Add collaborative surface drawing

2026-08-01 22:55:39 -04:00 · 2026-08-01 22:55:39 -04:00 · b92c55562d
commit b92c55562d
parent 6b7ef4c11b
28 changed files with 3504 additions and 2 deletions
--- a/drawing/surface_drawing_canvas.gd
+++ b/drawing/surface_drawing_canvas.gd
@ -0,0 +1,393 @@
+class_name SurfaceDrawingCanvas
+extends Node3D
+
+const GUIDED_PIXEL_FILL: float = 0.88
+const FINISHED_PIXEL_FILL: float = 1.0
+const SURFACE_OFFSET: float = 0.012
+const LAYER_OFFSET_STEP: float = 0.0002
+const SURFACE_SAMPLE_DEPTH: float = 0.45
+
+var canvas_id: String = ""
+var grid_width: int = 0
+var grid_height: int = 0
+var cell_size: float = 0.0
+var revision: int = -1
+var creator_fingerprint: String = ""
+
+var _surface_origin: Vector3
+var _surface_normal: Vector3
+var _surface_tangent: Vector3
+var _surface_bitangent: Vector3
+var _cells: Dictionary[int, Dictionary] = {}
+var _color_meshes: Dictionary[StringName, MultiMeshInstance3D] = {}
+var _grid_instance: MeshInstance3D
+var _guide_visible: bool = true
+var _finalized: bool = false
+var _layer: int = 0
+var _stencil_requested_visible: bool = false
+var _relationships: PlayerRelationshipStore
+var _solid_surface_mask: int = 1
+
+
+func setup(
+	data: Dictionary,
+	relationships: PlayerRelationshipStore,
+	solid_surface_mask: int = 1,
+) -> bool:
+	if not SurfaceDrawingProtocol.validate_canvas_state(data):
+		return false
+	canvas_id = str(data["canvas_id"])
+	grid_width = int(data["width"])
+	grid_height = int(data["height"])
+	cell_size = float(data["cell_size"])
+	revision = int(data["revision"])
+	creator_fingerprint = str(data["creator_fingerprint"])
+	_finalized = bool(data.get("finalized", false))
+	_guide_visible = bool(data.get("guide_visible", true)) and not _finalized
+	_layer = int(data.get("layer", 0))
+	_surface_origin = SurfaceDrawingProtocol.array_to_vector(data["origin"])
+	_surface_normal = SurfaceDrawingProtocol.array_to_vector(
+		data["normal"]
+	).normalized()
+	_surface_tangent = SurfaceDrawingProtocol.array_to_vector(
+		data["tangent"]
+	)
+	_surface_tangent = (
+		_surface_tangent - _surface_normal * _surface_tangent.dot(_surface_normal)
+	).normalized()
+	if _surface_normal.is_zero_approx() or _surface_tangent.is_zero_approx():
+		return false
+	_surface_bitangent = _surface_normal.cross(_surface_tangent).normalized()
+	_relationships = relationships
+	_solid_surface_mask = solid_surface_mask
+	_cells.clear()
+	for cell_value: Variant in data["cells"]:
+		var cell: Dictionary = cell_value
+		_cells[_cell_key(int(cell["x"]), int(cell["y"]))] = cell.duplicate(true)
+	_build_grid()
+	_rebuild_pixels()
+	return true
+
+
+func apply_update(data: Dictionary) -> bool:
+	if (
+		not SurfaceDrawingProtocol.validate_canvas_update(data)
+		or str(data["canvas_id"]) != canvas_id
+		or int(data["revision"]) <= revision
+	):
+		return false
+	for edit_value: Variant in data["edits"]:
+		var edit: Dictionary = edit_value
+		var key: int = _cell_key(int(edit["x"]), int(edit["y"]))
+		if str(edit["color_id"]).is_empty():
+			_cells.erase(key)
+		else:
+			_cells[key] = edit.duplicate(true)
+	revision = int(data["revision"])
+	_rebuild_pixels()
+	return true
+
+
+func apply_guide_update(data: Dictionary) -> bool:
+	if (
+		not SurfaceDrawingProtocol.validate_guide_update(data)
+		or str(data["canvas_id"]) != canvas_id
+		or int(data["revision"]) <= revision
+	):
+		return false
+	revision = int(data["revision"])
+	_finalized = bool(data["finalized"])
+	_guide_visible = bool(data["guide_visible"]) and not _finalized
+	_refresh_grid_visibility()
+	_rebuild_pixels()
+	return true
+
+
+func refresh_relationship_visibility() -> void:
+	_rebuild_pixels()
+
+
+func set_stencil_visible(should_be_visible: bool) -> void:
+	_stencil_requested_visible = should_be_visible
+	_refresh_grid_visibility()
+
+
+func is_guide_visible() -> bool:
+	return _guide_visible
+
+
+func is_finalized() -> bool:
+	return _finalized
+
+
+func get_rendered_pixel_size() -> float:
+	return cell_size * _pixel_fill()
+
+
+func contains_world_point(world_point: Vector3, tolerance: float = 0.3) -> bool:
+	var relative: Vector3 = world_point - _surface_origin
+	if absf(relative.dot(_surface_normal)) > tolerance:
+		return false
+	var half_width: float = float(grid_width) * cell_size * 0.5
+	var half_height: float = float(grid_height) * cell_size * 0.5
+	var horizontal: float = relative.dot(_surface_tangent)
+	var vertical: float = relative.dot(_surface_bitangent)
+	return (
+		horizontal >= -half_width
+		and horizontal < half_width
+		and vertical >= -half_height
+		and vertical < half_height
+	)
+
+
+func get_surface_plane_distance(world_point: Vector3) -> float:
+	return absf((world_point - _surface_origin).dot(_surface_normal))
+
+
+func cell_at_world_point(world_point: Vector3) -> Vector2i:
+	var relative: Vector3 = world_point - _surface_origin
+	var half_width: float = float(grid_width) * cell_size * 0.5
+	var half_height: float = float(grid_height) * cell_size * 0.5
+	var x: int = floori(
+		(relative.dot(_surface_tangent) + half_width) / cell_size
+	)
+	var y: int = floori(
+		(relative.dot(_surface_bitangent) + half_height) / cell_size
+	)
+	if x < 0 or x >= grid_width or y < 0 or y >= grid_height:
+		return Vector2i(-1, -1)
+	return Vector2i(x, y)
+
+
+func get_cell_world_position(x: int, y: int) -> Vector3:
+	var horizontal: float = (
+		(float(x) + 0.5 - float(grid_width) * 0.5) * cell_size
+	)
+	var vertical: float = (
+		(float(y) + 0.5 - float(grid_height) * 0.5) * cell_size
+	)
+	return (
+		_surface_origin
+		+ _surface_tangent * horizontal
+		+ _surface_bitangent * vertical
+	)
+
+
+func get_surface_normal() -> Vector3:
+	return _surface_normal
+
+
+func get_surface_tangent() -> Vector3:
+	return _surface_tangent
+
+
+func get_surface_bitangent() -> Vector3:
+	return _surface_bitangent
+
+
+func get_cell_surface_transform(
+	x: int,
+	y: int,
+	fill: float = 0.94,
+) -> Transform3D:
+	if x < 0 or x >= grid_width or y < 0 or y >= grid_height:
+		return Transform3D.IDENTITY
+	return global_transform * _sample_cell_transform(x, y, fill)
+
+
+func get_authoritative_cells() -> Array[Dictionary]:
+	var result: Array[Dictionary] = []
+	for value: Dictionary in _cells.values():
+		result.append(value.duplicate(true))
+	result.sort_custom(func(a: Dictionary, b: Dictionary) -> bool:
+		var a_key: int = _cell_key(int(a["x"]), int(a["y"]))
+		var b_key: int = _cell_key(int(b["x"]), int(b["y"]))
+		return a_key < b_key
+	)
+	return result
+
+
+func _build_grid() -> void:
+	if _grid_instance != null:
+		_grid_instance.queue_free()
+	var immediate := ImmediateMesh.new()
+	var material := StandardMaterial3D.new()
+	material.shading_mode = BaseMaterial3D.SHADING_MODE_UNSHADED
+	material.transparency = BaseMaterial3D.TRANSPARENCY_ALPHA
+	material.albedo_color = Color(0.75, 0.94, 0.96, 0.38)
+	material.no_depth_test = false
+	immediate.surface_begin(Mesh.PRIMITIVE_LINES, material)
+	var half_width: float = float(grid_width) * cell_size * 0.5
+	var half_height: float = float(grid_height) * cell_size * 0.5
+	for x: int in range(grid_width + 1):
+		var horizontal: float = -half_width + float(x) * cell_size
+		for y_segment: int in range(grid_height):
+			var start_vertical: float = (
+				-half_height + float(y_segment) * cell_size
+			)
+			_add_grid_vertex(
+				immediate,
+				_surface_origin + _surface_tangent * horizontal
+					+ _surface_bitangent * start_vertical,
+			)
+			_add_grid_vertex(
+				immediate,
+				_surface_origin + _surface_tangent * horizontal
+					+ _surface_bitangent * (start_vertical + cell_size),
+			)
+	for y: int in range(grid_height + 1):
+		var vertical: float = -half_height + float(y) * cell_size
+		for x_segment: int in range(grid_width):
+			var start_horizontal: float = (
+				-half_width + float(x_segment) * cell_size
+			)
+			_add_grid_vertex(
+				immediate,
+				_surface_origin + _surface_tangent * start_horizontal
+					+ _surface_bitangent * vertical,
+			)
+			_add_grid_vertex(
+				immediate,
+				_surface_origin
+					+ _surface_tangent * (start_horizontal + cell_size)
+					+ _surface_bitangent * vertical,
+			)
+	immediate.surface_end()
+	_grid_instance = MeshInstance3D.new()
+	_grid_instance.name = "PixelGrid"
+	_grid_instance.mesh = immediate
+	_grid_instance.cast_shadow = GeometryInstance3D.SHADOW_CASTING_SETTING_OFF
+	add_child(_grid_instance)
+	_refresh_grid_visibility()
+
+
+func _add_grid_vertex(immediate: ImmediateMesh, world_point: Vector3) -> void:
+	var sampled: Dictionary = _sample_surface(world_point)
+	var point: Vector3 = sampled["position"]
+	var normal: Vector3 = sampled["normal"]
+	immediate.surface_add_vertex(
+		to_local(point + normal * (_surface_offset() * 1.5))
+	)
+
+
+func _rebuild_pixels() -> void:
+	for instance: MultiMeshInstance3D in _color_meshes.values():
+		instance.queue_free()
+	_color_meshes.clear()
+	var cells_by_color: Dictionary[StringName, Array] = {}
+	for cell: Dictionary in _cells.values():
+		var author_fingerprint: String = str(cell.get("author_fingerprint", ""))
+		if (
+			_relationships != null
+			and _relationships.is_blocked(author_fingerprint)
+		):
+			continue
+		var color_id := StringName(str(cell.get("color_id", "")))
+		if not SurfaceDrawingPalette.has_color(color_id):
+			continue
+		var color_cells: Array = cells_by_color.get(color_id, [])
+		color_cells.append(cell)
+		cells_by_color[color_id] = color_cells
+	for color_id: StringName in cells_by_color:
+		_create_color_mesh(color_id, cells_by_color[color_id])
+
+
+func _create_color_mesh(color_id: StringName, cells: Array) -> void:
+	if cells.is_empty():
+		return
+	var quad := QuadMesh.new()
+	quad.size = Vector2.ONE
+	var material := StandardMaterial3D.new()
+	material.shading_mode = BaseMaterial3D.SHADING_MODE_UNSHADED
+	material.albedo_color = SurfaceDrawingPalette.get_color(color_id)
+	material.cull_mode = BaseMaterial3D.CULL_DISABLED
+	quad.material = material
+	var multimesh := MultiMesh.new()
+	multimesh.transform_format = MultiMesh.TRANSFORM_3D
+	multimesh.mesh = quad
+	multimesh.instance_count = cells.size()
+	for cell_index: int in range(cells.size()):
+		var cell: Dictionary = cells[cell_index]
+		multimesh.set_instance_transform(
+			cell_index,
+			_sample_cell_transform(
+				int(cell["x"]), int(cell["y"]), _pixel_fill()
+			),
+		)
+	var instance := MultiMeshInstance3D.new()
+	instance.name = "Pixels_%s" % color_id
+	instance.multimesh = multimesh
+	instance.cast_shadow = GeometryInstance3D.SHADOW_CASTING_SETTING_OFF
+	add_child(instance)
+	_color_meshes[color_id] = instance
+
+
+func _sample_cell_transform(
+	x: int,
+	y: int,
+	fill: float = GUIDED_PIXEL_FILL,
+) -> Transform3D:
+	var expected: Vector3 = get_cell_world_position(x, y)
+	var sampled: Dictionary = _sample_surface(expected)
+	var point: Vector3 = sampled["position"]
+	var normal: Vector3 = sampled["normal"]
+	var tangent: Vector3 = (
+		_surface_tangent - normal * _surface_tangent.dot(normal)
+	).normalized()
+	if tangent.is_zero_approx():
+		tangent = normal.cross(Vector3.UP).normalized()
+	if tangent.is_zero_approx():
+		tangent = Vector3.RIGHT
+	var bitangent: Vector3 = normal.cross(tangent).normalized()
+	var pixel_size: float = cell_size * clampf(fill, 0.05, 1.0)
+	var basis := Basis(
+		tangent * pixel_size,
+		bitangent * pixel_size,
+		normal,
+	)
+	return Transform3D(
+		basis,
+		to_local(point + normal * _surface_offset()),
+	)
+
+
+func _sample_surface(expected: Vector3) -> Dictionary:
+	var point: Vector3 = expected
+	var normal: Vector3 = _surface_normal
+	var world: World3D = get_world_3d()
+	if world != null:
+		var query := PhysicsRayQueryParameters3D.create(
+			expected + _surface_normal * SURFACE_SAMPLE_DEPTH,
+			expected - _surface_normal * SURFACE_SAMPLE_DEPTH,
+			_solid_surface_mask,
+		)
+		query.collide_with_areas = false
+		query.collide_with_bodies = true
+		var hit: Dictionary = world.direct_space_state.intersect_ray(query)
+		if (
+			not hit.is_empty()
+			and hit.get("collider") is StaticBody3D
+		):
+			var hit_normal: Vector3 = hit.get("normal", _surface_normal)
+			if hit_normal.dot(_surface_normal) >= 0.35:
+				var hit_position: Vector3 = hit.get("position", expected)
+				point = hit_position
+				normal = hit_normal.normalized()
+	return {"position": point, "normal": normal}
+
+
+func _pixel_fill() -> float:
+	return GUIDED_PIXEL_FILL if _guide_visible else FINISHED_PIXEL_FILL
+
+
+func _surface_offset() -> float:
+	return SURFACE_OFFSET + float(_layer) * LAYER_OFFSET_STEP
+
+
+func _refresh_grid_visibility() -> void:
+	if _grid_instance != null:
+		_grid_instance.visible = _stencil_requested_visible and _guide_visible
+
+
+func _cell_key(x: int, y: int) -> int:
+	return y * grid_width + x
--- a/drawing/surface_drawing_canvas.gd.uid
+++ b/drawing/surface_drawing_canvas.gd.uid
@ -0,0 +1 @@
+uid://cj2gq63l4mysl
--- a/drawing/surface_drawing_highlight.gdshader
+++ b/drawing/surface_drawing_highlight.gdshader
@ -0,0 +1,12 @@
+shader_type spatial;
+render_mode unshaded, cull_disabled, depth_draw_never;
+
+uniform sampler2D screen_texture : hint_screen_texture, repeat_disable, filter_nearest;
+
+
+void fragment() {
+	vec3 surface_color = textureLod(screen_texture, SCREEN_UV, 0.0).rgb;
+	vec3 inverted_color = vec3(1.0) - surface_color;
+	ALBEDO = inverted_color;
+	EMISSION = inverted_color;
+}
--- a/drawing/surface_drawing_highlight.gdshader.uid
