diff --git a/src/Autoload/Global.gd b/src/Autoload/Global.gd
index 314b0b012..21a04d459 100644
--- a/src/Autoload/Global.gd
+++ b/src/Autoload/Global.gd
@@ -71,7 +71,7 @@ enum EffectsMenu {
 	SHADER
 }
 ## Enumeration of items present in the Select Menu.
-enum SelectMenu { SELECT_ALL, CLEAR_SELECTION, INVERT, TILE_MODE }
+enum SelectMenu { SELECT_ALL, CLEAR_SELECTION, INVERT, TILE_MODE, MODIFY }
 ## Enumeration of items present in the Help Menu.
 enum HelpMenu {
 	VIEW_SPLASH_SCREEN,
diff --git a/src/Classes/SelectionMap.gd b/src/Classes/SelectionMap.gd
index 649a73299..cfb9591ee 100644
--- a/src/Classes/SelectionMap.gd
+++ b/src/Classes/SelectionMap.gd
@@ -1,7 +1,8 @@
 class_name SelectionMap
 extends Image
 
-var invert_shader := preload("res://src/Shaders/Effects/Invert.gdshader")
+const INVERT_SHADER := preload("res://src/Shaders/Effects/Invert.gdshader")
+const OUTLINE_INLINE_SHADER := preload("res://src/Shaders/Effects/OutlineInline.gdshader")
 
 
 func is_pixel_selected(pixel: Vector2i, calculate_offset := true) -> bool:
@@ -87,8 +88,7 @@ func clear() -> void:
 func invert() -> void:
 	var params := {"red": true, "green": true, "blue": true, "alpha": true}
 	var gen := ShaderImageEffect.new()
-	gen.generate_image(self, invert_shader, params, get_size())
-	self.convert(Image.FORMAT_LA8)
+	gen.generate_image(self, INVERT_SHADER, params, get_size())
 
 
 ## Returns a copy of itself that is cropped to [param size].
@@ -183,3 +183,24 @@ func resize_bitmap_values(
 	if new_bitmap_size != size:
 		crop(new_bitmap_size.x, new_bitmap_size.y)
 	blit_rect(smaller_image, Rect2i(Vector2i.ZERO, new_bitmap_size), dst)
+
+
+func expand(width: int, pattern: int) -> void:
+	var params := {
+		"color": Color(1, 1, 1, 1),
+		"width": width,
+		"pattern": pattern,
+	}
+	var gen := ShaderImageEffect.new()
+	gen.generate_image(self, OUTLINE_INLINE_SHADER, params, get_size())
+
+
+func shrink(width: int, pattern: int) -> void:
+	var params := {
+		"color": Color(0),
+		"inside": true,
+		"width": width,
+		"pattern": pattern,
+	}
+	var gen := ShaderImageEffect.new()
+	gen.generate_image(self, OUTLINE_INLINE_SHADER, params, get_size())
diff --git a/src/Classes/ShaderImageEffect.gd b/src/Classes/ShaderImageEffect.gd
index 04604fb0f..3a38b75e7 100644
--- a/src/Classes/ShaderImageEffect.gd
+++ b/src/Classes/ShaderImageEffect.gd
@@ -54,7 +54,7 @@ func generate_image(img: Image, shader: Shader, params: Dictionary, size: Vector
 	RenderingServer.free_rid(ci_rid)
 	RenderingServer.free_rid(mat_rid)
 	RenderingServer.free_rid(texture)
-	viewport_texture.convert(Image.FORMAT_RGBA8)
+	viewport_texture.convert(img.get_format())
 	img.copy_from(viewport_texture)
 	if resized_width:
 		img.crop(img.get_width() - 1, img.get_height())
diff --git a/src/Shaders/Effects/OutlineInline.gdshader b/src/Shaders/Effects/OutlineInline.gdshader
index 0eaa038a3..88325aa77 100644
--- a/src/Shaders/Effects/OutlineInline.gdshader
+++ b/src/Shaders/Effects/OutlineInline.gdshader
@@ -44,7 +44,12 @@ void fragment() {
 
 	if ((output.a > 0.0) == inside && has_contrary_neighbour(UV, TEXTURE_PIXEL_SIZE, TEXTURE)) {
 		output.rgb = inside ? mix(output.rgb, color.rgb, color.a) : color.rgb;
-		output.a += (1.0 - output.a) * color.a;
+		if (is_zero_approx(color.a)) {
+			output.a = color.a;
+		}
+		else {
+			output.a += (1.0 - output.a) * color.a;
+		}
 	}
 
