set(True)
        bpy.context.view_layer.objects.active = self.main_curve

        self.main_curve.name = "SURFSKIO_CRV_ORD"

        bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')
        bpy.ops.curve.select_all('INVOKE_REGION_WIN', action='DESELECT')
        bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')

        for _sp_idx in range(len(self.main_curve.data.splines)):
            self.main_curve.data.splines[0].bezier_points[0].select_control_point = True

            bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')
            bpy.ops.curve.separate('EXEC_REGION_WIN')
            bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')

        # Get the names of the separated splines objects in the original order
        splines_unordered = {}
        for o in bpy.data.objects:
            if o.name.find("SURFSKIO_CRV_ORD") != -1:
                spline_order_string = o.name.partition(".")[2]

                if spline_order_string != "" and int(spline_order_string) > 0:
                    spline_order_index = int(spline_order_string) - 1
                    splines_unordered[spline_order_index] = o.name

        # Join all splines objects in final order
        for order_idx in splines_new_order:
            bpy.ops.object.select_all('INVOKE_REGION_WIN', action='DESELECT')
            bpy.data.objects[splines_unordered[order_idx]].select_set(True)
            bpy.data.objects["SURFSKIO_CRV_ORD"].select_set(True)
            bpy.context.view_layer.objects.active = bpy.data.objects["SURFSKIO_CRV_ORD"]

            bpy.ops.object.join('INVOKE_REGION_WIN')

        # Go back to the original name of the curves object.
        bpy.context.object.name = curve_original_name

        # Delete all unused objects
        bpy.ops.object.delete({"selected_objects": objects_to_delete})

        bpy.ops.object.select_all('INVOKE_REGION_WIN', action='DESELECT')
        bpy.data.objects[curve_original_name].select_set(True)
        bpy.context.view_layer.objects.active = bpy.data.objects[curve_original_name]

        bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')
        bpy.ops.curve.select_all('INVOKE_REGION_WIN', action='DESELECT')

        try:
            bpy.context.scene.bsurfaces.SURFSK_gpencil.data.layers.active.clear()
        except:
            pass


        return {"FINISHED"}

    def invoke(self, context, event):
        self.main_curve = bpy.context.object
        there_are_GP_strokes = False

        try:
            # Get the active grease pencil layer
            strokes_num = len(self.main_curve.grease_pencil.layers.active.active_frame.strokes)

            if strokes_num > 0:
                there_are_GP_strokes = True
        except:
            pass

        if there_are_GP_strokes:
            self.execute(context)
            self.report({'INFO'}, "Splines have been reordered")
        else:
            self.report({'WARNING'}, "Draw grease pencil strokes to connect splines")

        return {"FINISHED"}

# ----------------------------
# Set first points operator
class CURVE_OT_SURFSK_first_points(Operator):
    bl_idname = "curve.surfsk_first_points"
    bl_label = "Bsurfaces set first points"
    bl_description = "Set the selected points as the first point of each spline"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context):
        splines_to_invert = []

        # Check non-cyclic splines to invert
        for i in range(len(self.main_curve.data.splines)):
            b_points = self.main_curve.data.splines[i].bezier_points

            if i not in self.cyclic_splines:  # Only for non-cyclic splines
                if b_points[len(b_points) - 1].select_control_point:
                    splines_to_invert.append(i)

        # Reorder points of cyclic splines, and set all handles to "Automatic"

        # Check first selected point
        cyclic_splines_new_first_pt = {}
        for i in self.cyclic_splines:
            sp = self.main_curve.data.splines[i]

            for t in range(len(sp.bezier_points)):
                bp = sp.bezier_points[t]
                if bp.select_control_point or bp.select_right_handle or bp.select_left_handle:
                    cyclic_splines_new_first_pt[i] = t
                    break  # To take only one if there are more