+++ b/drawing/surface_drawing_highlight.gdshader.uid
@ -0,0 +1 @@
+uid://k82me23ylsos
--- a/drawing/surface_drawing_palette.gd
+++ b/drawing/surface_drawing_palette.gd
@ -0,0 +1,57 @@
+class_name SurfaceDrawingPalette
+extends RefCounted
+
+const DEFAULT_COLOR_ID: StringName = &"chalk_white"
+
+# Stable IDs are the network and future progression boundary. New colors may
+# be appended without changing drawings created with an older palette.
+const COLORS: Array[Dictionary] = [
+	{"id": &"chalk_white", "name": "Chalk white", "color": Color("f5eed9")},
+	{"id": &"ocean_teal", "name": "Ocean teal", "color": Color("35b9c7")},
+	{"id": &"coral", "name": "Coral", "color": Color("ef5b62")},
+	{"id": &"sunny", "name": "Sunny", "color": Color("ffd166")},
+	{"id": &"leaf", "name": "Leaf", "color": Color("46c878")},
+	{"id": &"blue", "name": "Blue", "color": Color("5596f6")},
+	{"id": &"violet", "name": "Violet", "color": Color("b176e8")},
+	{"id": &"charcoal", "name": "Charcoal", "color": Color("28251f")},
+]
+
+
+static func has_color(color_id: StringName) -> bool:
+	return not get_entry(color_id).is_empty()
+
+
+static func get_entry(color_id: StringName) -> Dictionary:
+	for entry: Dictionary in COLORS:
+		if StringName(entry.get("id", &"")) == color_id:
+			return entry
+	return {}
+
+
+static func get_color(color_id: StringName) -> Color:
+	var entry: Dictionary = get_entry(color_id)
+	var value: Variant = entry.get("color", Color.WHITE)
+	return value if typeof(value) == TYPE_COLOR else Color.WHITE
+
+
+static func get_display_name(color_id: StringName) -> String:
+	return str(get_entry(color_id).get("name", "Unknown"))
+
+
+static func get_color_ids() -> Array[StringName]:
+	var result: Array[StringName] = []
+	for entry: Dictionary in COLORS:
+		result.append(StringName(entry.get("id", &"")))
+	return result
+
+
+static func filter_unlocked_ids(
+	unlocked_ids: Array[StringName],
+) -> Array[StringName]:
+	var result: Array[StringName] = []
+	for color_id: StringName in unlocked_ids:
+		if has_color(color_id) and color_id not in result:
+			result.append(color_id)
+	if result.is_empty():
+		result.append(DEFAULT_COLOR_ID)
+	return result
--- a/drawing/surface_drawing_palette.gd.uid
+++ b/drawing/surface_drawing_palette.gd.uid
@ -0,0 +1 @@
+uid://bk16cl6f3v887
--- a/drawing/surface_drawing_placement.gd
+++ b/drawing/surface_drawing_placement.gd
@ -0,0 +1,82 @@
+class_name SurfaceDrawingPlacement
+extends RefCounted
+
+const SNAP_DISTANCE: float = 0.55
+const SNAP_PLANE_DISTANCE: float = 0.18
+const SNAP_NORMAL_DOT: float = 0.94
+
+
+static func resolve(
+	origin: Vector3,
+	normal: Vector3,
+	fallback_tangent: Vector3,
+	canvas_states: Array[Dictionary],
+) -> Dictionary:
+	var surface_normal: Vector3 = normal.normalized()
+	var surface_tangent: Vector3 = _projected_tangent(
+		fallback_tangent, surface_normal
+	)
+	var best: Dictionary = {
+		"origin": origin,
+		"normal": surface_normal,
+		"tangent": surface_tangent,
+		"snapped": false,
+	}
+	var nearest_distance: float = SNAP_DISTANCE
+	for state: Dictionary in canvas_states:
+		if not SurfaceDrawingProtocol.validate_canvas_state(state):
+			continue
+		var anchor_normal: Vector3 = SurfaceDrawingProtocol.array_to_vector(
+			state["normal"]
+		).normalized()
+		if anchor_normal.dot(surface_normal) < SNAP_NORMAL_DOT:
+			continue
+		var anchor_origin: Vector3 = SurfaceDrawingProtocol.array_to_vector(
+			state["origin"]
+		)
+		var relative: Vector3 = origin - anchor_origin
+		if absf(relative.dot(anchor_normal)) > SNAP_PLANE_DISTANCE:
+			continue
+		var anchor_tangent: Vector3 = _projected_tangent(
+			SurfaceDrawingProtocol.array_to_vector(state["tangent"]),
+			anchor_normal,
+		)
+		var anchor_bitangent: Vector3 = anchor_normal.cross(
+			anchor_tangent
+		).normalized()
+		var width: float = float(state["width"]) * float(state["cell_size"])
+		var height: float = float(state["height"]) * float(state["cell_size"])
+		if width <= 0.0 or height <= 0.0:
+			continue
+		var horizontal_step: int = roundi(relative.dot(anchor_tangent) / width)
+		var vertical_step: int = roundi(relative.dot(anchor_bitangent) / height)
+		if horizontal_step == 0 and vertical_step == 0:
+			continue
+		var candidate: Vector3 = (
+			anchor_origin
+			+ anchor_tangent * float(horizontal_step) * width
+			+ anchor_bitangent * float(vertical_step) * height
+		)
+		var distance: float = candidate.distance_to(origin)
+		if distance > nearest_distance:
+			continue
+		nearest_distance = distance
+		best = {
+			"origin": candidate,
+			"normal": anchor_normal,
+			"tangent": anchor_tangent,
+			"snapped": true,
+		}
+	return best
+
+
+static func _projected_tangent(
+	value: Vector3,
+	normal: Vector3,
+) -> Vector3:
+	var tangent: Vector3 = (value - normal * value.dot(normal)).normalized()
+	if tangent.is_zero_approx():
+		tangent = Vector3.UP.cross(normal).normalized()
+	if tangent.is_zero_approx():
+		tangent = Vector3.RIGHT
+	return tangent
--- a/drawing/surface_drawing_placement.gd.uid
+++ b/drawing/surface_drawing_placement.gd.uid
@ -0,0 +1 @@
+uid://dgb7stq5k85d0
--- a/drawing/surface_drawing_protocol.gd
+++ b/drawing/surface_drawing_protocol.gd
@ -0,0 +1,243 @@
+class_name SurfaceDrawingProtocol
+extends RefCounted
+
+const CAPABILITY: StringName = &"surface_drawing_v1"
+const RELIABLE_CHANNEL: int = NetworkProtocol.ITEM_RELIABLE_CHANNEL
+const GRID_WIDTH: int = 32
+const GRID_HEIGHT: int = 32
+const CELL_SIZE: float = 0.075
+const MAX_ACTIVE_CANVASES: int = 24
+const MAX_CANVASES: int = 48
+const MAX_EDITS_PER_REQUEST: int = 16
+const MAX_CANVAS_ID_LENGTH: int = 64
+const MAX_REQUEST_ID_LENGTH: int = 64
+const MAX_STROKE_ID_LENGTH: int = 64
+const MAX_SESSION_ID_LENGTH: int = 96
+const MAX_COORDINATE: float = 10000.0
+
+
+static func validate_canvas_request(data: Variant) -> bool:
+	if typeof(data) != TYPE_DICTIONARY:
+		return false
+	var value: Dictionary = data
+	return (
+		_valid_common(value)
+		and _valid_vector(value.get("origin"))
+		and _valid_vector(value.get("normal"))
+		and _valid_vector(value.get("tangent"))
+		and typeof(value.get("width")) == TYPE_INT
+		and int(value["width"]) == GRID_WIDTH
+		and typeof(value.get("height")) == TYPE_INT
+		and int(value["height"]) == GRID_HEIGHT
+		and typeof(value.get("cell_size")) in [TYPE_FLOAT, TYPE_INT]
+		and is_equal_approx(float(value["cell_size"]), CELL_SIZE)
+	)
+
+
+static func validate_edit_request(data: Variant) -> bool:
+	if typeof(data) != TYPE_DICTIONARY:
+		return false
+	var value: Dictionary = data
+	if (
+		not _valid_common(value)
+		or typeof(value.get("canvas_id")) != TYPE_STRING
+		or str(value["canvas_id"]).is_empty()
+		or str(value["canvas_id"]).length() > MAX_CANVAS_ID_LENGTH
+		or not _valid_stroke_id(value.get("stroke_id"))
+		or typeof(value.get("edits")) != TYPE_ARRAY
+	):
+		return false
+	var edits: Array = value["edits"]
+	if edits.is_empty() or edits.size() > MAX_EDITS_PER_REQUEST:
+		return false
+	for edit_value: Variant in edits:
+		if not validate_cell_edit(edit_value):
+			return false
+	return true
+
+
+static func validate_guide_request(data: Variant) -> bool:
+	if typeof(data) != TYPE_DICTIONARY:
+		return false
+	var value: Dictionary = data
+	return (
+		_valid_common(value)
+		and _valid_canvas_id(value.get("canvas_id"))
+		and typeof(value.get("guide_visible")) == TYPE_BOOL
+		and typeof(value.get("finalized")) == TYPE_BOOL
+	)
+
+
+static func validate_undo_request(data: Variant) -> bool:
+	if typeof(data) != TYPE_DICTIONARY:
+		return false
+	var value: Dictionary = data
+	return _valid_common(value) and _valid_stroke_id(value.get("stroke_id"))
+
+
+static func validate_canvas_state(data: Variant) -> bool:
+	if typeof(data) != TYPE_DICTIONARY:
+		return false
+	var value: Dictionary = data
+	if (
+		typeof(value.get("session_id")) != TYPE_STRING
+		or str(value["session_id"]).is_empty()
+		or str(value["session_id"]).length() > MAX_SESSION_ID_LENGTH
+		or typeof(value.get("canvas_id")) != TYPE_STRING
+		or str(value["canvas_id"]).is_empty()
+		or str(value["canvas_id"]).length() > MAX_CANVAS_ID_LENGTH
+		or not _valid_vector(value.get("origin"))
+		or not _valid_vector(value.get("normal"))
+		or not _valid_vector(value.get("tangent"))
+		or typeof(value.get("width")) != TYPE_INT
+		or int(value["width"]) != GRID_WIDTH
+		or typeof(value.get("height")) != TYPE_INT
+		or int(value["height"]) != GRID_HEIGHT
+		or typeof(value.get("cell_size")) not in [TYPE_FLOAT, TYPE_INT]
+		or not is_equal_approx(float(value["cell_size"]), CELL_SIZE)
+		or typeof(value.get("revision")) != TYPE_INT
+		or int(value["revision"]) < 0
+		or typeof(value.get("guide_visible", true)) != TYPE_BOOL
+		or typeof(value.get("finalized", false)) != TYPE_BOOL
+		or typeof(value.get("layer", 0)) != TYPE_INT
+		or int(value.get("layer", 0)) < 0
+		or not NetworkIdentityCrypto.valid_fingerprint(
+			value.get("creator_fingerprint")
+		)
+		or typeof(value.get("cells")) != TYPE_ARRAY
+	):
+		return false
+	var cells: Array = value["cells"]
+	if cells.size() > GRID_WIDTH * GRID_HEIGHT:
+		return false
+	for cell_value: Variant in cells:
+		if not validate_authoritative_cell(cell_value):
+			return false
+	return true
+
+
+static func validate_guide_update(data: Variant) -> bool:
+	if typeof(data) != TYPE_DICTIONARY:
+		return false
+	var value: Dictionary = data
+	return (
+		typeof(value.get("session_id")) == TYPE_STRING
+		and not str(value["session_id"]).is_empty()
+		and str(value["session_id"]).length() <= MAX_SESSION_ID_LENGTH
+		and _valid_canvas_id(value.get("canvas_id"))
+		and typeof(value.get("revision")) == TYPE_INT
+		and int(value["revision"]) >= 1
+		and typeof(value.get("guide_visible")) == TYPE_BOOL
+		and typeof(value.get("finalized")) == TYPE_BOOL
+	)
+
+
+static func validate_canvas_update(data: Variant) -> bool:
+	if typeof(data) != TYPE_DICTIONARY:
+		return false
+	var value: Dictionary = data
+	if (
+		typeof(value.get("session_id")) != TYPE_STRING
+		or str(value["session_id"]).is_empty()
+		or str(value["session_id"]).length() > MAX_SESSION_ID_LENGTH
+		or typeof(value.get("canvas_id")) != TYPE_STRING
+		or str(value["canvas_id"]).is_empty()
+		or str(value["canvas_id"]).length() > MAX_CANVAS_ID_LENGTH
+		or typeof(value.get("revision")) != TYPE_INT
+		or int(value["revision"]) < 1
+		or typeof(value.get("edits")) != TYPE_ARRAY
+	):
+		return false
+	var edits: Array = value["edits"]
+	if edits.is_empty() or edits.size() > MAX_EDITS_PER_REQUEST:
+		return false
+	for edit_value: Variant in edits:
+		if not validate_authoritative_cell(edit_value, true):
+			return false
+	return true
+
+
+static func validate_cell_edit(data: Variant) -> bool:
+	if typeof(data) != TYPE_DICTIONARY:
+		return false
+	var value: Dictionary = data
+	if (
+		typeof(value.get("x")) != TYPE_INT
+		or int(value["x"]) < 0
+		or int(value["x"]) >= GRID_WIDTH
+		or typeof(value.get("y")) != TYPE_INT
+		or int(value["y"]) < 0
+		or int(value["y"]) >= GRID_HEIGHT
+		or typeof(value.get("color_id")) not in [TYPE_STRING, TYPE_STRING_NAME]
+	):
+		return false
+	var color_id := StringName(str(value["color_id"]))
+	return color_id.is_empty() or SurfaceDrawingPalette.has_color(color_id)
+
+
+static func validate_authoritative_cell(
+	data: Variant,
+	allow_erased: bool = false,
+) -> bool:
+	if not validate_cell_edit(data):
+		return false
+	var value: Dictionary = data
+	var color_id := StringName(str(value["color_id"]))
+	if color_id.is_empty():
+		return allow_erased and str(value.get("author_fingerprint", "")).is_empty()
+	return NetworkIdentityCrypto.valid_fingerprint(
+		value.get("author_fingerprint")
+	)
+
+
+static func vector_to_array(value: Vector3) -> Array[float]:
+	return [value.x, value.y, value.z]
+
+
+static func array_to_vector(value: Variant) -> Vector3:
+	if not _valid_vector(value):
+		return Vector3.ZERO
+	var array: Array = value
+	return Vector3(float(array[0]), float(array[1]), float(array[2]))
+
+
+static func _valid_common(value: Dictionary) -> bool:
+	return (
+		typeof(value.get("request_id")) == TYPE_STRING
+		and not str(value["request_id"]).is_empty()
+		and str(value["request_id"]).length() <= MAX_REQUEST_ID_LENGTH
+		and typeof(value.get("session_id")) == TYPE_STRING
+		and not str(value["session_id"]).is_empty()
+		and str(value["session_id"]).length() <= MAX_SESSION_ID_LENGTH
+	)
+
+
+static func _valid_canvas_id(value: Variant) -> bool:
+	return (
+		typeof(value) == TYPE_STRING
+		and not str(value).is_empty()
+		and str(value).length() <= MAX_CANVAS_ID_LENGTH
+	)
+
+
+static func _valid_stroke_id(value: Variant) -> bool:
+	return (
+		typeof(value) == TYPE_STRING
+		and not str(value).is_empty()
+		and str(value).length() <= MAX_STROKE_ID_LENGTH
+	)
+
+
+static func _valid_vector(value: Variant) -> bool:
+	if typeof(value) != TYPE_ARRAY:
+		return false
+	var array: Array = value
+	if array.size() != 3:
+		return false
+	for component: Variant in array:
+		if typeof(component) not in [TYPE_FLOAT, TYPE_INT]:
+			return false
+		var number: float = float(component)
+		if not is_finite(number) or absf(number) > MAX_COORDINATE:
+			return false
+	return true
--- a/drawing/surface_drawing_protocol.gd.uid
+++ b/drawing/surface_drawing_protocol.gd.uid
@ -0,0 +1 @@
+uid://c258sivd8k8nc
--- a/fishing/fishing_spot.gd
+++ b/fishing/fishing_spot.gd
@ -271,6 +271,15 @@ func can_open_fishing_shop() -> bool:
 	)