 	COLOR = mix(original_color, output, selection_color.a);
diff --git a/src/UI/Dialogs/ModifySelection.gd b/src/UI/Dialogs/ModifySelection.gd
new file mode 100644
index 000000000..d0195d083
--- /dev/null
+++ b/src/UI/Dialogs/ModifySelection.gd
@@ -0,0 +1,39 @@
+extends ConfirmationDialog
+
+enum Types { EXPAND, SHRINK }
+
+@export var type := Types.EXPAND:
+	set(value):
+		type = value
+		if type == Types.EXPAND:
+			title = "Expand Selection"
+		else:
+			title = "Shrink Selection"
+
+@onready var width_slider: ValueSlider = $GridContainer/WidthSlider
+@onready var brush_option_button: OptionButton = $GridContainer/BrushOptionButton
+@onready var selection_node := Global.canvas.selection
+
+
+func _on_visibility_changed() -> void:
+	if not visible:
+		Global.dialog_open(false)
+
+
+func _on_confirmed() -> void:
+	var project := Global.current_project
+	if !project.has_selection:
+		return
+	selection_node.transform_content_confirm()
+	var undo_data_tmp := selection_node.get_undo_data(false)
+	var width: int = width_slider.value
+	var brush := brush_option_button.selected
+	project.selection_map.crop(project.size.x, project.size.y)
+	if type == Types.EXPAND:
+		project.selection_map.expand(width, brush)
+	else:
+		project.selection_map.shrink(width, brush)
+	selection_node.big_bounding_rectangle = project.selection_map.get_used_rect()
+	project.selection_offset = Vector2.ZERO
+	selection_node.commit_undo("Modify Selection", undo_data_tmp)
+	selection_node.queue_redraw()
diff --git a/src/UI/Dialogs/ModifySelection.tscn b/src/UI/Dialogs/ModifySelection.tscn
new file mode 100644
index 000000000..27a38debd
--- /dev/null
+++ b/src/UI/Dialogs/ModifySelection.tscn
@@ -0,0 +1,60 @@
+[gd_scene load_steps=3 format=3 uid="uid://wcbpnsm7gptu"]
+
+[ext_resource type="Script" path="res://src/UI/Nodes/ValueSlider.gd" id="1_3jelw"]
+[ext_resource type="Script" path="res://src/UI/Dialogs/ModifySelection.gd" id="1_w6rs7"]
+
+[node name="ModifySelection" type="ConfirmationDialog"]
+title = "Expand selection"
+position = Vector2i(0, 36)
+size = Vector2i(260, 130)
+script = ExtResource("1_w6rs7")
+
+[node name="GridContainer" type="GridContainer" parent="."]
+offset_left = 8.0
+offset_top = 8.0
+offset_right = 252.0
+offset_bottom = 81.0
+columns = 2
+
+[node name="WidthLabel" type="Label" parent="GridContainer"]
+layout_mode = 2
+size_flags_horizontal = 3
+text = "Width:"
+
+[node name="WidthSlider" type="TextureProgressBar" parent="GridContainer"]
+custom_minimum_size = Vector2(0, 24)
+layout_mode = 2
+size_flags_horizontal = 3
+focus_mode = 2
+mouse_default_cursor_shape = 2
+theme_type_variation = &"ValueSlider"
+min_value = 1.0
+max_value = 25.0
+value = 1.0
+allow_greater = true
+nine_patch_stretch = true
+stretch_margin_left = 3
+stretch_margin_top = 3
+stretch_margin_right = 3
+stretch_margin_bottom = 3
+script = ExtResource("1_3jelw")
+
+[node name="BrushLabel" type="Label" parent="GridContainer"]
+layout_mode = 2
+size_flags_horizontal = 3
+text = "Brush:"
+
+[node name="BrushOptionButton" type="OptionButton" parent="GridContainer"]
+layout_mode = 2
+size_flags_horizontal = 3
+mouse_default_cursor_shape = 2
+selected = 0
+item_count = 3
+popup/item_0/text = "Diamond"
+popup/item_1/text = "Circle"
+popup/item_1/id = 1
+popup/item_2/text = "Square"
+popup/item_2/id = 2
+
+[connection signal="confirmed" from="." to="." method="_on_confirmed"]
+[connection signal="visibility_changed" from="." to="." method="_on_visibility_changed"]
diff --git a/src/UI/TopMenuContainer/TopMenuContainer.gd b/src/UI/TopMenuContainer/TopMenuContainer.gd
index b95524f00..a83257d54 100644
--- a/src/UI/TopMenuContainer/TopMenuContainer.gd
+++ b/src/UI/TopMenuContainer/TopMenuContainer.gd
@@ -17,6 +17,7 @@ var zen_mode := false
 var new_image_dialog := Dialog.new("res://src/UI/Dialogs/CreateNewImage.tscn")
 var project_properties_dialog := Dialog.new("res://src/UI/Dialogs/ProjectProperties.tscn")
 var preferences_dialog := Dialog.new("res://src/Preferences/PreferencesDialog.tscn")
+var modify_selection := Dialog.new("res://src/UI/Dialogs/ModifySelection.tscn")
 var offset_image_dialog := Dialog.new("res://src/UI/Dialogs/ImageEffects/OffsetImage.tscn")
 var scale_image_dialog := Dialog.new("res://src/UI/Dialogs/ImageEffects/ScaleImage.tscn")
 var resize_canvas_dialog := Dialog.new("res://src/UI/Dialogs/ImageEffects/ResizeCanvas.tscn")
@@ -54,6 +55,7 @@ var about_dialog := Dialog.new("res://src/UI/Dialogs/AboutDialog.tscn")
 