        # Reorder
        for spline_idx in cyclic_splines_new_first_pt:
            sp = self.main_curve.data.splines[spline_idx]

            spline_old_coords = []
            for bp_old in sp.bezier_points:
                coords = (bp_old.co[0], bp_old.co[1], bp_old.co[2])

                left_handle_type = str(bp_old.handle_left_type)
                left_handle_length = float(bp_old.handle_left.length)
                left_handle_xyz = (
                        float(bp_old.handle_left.x),
                        float(bp_old.handle_left.y),
                        float(bp_old.handle_left.z)
                        )
                right_handle_type = str(bp_old.handle_right_type)
                right_handle_length = float(bp_old.handle_right.length)
                right_handle_xyz = (
                        float(bp_old.handle_right.x),
                        float(bp_old.handle_right.y),
                        float(bp_old.handle_right.z)
                        )
                spline_old_coords.append(
                        [coords, left_handle_type,
                        right_handle_type, left_handle_length,
                        right_handle_length, left_handle_xyz,
                        right_handle_xyz]
                        )

            for t in range(len(sp.bezier_points)):
                bp = sp.bezier_points

                if t + cyclic_splines_new_first_pt[spline_idx] + 1 <= len(bp) - 1:
                    new_index = t + cyclic_splines_new_first_pt[spline_idx] + 1
                else:
                    new_index = t + cyclic_splines_new_first_pt[spline_idx] + 1 - len(bp)

                bp[t].co = Vector(spline_old_coords[new_index][0])

                bp[t].handle_left.length = spline_old_coords[new_index][3]
                bp[t].handle_right.length = spline_old_coords[new_index][4]

                bp[t].handle_left_type = "FREE"
                bp[t].handle_right_type = "FREE"

                bp[t].handle_left.x = spline_old_coords[new_index][5][0]
                bp[t].handle_left.y = spline_old_coords[new_index][5][1]
                bp[t].handle_left.z = spline_old_coords[new_index][5][2]

                bp[t].handle_right.x = spline_old_coords[new_index][6][0]
                bp[t].handle_right.y = spline_old_coords[new_index][6][1]
                bp[t].handle_right.z = spline_old_coords[new_index][6][2]

                bp[t].handle_left_type = spline_old_coords[new_index][1]
                bp[t].handle_right_type = spline_old_coords[new_index][2]

        # Invert the non-cyclic splines designated above
        for i in range(len(splines_to_invert)):
            bpy.ops.curve.select_all('INVOKE_REGION_WIN', action='DESELECT')

            bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')
            self.main_curve.data.splines[splines_to_invert[i]].bezier_points[0].select_control_point = True
            bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')

            bpy.ops.curve.switch_direction()

        bpy.ops.curve.select_all('INVOKE_REGION_WIN', action='DESELECT')

        # Keep selected the first vert of each spline
        bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')
        for i in range(len(self.main_curve.data.splines)):
            if not self.main_curve.data.splines[i].use_cyclic_u:
                bp = self.main_curve.data.splines[i].bezier_points[0]
            else:
                bp = self.main_curve.data.splines[i].bezier_points[
                                                        len(self.main_curve.data.splines[i].bezier_points) - 1
                                                        ]

            bp.select_control_point = True
            bp.select_right_handle = True
            bp.select_left_handle = True

        bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')

        return {'FINISHED'}

    def invoke(self, context, event):
        self.main_curve = bpy.context.object

        # Check if all curves are Bezier, and detect which ones are cyclic
        self.cyclic_splines = []
        for i in range(len(self.main_curve.data.splines)):
            if self.main_curve.data.splines[i].type != "BEZIER":
                self.report({'WARNING'}, "All splines must be Bezier type")

                return {'CANCELLED'}
            else:
                if self.main_curve.data.splines[i].use_cyclic_u:
                    self.cyclic_splines.append(i)

        self.execute(context)
        self.report({'INFO'}, "First points have been set")