+func can_use_surface_drawing() -> bool:
+	return (
+		_gameplay_input_enabled
+		and not _external_input_blocked
+		and state == FishingState.READY
+		and _active_player == null
+	)
+
+
 func is_ready_for_shop_transaction() -> bool:
 	return (
 		_can_use_shop_gameplay()
--- a/main/main.gd
+++ b/main/main.gd
@ -58,6 +58,9 @@ const NetworkItemUseServiceType = preload(
 const NetworkFishShowcaseServiceType = preload(
 	"res://network/network_fish_showcase_service.gd"
 )
+const NetworkSurfaceDrawingServiceType = preload(
+	"res://network/network_surface_drawing_service.gd"
+)
 const NetworkChatServiceType = preload(
 	"res://network/network_chat_service.gd"
 )
@ -130,6 +133,9 @@ const SHOP_PATTERN_SCALE: float = 1.75
@onready var _network_fish_showcase: NetworkFishShowcaseServiceType = (
 	%NetworkFishShowcaseService
 )
+@onready var _network_surface_drawing: NetworkSurfaceDrawingServiceType = (
+	%NetworkSurfaceDrawingService
+)
@onready var _network_chat: NetworkChatServiceType = %NetworkChatService
@onready var _network_mail: NetworkMailServiceType = %NetworkMailService
@onready var _network_player_list: NetworkPlayerListService = %NetworkPlayerListService
@ -141,6 +147,7 @@ const SHOP_PATTERN_SCALE: float = 1.75
 	%NetworkProfileService
 )
@onready var _players_root: Node3D = $Players
+@onready var _surface_drawings_root: Node3D = $SurfaceDrawings
@onready var _title_background: ColorRect = %TitleBackground
@onready var _player_menu_backdrop: ColorRect = %PlayerMenuBackdrop
@onready var _shop_backdrop: ColorRect = %ShopBackdrop
@ -351,6 +358,16 @@ func _initialize_after_data_root() -> void:
 		_player.inventory,
 		_player.hotbar,
 	)
+	_network_surface_drawing.setup(
+		_network_session,
+		_player_spawn_service,
+		_relationships,
+		_player,
+		_surface_drawings_root,
+	)
+	_network_player_list.set_surface_drawing_service(
+		_network_surface_drawing
+	)
 	_network_chat.setup(_network_session)
 	_network_fishing.setup(
 		_network_session,
@ -436,6 +453,7 @@ func _initialize_after_data_root() -> void:
 		_network_profile_service,
 		_network_player_list,
 		_settings_manager,
+		_network_surface_drawing,
 	)
 	_game_ui.setup_data_and_identity(
 		_data_root,
--- a/main/main.tscn
+++ b/main/main.tscn
@ -45,6 +45,7 @@
 [ext_resource type="Texture2D" uid="uid://b6xws1d2dnbnn" path="res://art/patterns/pattern_moneyfish.png" id="43_shop_pattern"]
 [ext_resource type="PackedScene" uid="uid://w7n4gjqq1juc" path="res://art/exported/characters/base/netfishing_base_character.glb" id="44_4pcu1"]
 [ext_resource type="Script" path="res://network/network_fish_showcase_service.gd" id="45_fish_showcase"]
+[ext_resource type="Script" path="res://network/network_surface_drawing_service.gd" id="46_surface_drawing"]