 @onready var greyscale_vision: ColorRect = main_ui.find_child("GreyscaleVision")
 @onready var tile_mode_submenu := PopupMenu.new()
+@onready var selection_modify_submenu := PopupMenu.new()
 @onready var snap_to_submenu := PopupMenu.new()
 @onready var panels_submenu := PopupMenu.new()
 @onready var layouts_submenu := PopupMenu.new()
@@ -440,17 +442,29 @@ func _setup_select_menu() -> void:
 		"All": "select_all",
 		"Clear": "clear_selection",
 		"Invert": "invert_selection",
-		"Tile Mode": ""
+		"Tile Mode": "",
+		"Modify": ""
 	}
 	for i in select_menu_items.size():
 		var item: String = select_menu_items.keys()[i]
 		if item == "Tile Mode":
 			select_menu.add_check_item(item, i)
+		elif item == "Modify":
+			_setup_selection_modify_submenu(item)
 		else:
 			_set_menu_shortcut(select_menu_items[item], select_menu, i, item)
 	select_menu.id_pressed.connect(select_menu_id_pressed)
 
 
+func _setup_selection_modify_submenu(item: String) -> void:
+	selection_modify_submenu.set_name("selection_modify_submenu")
+	selection_modify_submenu.add_item("Expand")
+	selection_modify_submenu.add_item("Shrink")
+	selection_modify_submenu.id_pressed.connect(_selection_modify_submenu_id_pressed)
+	select_menu.add_child(selection_modify_submenu)
+	select_menu.add_submenu_item(item, selection_modify_submenu.get_name())
+
+
 func _setup_help_menu() -> void:
 	# Order as in Global.HelpMenu enum
 	var help_menu_items := {
@@ -667,6 +681,11 @@ func _tile_mode_submenu_id_pressed(id: Tiles.MODE) -> void:
 	get_tree().current_scene.tile_mode_offsets_dialog.change_mask()
 
 
+func _selection_modify_submenu_id_pressed(id: int) -> void:
+	modify_selection.popup()
+	modify_selection.node.type = id
+
+
 func _snap_to_submenu_id_pressed(id: int) -> void:
 	if id == 0:
 		Global.snap_to_rectangular_grid_boundary = !Global.snap_to_rectangular_grid_boundary