        return {'FINISHED'}


# Add-ons Preferences Update Panel

# Define Panel classes for updating
panels = (
        VIEW3D_PT_tools_SURFSK_mesh,
        VIEW3D_PT_tools_SURFSK_curve
        )


def conver_gpencil_to_curve(self, context, pencil, type):
    newCurve = bpy.data.curves.new(type + '_curve', type='CURVE')
    newCurve.dimensions = '3D'
    CurveObject = object_utils.object_data_add(context, newCurve)
    error = False

    if type == 'GPensil':
        try:
            strokes = pencil.data.layers.active.active_frame.strokes
        except:
            error = True
        CurveObject.location = pencil.location
        CurveObject.rotation_euler = pencil.rotation_euler
        CurveObject.scale = pencil.scale
    elif type == 'Annotation':
        try:
            strokes = bpy.context.annotation_data.layers.active.active_frame.strokes
        except:
            error = True
        CurveObject.location = (0.0, 0.0, 0.0)
        CurveObject.rotation_euler = (0.0, 0.0, 0.0)
        CurveObject.scale = (1.0, 1.0, 1.0)

    if not error:
        for i, _stroke in enumerate(strokes):
            stroke_points = strokes[i].points
            data_list = [ (point.co.x, point.co.y, point.co.z)
                            for point in stroke_points ]
            points_to_add = len(data_list)-1

            flat_list = []
            for point in data_list:
                flat_list.extend(point)

            spline = newCurve.splines.new(type='BEZIER')
            spline.bezier_points.add(points_to_add)
            spline.bezier_points.foreach_set("co", flat_list)

            for point in spline.bezier_points:
                point.handle_left_type="AUTO"
                point.handle_right_type="AUTO"

        return CurveObject
    else:
        return None


def update_panel(self, context):
    message = "Bsurfaces GPL Edition: Updating Panel locations has failed"
    try:
        for panel in panels:
            if "bl_rna" in panel.__dict__:
                bpy.utils.unregister_class(panel)

        for panel in panels:
            category = context.preferences.addons[__name__].preferences.category
            if category != 'Tool':
                panel.bl_category = context.preferences.addons[__name__].preferences.category
            else:
                context.preferences.addons[__name__].preferences.category = 'Edit'
                panel.bl_category = 'Edit'
                raise ValueError("You can not install add-ons in the Tool panel")
            bpy.utils.register_class(panel)

    except Exception as e:
        print("\n[{}]\n{}\n\nError:\n{}".format(__name__, message, e))
        pass

def makeMaterial(name, diffuse):

    if name in bpy.data.materials:
        material = bpy.data.materials[name]
        material.diffuse_color = diffuse
    else:
        material = bpy.data.materials.new(name)
        material.diffuse_color = diffuse

    return material

def update_mesh(self, context):
    try:
        bpy.ops.object.mode_set('INVOKE_REGION_WIN', mode='OBJECT')
        bpy.ops.object.select_all(action='DESELECT')
        bpy.context.view_layer.update()
        global global_mesh_object
        global_mesh_object = bpy.context.scene.bsurfaces.SURFSK_mesh.name
        bpy.data.objects[global_mesh_object].select_set(True)
        bpy.context.view_layer.objects.active = bpy.data.objects[global_mesh_object]
    except:
        print("Select mesh object")

def update_gpencil(self, context):
    try:
        bpy.ops.object.mode_set('INVOKE_REGION_WIN', mode='OBJECT')
        bpy.ops.object.select_all(action='DESELECT')
        bpy.context.view_layer.update()
        global global_gpencil_object
        global_gpencil_object = bpy.context.scene.bsurfaces.SURFSK_gpencil.name
        bpy.data.objects[global_gpencil_object].select_set(True)
        bpy.context.view_layer.objects.active = bpy.data.objects[global_gpencil_object]
    except:
        print("Select gpencil object")