 [sub_resource type="ShaderMaterial" id="ShaderMaterial_title_water_native"]
 shader = ExtResource("40_title_water")
@ -188,6 +189,10 @@ script = ExtResource("24_network_item")
 unique_name_in_owner = true
 script = ExtResource("45_fish_showcase")

+[node name="NetworkSurfaceDrawingService" type="Node" parent="."]
+unique_name_in_owner = true
+script = ExtResource("46_surface_drawing")
+
 [node name="NetworkChatService" type="Node" parent="." unique_id=1956959711]
 unique_name_in_owner = true
 script = ExtResource("25_network_chat")
@ -209,6 +214,8 @@ transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, -0.0024132729, 0, -0.00093126

 [node name="Players" type="Node3D" parent="." unique_id=550630144]

+[node name="SurfaceDrawings" type="Node3D" parent="."]
+
 [node name="Player" parent="Players" unique_id=485429332 instance=ExtResource("2_player")]
 unique_name_in_owner = true
 transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 16)
--- a/network/network_player_list_service.gd
+++ b/network/network_player_list_service.gd
@ -11,6 +11,7 @@ var _known: KnownPlayerStore
 var _spawn: PlayerSpawnService
 var _chat: NetworkChatService
 var _mail: NetworkMailService
+var _surface_drawing: NetworkSurfaceDrawingService
 var _revision := 0
 var _host_block_pairs: Dictionary[String, bool] = {}
 var _peer_fingerprints: Dictionary[int, String] = {}
@ -96,6 +97,42 @@ func get_bans() -> Array[Dictionary]:
 	return _bans.get_bans(_session.get_host_identity_fingerprint()) if _session.is_host() else []


+func set_surface_drawing_service(
+	service: NetworkSurfaceDrawingService,
+) -> void:
+	_surface_drawing = service
+	if (
+		_surface_drawing != null
+		and not _surface_drawing.session_artwork_changed.is_connected(
+			_on_session_artwork_changed
+		)
+	):
+		_surface_drawing.session_artwork_changed.connect(
+			_on_session_artwork_changed
+		)
+	_changed()
+
+
+func get_session_artwork_counts() -> Vector2i:
+	if _surface_drawing == null:
+		return Vector2i.ZERO
+	return Vector2i(
+		_surface_drawing.get_canvas_count(),
+		_surface_drawing.get_painted_cell_count(),
+	)
+
+
+func reset_session_artwork() -> bool:
+	if not _session.is_host() or _surface_drawing == null:
+		return false
+	var ok: bool = _surface_drawing.clear_session_artwork()
+	moderation_finished.emit(
+		ok,
+		"Session artwork cleared." if ok else "Session artwork could not be cleared.",
+	)
+	return ok
+
+
 func set_muted(fingerprint: String, display_name: String, value: bool) -> bool:
 	if fingerprint == _session.get_local_identity_fingerprint():
 		return false
@ -269,3 +306,10 @@ func _entry_before(a: PlayerListEntry, b: PlayerListEntry) -> bool:
 func _changed() -> void:
 	_revision += 1
 	entries_changed.emit()
+
+
+func _on_session_artwork_changed(
+	_canvas_count: int,
+	_painted_cell_count: int,
+) -> void:
+	_changed()
--- a/network/network_protocol.gd
+++ b/network/network_protocol.gd
@ -10,7 +10,7 @@ const MAX_PUBLIC_KEY_LENGTH: int = 8192
 const MAX_SIGNATURE_LENGTH: int = 2048
 # ENet channels: 0 reliable lifecycle, 1 movement input, 2 movement
 # snapshots, 3 fishing input, 4 fishing snapshots, 5 reliable sales,
-# 6 reliable shop transactions, 7 reliable item/equipment/showcase lifecycle,
+# 6 reliable shop transactions, 7 reliable item/equipment/showcase/drawing,
 # 8 reliable ordered session chat, 9 reliable private session mail.
 const SALE_RELIABLE_CHANNEL: int = 5
 const SHOP_RELIABLE_CHANNEL: int = 6
@ -18,6 +18,7 @@ const ITEM_RELIABLE_CHANNEL: int = 7
 const CHAT_RELIABLE_CHANNEL: int = 8
 const MAIL_RELIABLE_CHANNEL: int = 9
 const ENET_CHANNEL_COUNT: int = 10
+const SURFACE_DRAWING_CAPABILITY: String = "surface_drawing_v1"

 enum RejectionCode {
 	NONE,
@ -83,7 +84,9 @@ static func make_client_hello(
 		"local_profile_id": profile_id,
 		"display_name": display_name,
 		"client_nonce": client_nonce,
-		"capability_flags": PackedStringArray(),
+		"capability_flags": PackedStringArray([
+			SURFACE_DRAWING_CAPABILITY,
+		]),
 		"cosmetic_snapshot": cosmetic_snapshot,
 		"identity_fingerprint": identity_fingerprint,
 		"identity_signature": identity_signature,
@ -122,6 +125,16 @@ static func validate_client_hello(data: Variant) -> String:
 		TYPE_ARRAY,
 	]:
 		return "Capabilities are invalid."
+	var capabilities: Variant = payload["capability_flags"]
+	if capabilities.size() > 32:
+		return "Capabilities are invalid."
+	for capability: Variant in capabilities:
+		if (
+			typeof(capability) not in [TYPE_STRING, TYPE_STRING_NAME]
+			or str(capability).is_empty()
+			or str(capability).length() > 64
+		):
+			return "Capabilities are invalid."
 	if typeof(payload["cosmetic_snapshot"]) != TYPE_DICTIONARY:
 		return "Cosmetic snapshot is invalid."
 	if (
@ -188,6 +201,7 @@ static func make_server_hello(
 			"item_use_v1",
 			"equipment_v1",
 			"fish_showcase_v1",
+			SURFACE_DRAWING_CAPABILITY,
 			"chat_v1",
 			"mail_v1",
 			"profile_v1",
--- a/network/network_session.gd
+++ b/network/network_session.gd
@ -168,6 +168,7 @@ func start_private_host(port: int = DEFAULT_PORT) -> bool:
 		NetworkProtocol.PROTOCOL_VERSION,
 		_player_identity.fingerprint,
 		_player_identity.public_pem,
+		PackedStringArray([NetworkProtocol.SURFACE_DRAWING_CAPABILITY]),
 	)
 	_registry.update_appearance(1, _local_appearance_snapshot)
 	var host_profile_hello := NetworkProtocol.make_client_hello(
@ -361,6 +362,7 @@ func supports_server_capability(capability: StringName) -> bool:
 		return str(capability) in PackedStringArray([
 			"movement_v1", "fishing_v1", "sale_v1", "shop_v1",
 			"item_use_v1", "equipment_v1", "fish_showcase_v1",
+			NetworkProtocol.SURFACE_DRAWING_CAPABILITY,
 			"chat_v1",
 			"mail_v1",
 			"profile_v1",
@ -373,6 +375,14 @@ func get_peer_record(peer_id: int) -> PeerRegistry.PeerRecord:
 	return _registry.get_peer(peer_id)