def update_curve(self, context):
    try:
        bpy.ops.object.mode_set('INVOKE_REGION_WIN', mode='OBJECT')
        bpy.ops.object.select_all(action='DESELECT')
        bpy.context.view_layer.update()
        global global_curve_object
        global_curve_object = bpy.context.scene.bsurfaces.SURFSK_curve.name
        bpy.data.objects[global_curve_object].select_set(True)
        bpy.context.view_layer.objects.active = bpy.data.objects[global_curve_object]
    except:
        print("Select curve object")

def update_color(self, context):
    try:
        global global_color
        global global_mesh_object
        material = makeMaterial("BSurfaceMesh", bpy.context.scene.bsurfaces.SURFSK_mesh_color)
        if bpy.data.objects[global_mesh_object].data.materials:
            bpy.data.objects[global_mesh_object].data.materials[0] = material
        else:
            bpy.data.objects[global_mesh_object].data.materials.append(material)
        diffuse_color = material.diffuse_color
        global_color = (diffuse_color[0], diffuse_color[1], diffuse_color[2], diffuse_color[3])
    except:
        print("Select mesh object")

def update_Shrinkwrap_offset(self, context):
    try:
        global global_offset
        global_offset = bpy.context.scene.bsurfaces.SURFSK_Shrinkwrap_offset
        global global_mesh_object
        modifier = bpy.data.objects[global_mesh_object].modifiers["Shrinkwrap"]
        modifier.offset = global_offset
    except:
        print("Shrinkwrap modifier not found")

def update_in_front(self, context):
    try:
        global global_in_front
        global_in_front = bpy.context.scene.bsurfaces.SURFSK_in_front
        global global_mesh_object
        bpy.data.objects[global_mesh_object].show_in_front = global_in_front
    except:
        print("Select mesh object")

def update_show_wire(self, context):
    try:
        global global_show_wire
        global_show_wire = bpy.context.scene.bsurfaces.SURFSK_show_wire
        global global_mesh_object
        bpy.data.objects[global_mesh_object].show_wire = global_show_wire
    except:
        print("Select mesh object")

def update_shade_smooth(self, context):
    try:
        global global_shade_smooth
        global_shade_smooth = bpy.context.scene.bsurfaces.SURFSK_shade_smooth

        contex_mode = bpy.context.mode

        if bpy.ops.object.mode_set.poll():
            bpy.ops.object.mode_set('INVOKE_REGION_WIN', mode='OBJECT')

        bpy.ops.object.select_all(action='DESELECT')
        global global_mesh_object
        global_mesh_object = bpy.context.scene.bsurfaces.SURFSK_mesh.name
        bpy.data.objects[global_mesh_object].select_set(True)

        if global_shade_smooth:
            bpy.ops.object.shade_smooth()
        else:
            bpy.ops.object.shade_flat()

        if contex_mode == "EDIT_MESH":
            bpy.ops.object.editmode_toggle('INVOKE_REGION_WIN')

    except:
        print("Select mesh object")


class BsurfPreferences(AddonPreferences):
    # this must match the addon name, use '__package__'
    # when defining this in a submodule of a python package.
    bl_idname = __name__

    category: StringProperty(
            name="Tab Category",
            description="Choose a name for the category of the panel",
            default="Edit",
            update=update_panel
            )

    def draw(self, context):
        layout = self.layout

        row = layout.row()
        col = row.column()
        col.label(text="Tab Category:")
        col.prop(self, "category", text="")