+func peer_supports_capability(
+	peer_id: int,
+	capability: StringName,
+) -> bool:
+	var record: PeerRegistry.PeerRecord = _registry.get_peer(peer_id)
+	return record != null and str(capability) in record.capability_flags
+
+
 func get_authenticated_peer_ids() -> Array[int]:
 	return _registry.get_peer_ids()

@ -931,6 +941,7 @@ func submit_client_hello(data: Dictionary) -> void:
 		NetworkProtocol.PROTOCOL_VERSION,
 		identity["fingerprint"],
 		identity["public_key"],
+		_sanitized_capabilities(data.get("capability_flags", [])),
 	):
 		_reject_peer(
 			sender_id,
@ -1062,6 +1073,7 @@ func receive_server_hello(data: Dictionary) -> void:
 		NetworkProtocol.PROTOCOL_VERSION,
 		_player_identity.fingerprint,
 		_player_identity.public_pem,
+		PackedStringArray([NetworkProtocol.SURFACE_DRAWING_CAPABILITY]),
 	)
 	_registry.update_appearance(local_peer_id, _local_appearance_snapshot)
 	var local_record := _registry.get_peer(local_peer_id)
@ -1143,6 +1155,7 @@ func _apply_spawn_entry(entry: Dictionary) -> void:
 			NetworkProtocol.PROTOCOL_VERSION,
 			str(entry.get("identity_fingerprint", "")),
 			str(entry.get("identity_public_key", "")),
+			_sanitized_capabilities(entry.get("capability_flags", [])),
 		)
 		var added_record := _registry.get_peer(peer_id)
 		if added_record != null and added_record.identity_authenticated:
@ -1218,9 +1231,28 @@ func _make_spawn_entry(
 			record.profile_authorization.duplicate(true)
 			if record != null else {}
 		),
+		"capability_flags": (
+			record.capability_flags.duplicate()
+			if record != null else PackedStringArray()
+		),
 	}


+func _sanitized_capabilities(value: Variant) -> PackedStringArray:
+	var result := PackedStringArray()
+	if typeof(value) not in [TYPE_ARRAY, TYPE_PACKED_STRING_ARRAY]:
+		return result
+	for capability: Variant in value:
+		if (
+			typeof(capability) in [TYPE_STRING, TYPE_STRING_NAME]
+			and not str(capability).is_empty()
+			and str(capability).length() <= 64
+			and str(capability) not in result
+		):
+			result.append(str(capability))
+	return result
+
+
 func _verify_spawn_identity(entry: Dictionary) -> bool:
 	var fingerprint := str(entry.get("identity_fingerprint", ""))
 	var public_pem := NetworkIdentityCrypto.normalize_public_pem(
--- a/network/network_surface_drawing_service.gd
+++ b/network/network_surface_drawing_service.gd
--- a/network/network_surface_drawing_service.gd.uid
+++ b/network/network_surface_drawing_service.gd.uid
@ -0,0 +1 @@
+uid://bkiepgu8j5r6t
--- a/network/peer_registry.gd
+++ b/network/peer_registry.gd
@ -16,6 +16,7 @@ class PeerRecord:
 	var identity_public_key: String = ""
 	var identity_authenticated: bool = false
 	var profile_authorization: Dictionary = {}
+	var capability_flags: PackedStringArray = PackedStringArray()


 var _records: Dictionary[int, PeerRecord] = {}
@ -28,6 +29,7 @@ func add_peer(
 	protocol_version: int,
 	identity_fingerprint: String = "",
 	identity_public_key: String = "",
+	capability_flags: PackedStringArray = PackedStringArray(),
 ) -> bool:
 	if (
 		peer_id <= 0
@ -49,6 +51,7 @@ func add_peer(
 	record.identity_authenticated = NetworkIdentityCrypto.valid_fingerprint(
 		identity_fingerprint
 	)
+	record.capability_flags = capability_flags.duplicate()
 	_records[peer_id] = record
 	return true

--- a/tests/surface_drawing_multiplayer_validation.gd
+++ b/tests/surface_drawing_multiplayer_validation.gd
@ -0,0 +1,252 @@
+extends SceneTree
+
+const MainScene: PackedScene = preload("res://main/main.tscn")
+const TEST_PORT: int = 18133
+const WAIT_MSEC: int = 20000
+
+
+func _initialize() -> void:
+	call_deferred("_run")
+
+
+func _run() -> void:
+	var arguments: PackedStringArray = OS.get_cmdline_user_args()
+	if arguments.has("host"):
+		await _run_host()
+		return
+	if arguments.has("client"):
+		await _run_client()
+		return
+	push_error("Surface drawing multiplayer validation needs host or client mode.")
+	quit(1)
+
+
+func _run_host() -> void:
+	var main: Node = await _create_initialized_main()
+	var session := main.get_node("%NetworkSession") as NetworkSession
+	assert(session.start_private_host(TEST_PORT))
+	var save_manager := main.get("_save_manager") as PlayerSaveManager
+	assert(save_manager.initialize_new_game())
+	main.call("_enter_gameplay")
+	await physics_frame
+	await physics_frame
+
+	var service := main.get_node(
+		"%NetworkSurfaceDrawingService"
+	) as NetworkSurfaceDrawingService
+	var surface: Dictionary = _surface_below_local_player(main)
+	assert(not surface.is_empty())
+	assert(service.request_canvas_at_surface(
+		surface["position"], surface["normal"], Vector3.RIGHT
+	))
+	var canvas_id: String = service.get_canvas_ids()[0]
+	assert(service.request_cell_edits(canvas_id, [{
+		"x": 7,
+		"y": 7,
+		"color_id": "coral",
+	}]))
+	assert(session.set_host_open(true))
+
+	var remote_peer_id: int = await _wait_for_remote_peer(session)
+	assert(remote_peer_id > 1)
+	var remote_record: PeerRegistry.PeerRecord = session.get_peer_record(
+		remote_peer_id
+	)
+	assert(remote_record != null)
+	assert(session.peer_supports_capability(
+		remote_peer_id, SurfaceDrawingProtocol.CAPABILITY
+	))
+
+	var overwrite_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < overwrite_deadline:
+		await process_frame
+		var state: Dictionary = service.get_canvas_state(canvas_id)
+		if int(state.get("revision", 0)) < 2:
+			continue
+		var cell: Dictionary = _find_cell(state, 7, 7)
+		if (
+			str(cell.get("color_id", "")) == "blue"
+			and str(cell.get("author_fingerprint", ""))
+				== remote_record.identity_fingerprint
+		):
+			break
+	var overwritten_state: Dictionary = service.get_canvas_state(canvas_id)
+	assert(int(overwritten_state["revision"]) >= 2)
+	var overwritten_cell: Dictionary = _find_cell(overwritten_state, 7, 7)
+	assert(str(overwritten_cell["color_id"]) == "blue")
+	assert(
+		str(overwritten_cell["author_fingerprint"])
+		== remote_record.identity_fingerprint
+	)
+
+	assert(service.request_cell_edits(canvas_id, [{
+		"x": 8,
+		"y": 7,
+		"color_id": "sunny",
+	}]))
+	var undo_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < undo_deadline:
+		await process_frame
+		var state: Dictionary = service.get_canvas_state(canvas_id)
+		if str(_find_cell(state, 7, 7).get("color_id", "")) == "coral":
+			break
+	assert(
+		str(_find_cell(service.get_canvas_state(canvas_id), 7, 7)["color_id"])
+		== "coral"
+	)
+	var guide_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < guide_deadline:
+		await process_frame
+		if not bool(service.get_canvas_state(canvas_id).get("guide_visible", true)):
+			break
+	assert(not bool(service.get_canvas_state(canvas_id)["guide_visible"]))
+	# Keep the shared hidden state observable before publishing the restore.
+	await create_timer(1.0).timeout
+	assert(service.request_guide_visibility(canvas_id, true))
+	await create_timer(1.0).timeout
+	assert(service.request_guide_visibility(canvas_id, false, true))
+	await create_timer(1.0).timeout
+	assert(service.clear_session_artwork())
+	await create_timer(1.0).timeout
+	print("Surface drawing multiplayer host validation: PASS")
+	session.disconnect_session("")
+	main.queue_free()
+	await process_frame
+	quit()
+
+
+func _run_client() -> void:
+	var main: Node = await _create_initialized_main()
+	main.call(
+		"_on_title_join_game_requested",
+		"127.0.0.1:%d" % TEST_PORT,
+	)
+	var session := main.get_node("%NetworkSession") as NetworkSession
+	var join_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < join_deadline:
+		await process_frame
+		if session.state == NetworkSession.State.VERIFYING_SERVER_IDENTITY:
+			main.call("_confirm_server_trust")
+		if session.is_joined_client() and bool(main.get("_gameplay_started")):
+			break
+	assert(session.is_joined_client())
+	assert(session.supports_server_capability(
+		SurfaceDrawingProtocol.CAPABILITY
+	))
+
+	var service := main.get_node(
+		"%NetworkSurfaceDrawingService"
+	) as NetworkSurfaceDrawingService
+	var snapshot_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while (
+		Time.get_ticks_msec() < snapshot_deadline
+		and service.get_canvas_ids().is_empty()
+	):
+		await process_frame
+	assert(service.get_canvas_ids().size() == 1)
+	var canvas_id: String = service.get_canvas_ids()[0]
+	var snapshot: Dictionary = service.get_canvas_state(canvas_id)
+	assert(str(_find_cell(snapshot, 7, 7).get("color_id", "")) == "coral")
+	assert(service.request_cell_edits(canvas_id, [{
+		"x": 7,
+		"y": 7,
+		"color_id": "blue",
+	}]))
+
+	var shared_edit_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < shared_edit_deadline:
+		await process_frame
+		var state: Dictionary = service.get_canvas_state(canvas_id)
+		if str(_find_cell(state, 8, 7).get("color_id", "")) == "sunny":
+			break
+	var final_state: Dictionary = service.get_canvas_state(canvas_id)
+	assert(str(_find_cell(final_state, 7, 7)["color_id"]) == "blue")
+	assert(str(_find_cell(final_state, 8, 7)["color_id"]) == "sunny")
+	assert(service.request_undo_last_stroke())
+	var undo_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < undo_deadline:
+		await process_frame
+		var state: Dictionary = service.get_canvas_state(canvas_id)
+		if str(_find_cell(state, 7, 7).get("color_id", "")) == "coral":
+			break
+	assert(
+		str(_find_cell(service.get_canvas_state(canvas_id), 7, 7)["color_id"])
+		== "coral"
+	)
+	assert(service.request_guide_visibility(canvas_id, false))
+	var guide_hidden_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < guide_hidden_deadline:
+		await process_frame
+		if not bool(service.get_canvas_state(canvas_id).get("guide_visible", true)):
+			break
+	assert(not bool(service.get_canvas_state(canvas_id)["guide_visible"]))
+	var guide_restored_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < guide_restored_deadline:
+		await process_frame
+		if bool(service.get_canvas_state(canvas_id).get("guide_visible", false)):
+			break
+	assert(bool(service.get_canvas_state(canvas_id)["guide_visible"]))
+	var finalized_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < finalized_deadline:
+		await process_frame
+		if bool(service.get_canvas_state(canvas_id).get("finalized", false)):
+			break
+	assert(bool(service.get_canvas_state(canvas_id)["finalized"]))
+	var reset_deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < reset_deadline:
+		await process_frame
+		if service.get_canvas_ids().is_empty():
+			break
+	assert(service.get_canvas_ids().is_empty())
+	print("Surface drawing multiplayer client validation: PASS")
+	session.disconnect_session("")
+	main.queue_free()
+	await process_frame
+	quit()
+
+
+func _surface_below_local_player(main: Node) -> Dictionary:
+	var player := main.get("_player") as Player
+	var query := PhysicsRayQueryParameters3D.create(
+		player.global_position + Vector3.UP * 3.0,
+		player.global_position + Vector3.DOWN * 6.0,
+		1,
+	)
+	query.collide_with_areas = false
+	query.collide_with_bodies = true
+	query.exclude = [player.get_rid()]
+	return player.get_world_3d().direct_space_state.intersect_ray(query)
+
+
+func _wait_for_remote_peer(session: NetworkSession) -> int:
+	var deadline: int = Time.get_ticks_msec() + WAIT_MSEC
+	while Time.get_ticks_msec() < deadline:
+		await process_frame
+		for peer_id: int in session.get_authenticated_peer_ids():
+			if peer_id != session.get_local_peer_id():
+				return peer_id
+	return 0
+
+
+func _find_cell(state: Dictionary, x: int, y: int) -> Dictionary:
+	for value: Variant in state.get("cells", []):
+		if typeof(value) != TYPE_DICTIONARY:
+			continue
+		var cell: Dictionary = value
+		if int(cell.get("x", -1)) == x and int(cell.get("y", -1)) == y:
+			return cell
+	return {}
+
+
+func _create_initialized_main() -> Node:
+	root.size = Vector2i(1280, 720)
+	var main: Node = MainScene.instantiate()
+	root.add_child(main)
+	for _frame: int in 4:
+		await process_frame
+	if not bool(main.get("_application_initialized")):
+		main.call("_activate_selected_data_path", "", true)
+	for _frame: int in 8:
+		await process_frame
+	assert(bool(main.get("_application_initialized")))
+	return main
--- a/tests/surface_drawing_multiplayer_validation.gd.uid
+++ b/tests/surface_drawing_multiplayer_validation.gd.uid
@ -0,0 +1 @@
+uid://bj5dje2scya34
--- a/tests/surface_drawing_runtime_validation.gd
+++ b/tests/surface_drawing_runtime_validation.gd
@ -0,0 +1,161 @@
+extends SceneTree
+
+const MainScene: PackedScene = preload("res://main/main.tscn")
+const TEST_PORT: int = 18132
+
+
+func _initialize() -> void:
+	call_deferred("_run")
+
+
+func _run() -> void:
+	root.size = Vector2i(1280, 720)
+	var main: Node = MainScene.instantiate()
+	root.add_child(main)
+	for _frame: int in 4:
+		await process_frame
+	if not bool(main.get("_application_initialized")):
+		main.call("_activate_selected_data_path", "", true)
+	for _frame: int in 8:
+		await process_frame
+	assert(bool(main.get("_application_initialized")))
+	var session := main.get_node("%NetworkSession") as NetworkSession
+	assert(session.start_private_host(TEST_PORT))
+	var save_manager := main.get("_save_manager") as PlayerSaveManager
+	assert(save_manager.initialize_new_game())
+	main.call("_enter_gameplay")
+	await physics_frame
+	await physics_frame
+
+	var player := main.get("_player") as Player
+	var service := main.get_node(
+		"%NetworkSurfaceDrawingService"
+	) as NetworkSurfaceDrawingService
+	assert(service != null)
+	assert(session.supports_server_capability(SurfaceDrawingProtocol.CAPABILITY))
+	_validate_marker_controls(service, player)
+	var query := PhysicsRayQueryParameters3D.create(
+		player.global_position + Vector3.UP * 3.0,
+		player.global_position + Vector3.DOWN * 6.0,
+		1,
+	)
+	query.collide_with_areas = false
+	query.collide_with_bodies = true
+	query.exclude = [player.get_rid()]
+	var hit: Dictionary = player.get_world_3d().direct_space_state.intersect_ray(query)
+	assert(not hit.is_empty())
+	assert(service.request_canvas_at_surface(
+		hit["position"], hit["normal"], Vector3.RIGHT
+	))
+	assert(service.get_canvas_ids().size() == 1)
+	var canvas_id: String = service.get_canvas_ids()[0]
+	assert(service.request_cell_edits(canvas_id, [{
+		"x": 8,
+		"y": 8,
+		"color_id": "ocean_teal",
+	}]))
+	var state: Dictionary = service.get_canvas_state(canvas_id)
+	assert(int(state["revision"]) == 1)
+	assert((state["cells"] as Array).size() == 1)
+	assert(state["cells"][0]["color_id"] == "ocean_teal")
+	assert(
+		state["cells"][0]["author_fingerprint"]
+		== session.get_local_identity_fingerprint()
+	)
+	assert(service.request_guide_visibility(canvas_id, false))
+	state = service.get_canvas_state(canvas_id)
+	assert(not bool(state["guide_visible"]))
+	var canvas_nodes: Dictionary = service.get("_canvas_nodes")
+	var canvas := canvas_nodes[canvas_id] as SurfaceDrawingCanvas
+	assert(not canvas.is_guide_visible())
+	assert(service.request_guide_visibility(canvas_id, true))
+	assert(bool(service.get_canvas_state(canvas_id)["guide_visible"]))
+	assert(service.request_guide_visibility(canvas_id, false, true))
+	assert(bool(service.get_canvas_state(canvas_id)["finalized"]))
+	assert(canvas.is_finalized())
+	assert(service.request_canvas_at_surface(
+		hit["position"], hit["normal"], Vector3.RIGHT
+	))
+	assert(service.get_canvas_ids().size() == 2)
+	var replacement_canvas_id: String = ""
+	for candidate_id: String in service.get_canvas_ids():
+		if candidate_id != canvas_id:
+			replacement_canvas_id = candidate_id
+	assert(not replacement_canvas_id.is_empty())
+	assert(service.request_cell_edits(replacement_canvas_id, [{
+		"x": 8,
+		"y": 8,
+		"color_id": "blue",
+	}]))
+	assert((service.get_canvas_state(canvas_id)["cells"] as Array).is_empty())
+	assert(
+		str(service.get_canvas_state(replacement_canvas_id)["cells"][0]["color_id"])