# Properties
class BsurfacesProps(PropertyGroup):
    SURFSK_guide: EnumProperty(
        name="Guide:",
        items=[
                ('Annotation', 'Annotation', 'Annotation'),
                ('GPencil', 'GPencil', 'GPencil'),
                ('Curve', 'Curve', 'Curve')
              ],
        default="Annotation"
        )
    SURFSK_edges_U: IntProperty(
                    name="Cross",
                    description="Number of face-loops crossing the strokes",
                    default=5,
                    min=1,
                    max=200
                    )
    SURFSK_edges_V: IntProperty(
                    name="Follow",
                    description="Number of face-loops following the strokes",
                    default=1,
                    min=1,
                    max=200
                    )
    SURFSK_cyclic_cross: BoolProperty(
                name="Cyclic Cross",
                description="Make cyclic the face-loops crossing the strokes",
                default=False
                )
    SURFSK_cyclic_follow: BoolProperty(
                name="Cyclic Follow",
                description="Make cyclic the face-loops following the strokes",
                default=False
                )
    SURFSK_keep_strokes: BoolProperty(
                name="Keep strokes",
                description="Keeps the sketched strokes or curves after adding the surface",
                default=False
                )
    SURFSK_automatic_join: BoolProperty(
                name="Automatic join",
                description="Join automatically vertices of either surfaces "
                            "generated by crosshatching, or from the borders of closed shapes",
                default=True
                )
    SURFSK_loops_on_strokes: BoolProperty(
                name="Loops on strokes",
                description="Make the loops match the paths of the strokes",
                default=True
                )
    SURFSK_precision: IntProperty(
                name="Precision",
                description="Precision level of the surface calculation",
                default=2,
                min=1,
                max=100
                )
    SURFSK_mesh: PointerProperty(
                name="Mesh of BSurface",
                type=bpy.types.Object,
                description="Mesh of BSurface",
                update=update_mesh,
                )
    SURFSK_gpencil: PointerProperty(
                name="GreasePencil object",
                type=bpy.types.Object,
                description="GreasePencil object",
                update=update_gpencil,
                )
    SURFSK_curve: PointerProperty(
                name="Curve object",
                type=bpy.types.Object,
                description="Curve object",
                update=update_curve,
                )
    SURFSK_mesh_color: FloatVectorProperty(
                name="Mesh color",
                default=(1.0, 0.0, 0.0, 0.3),
                size=4,
                subtype="COLOR",
                min=0,
                max=1,
                update=update_color,
                description="Mesh color",
                )
    SURFSK_Shrinkwrap_offset: FloatProperty(
                name="Shrinkwrap offset",
                default=0.01,
                precision=3,
                description="Distance to keep from the target",
                update=update_Shrinkwrap_offset,
                )
    SURFSK_in_front: BoolProperty(
                name="In Front",
                description="Make the object draw in front of others",
                default=False,
                update=update_in_front,
                )
    SURFSK_show_wire: BoolProperty(
                name="Show wire",
                description="Add the object’s wireframe over solid drawing",
                default=False,
                update=update_show_wire,
                )
    SURFSK_shade_smooth: BoolProperty(
                name="Shade smooth",
                description="Render and display faces smooth, using interpolated Vertex Normals",
                default=False,
                update=update_shade_smooth,
                )

classes = (
    MESH_OT_SURFSK_init,
    MESH_OT_SURFSK_add_modifiers,
    MESH_OT_SURFSK_add_surface,
    MESH_OT_SURFSK_edit_surface,
    GPENCIL_OT_SURFSK_add_strokes,
    GPENCIL_OT_SURFSK_edit_strokes,
    GPENCIL_OT_SURFSK_strokes_to_curves,
    GPENCIL_OT_SURFSK_annotation_to_curves,
    GPENCIL_OT_SURFSK_add_annotation,
    CURVE_OT_SURFSK_edit_curve,
    CURVE_OT_SURFSK_reorder_splines,
    CURVE_OT_SURFSK_first_points,
    BsurfPreferences,
    BsurfacesProps
    )

def register():
    for cls in classes:
        bpy.utils.register_class(cls)

    for panel in panels:
        bpy.utils.register_class(panel)

    bpy.types.Scene.bsurfaces = PointerProperty(type=BsurfacesProps)
    update_panel(None, bpy.context)

def unregister():
    for panel in panels:
        bpy.utils.unregister_class(panel)

    for cls in classes:
        bpy.utils.unregister_class(cls)

    del bpy.types.Scene.bsurfaces

if __name__ == "__main__":
    register()