+		== "blue"
+	)
+	assert(service.request_undo_last_stroke())
+	assert((service.get_canvas_state(replacement_canvas_id)["cells"] as Array).is_empty())
+	assert(
+		str(service.get_canvas_state(canvas_id)["cells"][0]["color_id"])
+		== "ocean_teal"
+	)
+	var player_list := main.get_node(
+		"%NetworkPlayerListService"
+	) as NetworkPlayerListService
+	assert(player_list.get_session_artwork_counts() == Vector2i(2, 1))
+	assert(player_list.reset_session_artwork())
+	assert(service.get_canvas_ids().is_empty())
+
+	print("Surface drawing runtime validation: PASS")
+	session.disconnect_session("")
+	main.queue_free()
+	await process_frame
+	quit()
+
+
+func _validate_marker_controls(
+	service: NetworkSurfaceDrawingService,
+	player: Player,
+) -> void:
+	var prior_mouse_mode: Input.MouseMode = Input.mouse_mode
+	service.activate()
+	assert(service.is_active())
+	assert(not service.is_placement_mode())
+	if DisplayServer.get_name() != "headless":
+		assert(Input.mouse_mode == Input.MOUSE_MODE_CAPTURED)
+
+	var placement_key := InputEventKey.new()
+	placement_key.physical_keycode = KEY_R
+	placement_key.pressed = true
+	assert(service.handle_input(placement_key, true))
+	assert(service.is_placement_mode())
+
+	var pointer_before: Vector2 = service.get_pointer_screen_position()
+	var pointer_motion := InputEventMouseMotion.new()
+	pointer_motion.screen_relative = Vector2(30.0, -12.0)
+	assert(service.handle_input(pointer_motion, true))
+	assert(service.get_pointer_screen_position() != pointer_before)
+	var zoom_event := InputEventMouseButton.new()
+	zoom_event.button_index = MOUSE_BUTTON_WHEEL_UP
+	zoom_event.shift_pressed = true
+	zoom_event.pressed = true
+	assert(not service.handle_input(zoom_event, true))
+	var prior_zoom: float = float(player.get("_target_zoom"))
+	player.call("_unhandled_input", zoom_event)
+	assert(float(player.get("_target_zoom")) < prior_zoom)
+
+	var camera_press := InputEventMouseButton.new()
+	camera_press.button_index = MOUSE_BUTTON_RIGHT
+	camera_press.pressed = true
+	assert(not service.handle_input(camera_press, true))
+	var pointer_before_camera: Vector2 = service.get_pointer_screen_position()
+	assert(not service.handle_input(pointer_motion, true))
+	assert(service.get_pointer_screen_position() == pointer_before_camera)
+	var camera_release := InputEventMouseButton.new()
+	camera_release.button_index = MOUSE_BUTTON_RIGHT
+	camera_release.pressed = false
+	assert(not service.handle_input(camera_release, true))
+
+	assert(service.handle_input(placement_key, true))
+	assert(not service.is_placement_mode())
+	service.deactivate()
+	assert(Input.mouse_mode == prior_mouse_mode)
--- a/tests/surface_drawing_runtime_validation.gd.uid
+++ b/tests/surface_drawing_runtime_validation.gd.uid
@ -0,0 +1 @@
+uid://8c30wxeu4lxv
--- a/tests/surface_drawing_validation.gd
+++ b/tests/surface_drawing_validation.gd
@ -0,0 +1,284 @@
+extends SceneTree
+
+const FINGERPRINT_A: String = (
+	"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+)
+const FINGERPRINT_B: String = (
+	"bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
+)
+
+
+func _initialize() -> void:
+	call_deferred("_run")
+
+
+func _run() -> void:
+	_validate_palette()
+	_validate_protocol_bounds()
+	_validate_grid_snapping()
+	await _validate_canvas_geometry_and_collaboration()
+	await _validate_blocked_author_visibility()
+	_validate_peer_capability_tracking()
+	print("Surface drawing validation: PASS")
+	quit()
+
+
+func _validate_palette() -> void:
+	var ids: Array[StringName] = SurfaceDrawingPalette.get_color_ids()
+	assert(ids.size() == 8)
+	assert(ids.front() == SurfaceDrawingPalette.DEFAULT_COLOR_ID)
+	for color_id: StringName in ids:
+		assert(SurfaceDrawingPalette.has_color(color_id))
+		assert(not SurfaceDrawingPalette.get_display_name(color_id).is_empty())
+	assert(
+		SurfaceDrawingPalette.filter_unlocked_ids([])
+		== [SurfaceDrawingPalette.DEFAULT_COLOR_ID]
+	)
+
+
+func _validate_protocol_bounds() -> void:
+	assert(SurfaceDrawingProtocol.GRID_WIDTH == 32)
+	assert(SurfaceDrawingProtocol.GRID_HEIGHT == 32)
+	assert(is_equal_approx(SurfaceDrawingProtocol.CELL_SIZE, 0.075))
+	assert(is_equal_approx(
+		float(SurfaceDrawingProtocol.GRID_WIDTH)
+			* SurfaceDrawingProtocol.CELL_SIZE,
+		2.4,
+	))
+	var canvas_request: Dictionary = {
+		"request_id": "canvas-1",
+		"session_id": "session",
+		"origin": [0.0, 1.0, 2.0],
+		"normal": [0.0, 1.0, 0.0],
+		"tangent": [1.0, 0.0, 0.0],
+		"width": SurfaceDrawingProtocol.GRID_WIDTH,
+		"height": SurfaceDrawingProtocol.GRID_HEIGHT,
+		"cell_size": SurfaceDrawingProtocol.CELL_SIZE,
+	}
+	assert(SurfaceDrawingProtocol.validate_canvas_request(canvas_request))
+	var malformed: Dictionary = canvas_request.duplicate(true)
+	malformed["origin"] = [INF, 0.0, 0.0]
+	assert(not SurfaceDrawingProtocol.validate_canvas_request(malformed))
+	var edit_request: Dictionary = {
+		"request_id": "edit-1",
+		"session_id": "session",
+		"canvas_id": "canvas-1",
+		"stroke_id": "stroke-1",
+		"edits": [{"x": 0, "y": 0, "color_id": "ocean_teal"}],
+	}
+	assert(SurfaceDrawingProtocol.validate_edit_request(edit_request))
+	edit_request["edits"] = [{
+		"x": SurfaceDrawingProtocol.GRID_WIDTH,
+		"y": 0,
+		"color_id": "ocean_teal",
+	}]
+	assert(not SurfaceDrawingProtocol.validate_edit_request(edit_request))
+	var guide_request: Dictionary = {
+		"request_id": "guide-1",
+		"session_id": "session",
+		"canvas_id": "canvas-1",
+		"guide_visible": false,
+		"finalized": false,
+	}
+	assert(SurfaceDrawingProtocol.validate_guide_request(guide_request))
+	guide_request["guide_visible"] = 0
+	assert(not SurfaceDrawingProtocol.validate_guide_request(guide_request))
+	var undo_request: Dictionary = {
+		"request_id": "undo-1",
+		"session_id": "session",
+		"stroke_id": "stroke-1",
+	}
+	assert(SurfaceDrawingProtocol.validate_undo_request(undo_request))
+
+
+func _validate_grid_snapping() -> void:
+	var anchor: Dictionary = {
+		"session_id": "session",
+		"canvas_id": "anchor",
+		"origin": [0.0, 0.0, 0.0],
+		"normal": [0.0, 1.0, 0.0],
+		"tangent": [1.0, 0.0, 0.0],
+		"width": SurfaceDrawingProtocol.GRID_WIDTH,
+		"height": SurfaceDrawingProtocol.GRID_HEIGHT,
+		"cell_size": SurfaceDrawingProtocol.CELL_SIZE,
+		"revision": 0,
+		"guide_visible": false,
+		"finalized": true,
+		"layer": 1,
+		"creator_fingerprint": FINGERPRINT_A,
+		"cells": [],
+	}
+	var snapped: Dictionary = SurfaceDrawingPlacement.resolve(
+		Vector3(2.28, 0.02, 0.08),
+		Vector3.UP,
+		Vector3.RIGHT,
+		[anchor],
+	)
+	assert(bool(snapped["snapped"]))
+	var snapped_origin: Vector3 = snapped["origin"]
+	assert(snapped_origin.is_equal_approx(Vector3(2.4, 0.0, 0.0)))
+	var unsnapped: Dictionary = SurfaceDrawingPlacement.resolve(
+		Vector3(1.2, 0.0, 0.0),
+		Vector3.UP,
+		Vector3.RIGHT,
+		[anchor],
+	)
+	assert(not bool(unsnapped["snapped"]))
+
+
+func _validate_canvas_geometry_and_collaboration() -> void:
+	var world := Node3D.new()
+	root.add_child(world)
+	var canvas := SurfaceDrawingCanvas.new()
+	world.add_child(canvas)
+	var state: Dictionary = {
+		"session_id": "session",
+		"canvas_id": "canvas-1",
+		"origin": [0.0, 0.0, 0.0],
+		"normal": [0.0, 1.0, 0.0],
+		"tangent": [1.0, 0.0, 0.0],
+		"width": SurfaceDrawingProtocol.GRID_WIDTH,
+		"height": SurfaceDrawingProtocol.GRID_HEIGHT,
+		"cell_size": SurfaceDrawingProtocol.CELL_SIZE,
+		"revision": 0,
+		"creator_fingerprint": FINGERPRINT_A,
+		"cells": [],
+	}
+	assert(canvas.setup(state, null))
+	var first_center: Vector3 = canvas.get_cell_world_position(0, 0)
+	assert(canvas.cell_at_world_point(first_center) == Vector2i(0, 0))
+	assert(canvas.contains_world_point(first_center))
+	var first_update: Dictionary = {
+		"session_id": "session",
+		"canvas_id": "canvas-1",
+		"revision": 1,
+		"edits": [{
+			"x": 3,
+			"y": 4,
+			"color_id": "coral",
+			"author_fingerprint": FINGERPRINT_A,
+		}],
+	}
+	assert(canvas.apply_update(first_update))
+	var finish_update: Dictionary = {
+		"session_id": "session",
+		"canvas_id": "canvas-1",
+		"revision": 2,
+		"guide_visible": false,
+		"finalized": true,
+	}
+	assert(canvas.apply_guide_update(finish_update))
+	assert(not canvas.is_guide_visible())
+	assert(canvas.is_finalized())
+	await process_frame
+	var finished_pixel_scale: float = canvas.get_rendered_pixel_size()
+	assert(
+		is_equal_approx(
+			finished_pixel_scale, SurfaceDrawingProtocol.CELL_SIZE
+		),
+		"finished pixel scale %f does not match cell size %f"
+		% [finished_pixel_scale, SurfaceDrawingProtocol.CELL_SIZE],
+	)
+	var second_update: Dictionary = {
+		"session_id": "session",
+		"canvas_id": "canvas-1",
+		"revision": 3,
+		"edits": [{
+			"x": 3,
+			"y": 4,
+			"color_id": "blue",
+			"author_fingerprint": FINGERPRINT_B,
+		}],
+	}
+	assert(canvas.apply_update(second_update))
+	var cells: Array[Dictionary] = canvas.get_authoritative_cells()
+	assert(cells.size() == 1)
+	assert(cells[0]["color_id"] == "blue")
+	assert(cells[0]["author_fingerprint"] == FINGERPRINT_B)
+	var erase_update: Dictionary = {
+		"session_id": "session",
+		"canvas_id": "canvas-1",
+		"revision": 4,
+		"edits": [{
+			"x": 3,
+			"y": 4,
+			"color_id": "",
+			"author_fingerprint": "",
+		}],
+	}
+	assert(canvas.apply_update(erase_update))
+	assert(canvas.get_authoritative_cells().is_empty())
+	world.queue_free()
+	await process_frame
+
+
+func _validate_peer_capability_tracking() -> void:
+	var registry := PeerRegistry.new()
+	assert(registry.add_peer(
+		1,
+		"profile",
+		"Voyager",
+		NetworkProtocol.PROTOCOL_VERSION,
+		FINGERPRINT_A,
+		"public key fixture",
+		PackedStringArray([str(SurfaceDrawingProtocol.CAPABILITY)]),
+	))
+	var record: PeerRegistry.PeerRecord = registry.get_peer(1)
+	assert(record != null)
+	assert(str(SurfaceDrawingProtocol.CAPABILITY) in record.capability_flags)
+
+
+func _validate_blocked_author_visibility() -> void:
+	var relationships := PlayerRelationshipStore.new()
+	relationships.set("_loaded", true)
+	relationships.set("_records", {
+		FINGERPRINT_B: {"blocked": true, "muted": true},
+	})
+	var world := Node3D.new()
+	root.add_child(world)
+	var canvas := SurfaceDrawingCanvas.new()
+	world.add_child(canvas)
+	var state: Dictionary = {
+		"session_id": "session",
+		"canvas_id": "blocked-author-canvas",
+		"origin": [0.0, 0.0, 0.0],
+		"normal": [0.0, 1.0, 0.0],
+		"tangent": [1.0, 0.0, 0.0],
+		"width": SurfaceDrawingProtocol.GRID_WIDTH,
+		"height": SurfaceDrawingProtocol.GRID_HEIGHT,
+		"cell_size": SurfaceDrawingProtocol.CELL_SIZE,
+		"revision": 1,
+		"creator_fingerprint": FINGERPRINT_A,
+		"cells": [
+			{
+				"x": 1,
+				"y": 1,
+				"color_id": "coral",
+				"author_fingerprint": FINGERPRINT_A,
+			},
+			{
+				"x": 2,
+				"y": 1,
+				"color_id": "blue",
+				"author_fingerprint": FINGERPRINT_B,
+			},
+		],
+	}
+	assert(canvas.setup(state, relationships))
+	assert(_rendered_pixel_count(canvas) == 1)
+	relationships.set("_records", {})
+	canvas.refresh_relationship_visibility()
+	await process_frame
+	assert(_rendered_pixel_count(canvas) == 2)
+	world.queue_free()
+	await process_frame
+
+
+func _rendered_pixel_count(canvas: SurfaceDrawingCanvas) -> int:
+	var count: int = 0
+	for child: Node in canvas.get_children():
+		if child is MultiMeshInstance3D:
+			var instance := child as MultiMeshInstance3D
+			if instance.multimesh != null:
+				count += instance.multimesh.instance_count
+	return count
--- a/tests/surface_drawing_validation.gd.uid
+++ b/tests/surface_drawing_validation.gd.uid
@ -0,0 +1 @@
+uid://dui5ueufqooul
--- a/ui/game_ui.gd
+++ b/ui/game_ui.gd
@ -65,6 +65,10 @@ signal shop_backdrop_visibility_changed(is_visible: bool)
@onready var _effect_status: Label = %EffectStatus
@onready var _chat_ui: ChatUIType = %ChatUI
@onready var _emote_radial_menu: EmoteRadialMenuType = %EmoteRadialMenu
+@onready var _marker_hud: PanelContainer = %MarkerHUD
+@onready var _marker_swatch: ColorRect = %MarkerSwatch
+@onready var _marker_summary: Label = %MarkerSummary
+@onready var _marker_help: Label = %MarkerHelp
@onready var _title_settings_panel: SettingsPanelType = (
 	$UIRoot/TitleScreen/ResponsiveTitleStage/TitlePresentationScaleRoot/SettingsPanel
 )
@ -84,6 +88,7 @@ var _player_menu_hotbar_visible: bool = false
 var _item_effects: PlayerItemEffectsType
 var _main_shop_buyer: FishBuyerProfileType
 var _shop_interaction: ShopInteractionType
+var _surface_drawing: NetworkSurfaceDrawingService


 func _ready() -> void:
@ -136,6 +141,7 @@ func setup(
 	network_profile_service: NetworkProfileService,
 	network_player_list: NetworkPlayerListService,
 	settings_manager: PlayerSettingsManagerType,
+	surface_drawing: NetworkSurfaceDrawingService,
 ) -> void:
 	_player = player
 	_fishing_spot = fishing_spot
@ -206,9 +212,30 @@ func setup(
 	)
 	_main_shop_buyer = main_shop_buyer
 	_shop_interaction = shop_interaction
+	_surface_drawing = surface_drawing
+	if _surface_drawing != null:
+		_surface_drawing.hud_state_changed.connect(
+			_on_surface_drawing_hud_state_changed
+		)


 func _input(event: InputEvent) -> void:
+	var drawing_can_open: bool = (
+		_gameplay_ui_enabled
+		and not _system_menu_open
+		and not _player_menu_open
+		and not _shop_open
+		and not _chat_input_open
+		and not _showcase_active
+		and _fishing_spot != null
+		and _fishing_spot.can_use_surface_drawing()
+	)
+	if (
+		_surface_drawing != null
+		and _surface_drawing.handle_input(event, drawing_can_open)
+	):
+		get_viewport().set_input_as_handled()
+		return
 	if _emote_radial_menu == null:
 		return
 	var can_open: bool = (
@ -306,6 +333,8 @@ func set_gameplay_ui_enabled(enabled: bool) -> void:
 	_gameplay_transient_hud.visible = enabled and not _player_menu_open
 	_refresh_chat_availability()
 	if not enabled:
+		if _surface_drawing != null:
+			_surface_drawing.deactivate()
 		close_player_menu_for_session_end()
 		_fishing_shop.close_for_session_end()
 		_fishing_panel.visible = false
@ -320,6 +349,8 @@ func set_gameplay_ui_enabled(enabled: bool) -> void:

 func set_system_menu_open(is_open: bool) -> void:
 	_system_menu_open = is_open
+	if is_open and _surface_drawing != null:
+		_surface_drawing.deactivate()
 	_refresh_chat_availability()
 	_refresh_hotbar_visibility()
 	_hotbar_ui.set_gameplay_input_enabled(
@ -532,6 +563,8 @@ func _on_showcase_changed(

 func _on_player_menu_visibility_changed(is_open: bool) -> void:
 	_player_menu_open = is_open
+	if is_open and _surface_drawing != null:
+		_surface_drawing.deactivate()
 	_gameplay_transient_hud.visible = _gameplay_ui_enabled and not is_open
 	if is_open:
 		_hotbar_ui.set_drag_enabled(false)
@ -577,6 +610,8 @@ func _on_player_menu_exit_started() -> void:

 func _on_shop_visibility_changed(is_open: bool) -> void:
 	_shop_open = is_open
+	if is_open and _surface_drawing != null:
+		_surface_drawing.deactivate()
 	shop_backdrop_visibility_changed.emit(is_open)
 	if not is_open and _player_menu.is_shop_cooler_mounted():
 		_player_menu.unmount_shop_cooler()
@ -679,9 +714,40 @@ func _emit_interactive_pointer_ui_changed() -> void:

 func _on_chat_text_entry_ownership_changed(active: bool) -> void:
 	_chat_input_open = active
+	if active and _surface_drawing != null:
+		_surface_drawing.deactivate()
 	_emit_interactive_pointer_ui_changed()


+func _on_surface_drawing_hud_state_changed(
+	is_active: bool,
+	mode_name: String,
+	color_name: String,
+	color_value: Color,
+	brush_size: int,
+	status: String,
+) -> void:
+	_marker_hud.visible = is_active and _gameplay_ui_enabled
+	_marker_swatch.visible = mode_name != "place grid"
+	_marker_swatch.color = color_value
+	_marker_summary.text = (
+		"place shared grid"
+		if mode_name == "place grid"
+		else "marker • %s • brush %d" % [
+			color_name.to_lower(), brush_size,
+		]
+	)
+	_marker_help.text = (
+		status
+		if not status.is_empty()
+		else (
+			"click place/restore • shift hide • ctrl shift finish • shift scroll zoom"
+			if mode_name == "place grid"
+			else "click draw • shift click erase • ctrl z undo • shift scroll zoom"
+		)
+	)
+
+
 func _refresh_chat_availability() -> void:
 	if _chat_ui == null:
 		return
--- a/ui/game_ui.tscn
+++ b/ui/game_ui.tscn
@ -34,6 +34,17 @@ corner_radius_bottom_left = 12
 [sub_resource type="StyleBoxFlat" id="StyleBox_transparent"]
 bg_color = Color(0, 0, 0, 0)

+[sub_resource type="StyleBoxFlat" id="StyleBox_marker_panel"]
+bg_color = Color(0.051, 0.173, 0.227, 0.96)
+corner_radius_top_left = 12
+corner_radius_top_right = 12
+corner_radius_bottom_right = 12
+corner_radius_bottom_left = 12
+content_margin_left = 14.0
+content_margin_top = 9.0
+content_margin_right = 14.0
+content_margin_bottom = 9.0
+
 [node name="GameUI" type="CanvasLayer"]
 script = ExtResource("1_ui")

@ -254,6 +265,51 @@ theme_override_colors/font_shadow_color = Color(0, 0, 0, 0.9)
 theme_override_constants/shadow_offset_x = 2
 theme_override_constants/shadow_offset_y = 2

+[node name="MarkerHUD" type="PanelContainer" parent="UIRoot/CanonicalStage/GameplayTransientHUD"]
+unique_name_in_owner = true
+visible = false
+z_index = 58
+anchors_preset = 10
+anchor_left = 1.0
+anchor_right = 1.0
+offset_left = -510.0
+offset_top = 18.0
+offset_right = -18.0
+offset_bottom = 92.0
+grow_horizontal = 0
+mouse_filter = 2
+theme = ExtResource("3_theme")
+theme_override_styles/panel = SubResource("StyleBox_marker_panel")
+
+[node name="Layout" type="HBoxContainer" parent="UIRoot/CanonicalStage/GameplayTransientHUD/MarkerHUD"]
+layout_mode = 2
+theme_override_constants/separation = 12
+
+[node name="MarkerSwatch" type="ColorRect" parent="UIRoot/CanonicalStage/GameplayTransientHUD/MarkerHUD/Layout"]
+unique_name_in_owner = true
+custom_minimum_size = Vector2(42, 42)
+layout_mode = 2
+mouse_filter = 2
+color = Color(0.960784, 0.933333, 0.85098, 1)
+
+[node name="Text" type="VBoxContainer" parent="UIRoot/CanonicalStage/GameplayTransientHUD/MarkerHUD/Layout"]
+layout_mode = 2
+size_flags_horizontal = 3
+theme_override_constants/separation = 0
+
+[node name="MarkerSummary" type="Label" parent="UIRoot/CanonicalStage/GameplayTransientHUD/MarkerHUD/Layout/Text"]
+unique_name_in_owner = true
+layout_mode = 2
+text = "marker • chalk white • brush 1"
+theme_override_font_sizes/font_size = 18
+
+[node name="MarkerHelp" type="Label" parent="UIRoot/CanonicalStage/GameplayTransientHUD/MarkerHUD/Layout/Text"]
+unique_name_in_owner = true
+layout_mode = 2
+text = "click draw • shift erase • ctrl z undo • shift scroll zoom"
+theme_override_colors/font_color = Color(0.623529, 0.811765, 0.823529, 1)
+theme_override_font_sizes/font_size = 13
+
 [node name="ChatUI" type="Control" parent="UIRoot"]
 unique_name_in_owner = true
 layout_mode = 1
--- a/ui/players_page.gd
+++ b/ui/players_page.gd
@ -105,6 +105,8 @@ func _refresh() -> void:


 func _build_active_rows() -> void:
+	if _service.is_local_host():
+		_build_session_artwork_controls()
 	var entries := _service.get_entries()
 	if entries.is_empty():
 		_add_empty("No authenticated players.")
@ -169,6 +171,33 @@ func _build_active_rows() -> void:
 		_list.add_child(row)


+func _build_session_artwork_controls() -> void:
+	var counts: Vector2i = _service.get_session_artwork_counts()
+	var row := _make_row()
+	var label := Label.new()
+	label.custom_minimum_size.x = 830
+	label.text = "session artwork · %d layers · %d painted pixels" % [
+		counts.x, counts.y,
+	]
+	label.add_theme_color_override(
+		"font_color", UtilityPageStyle.OCEAN_TEXT_PRIMARY
+	)
+	row.add_child(label)
+	var reset := Button.new()
+	reset.text = "reset paint"
+	reset.disabled = counts.x == 0
+	reset.tooltip_text = "Clears all shared artwork from this session."
+	reset.pressed.connect(func() -> void:
+		_confirm(
+			"Clear all shared paint from this session?\nThis cannot be undone.",
+			_service.reset_session_artwork,
+		)
+	)
+	UtilityPageStyle.apply_ocean_button(reset)
+	row.add_child(reset)
+	_list.add_child(row)
+
+
 func _build_relationship_rows() -> void:
 	var records := _service.get_relationships()
 	if records.is_empty():