grunk/src/world/gunk_body.gd

class_name GunkBody extends StaticBody3D
## StaticBody3D with an associated "gunkable" mesh.

const MASK_DIM := 512
const FACE_EPSILON := 0.4
const MASK_COLOR := Color.RED

@export var mask_image: Image

var meshtool := MeshDataTool.new()

@onready var mesh_instance: MeshInstance3D = $MeshInstance3D
@onready var mesh: ArrayMesh = mesh_instance.mesh
@onready var gunk_mat: ShaderMaterial = mesh_instance.get_surface_override_material(0).next_pass

@onready var debug_draw: DebugDraw = %DebugDraw


func _ready() -> void:
	if not mask_image:
		mask_image = Image.create_empty(MASK_DIM, MASK_DIM, false, Image.FORMAT_L8)
		_update_gunk_material()

	meshtool.create_from_surface(mesh, 0)


## Must be called internally whenever `mask_image` is updated
func _update_gunk_material() -> void:
	gunk_mat.set_shader_parameter("gunk_mask", ImageTexture.create_from_image(mask_image))


## Transform cartesian coordinates to barycentric wrt the given triangle
func _barycentric(p: Vector3, a: Vector3, b: Vector3, c: Vector3) -> Vector3:
	var v0 := b - a
	var v1 := c - a
	var v2 := p - a
	var d00 := v0.dot(v0)
	var d01 := v0.dot(v1)
	var d11 := v1.dot(v1)
	var d20 := v2.dot(v0)
	var d21 := v2.dot(v1)
	var denom := d00 * d11 - d01 * d01
	var v := (d11 * d20 - d01 * d21) / denom
	var w := (d00 * d21 - d01 * d20) / denom
	var u := 1.0 - v - w
	return Vector3(u, v, w)


func _is_in_triangle(point: Vector3, v1: Vector3, v2: Vector3, v3: Vector3) -> bool:
	var bc := _barycentric(point, v1, v2, v3)
	return (bc.x > 0 and bc.x < 1) and (bc.y > 0 and bc.y < 1) and (bc.z > 0 and bc.z < 1)


func _get_face(point: Vector3, normal: Vector3) -> int:
	for i in range(meshtool.get_face_count()):
		if meshtool.get_face_normal(i).distance_squared_to(normal) > FACE_EPSILON:
			continue
		# Normals match, so check if the point is on this face
		var v1 := meshtool.get_vertex(meshtool.get_face_vertex(i, 0))
		var v2 := meshtool.get_vertex(meshtool.get_face_vertex(i, 1))
		var v3 := meshtool.get_vertex(meshtool.get_face_vertex(i, 2))
		if _is_in_triangle(point, v1, v2, v3):
			return i
	return -1


func _get_uv(point: Vector3, normal: Vector3) -> Vector2:
	var face := _get_face(point, normal)
	if face < 0:
		return Vector2.INF
	var fv0 := meshtool.get_face_vertex(face, 0)
	var fv1 := meshtool.get_face_vertex(face, 1)
	var fv2 := meshtool.get_face_vertex(face, 2)
	var v1 := meshtool.get_vertex(fv0)
	var v2 := meshtool.get_vertex(fv1)
	var v3 := meshtool.get_vertex(fv2)
	var bc := _barycentric(point, v1, v2, v3)  # TODO memoize
	var uv1 := meshtool.get_vertex_uv(fv0)
	var uv2 := meshtool.get_vertex_uv(fv1)
	var uv3 := meshtool.get_vertex_uv(fv2)
	return (uv1 * bc.x) + (uv2 * bc.y) + (uv3 * bc.z)


## Paint a rectangle on the mask at a given point & normal on the mesh.
func paint_mask(point: Vector3, normal: Vector3, radius: int) -> void:
	# debug_draw.draw_vector(normal, point)
	var local_point := point * global_transform
	var local_normal := normal * global_basis
	var uv := _get_uv(local_point, local_normal)
	if uv == Vector2.INF:
		return
	var px_center: Vector2i = Vector2i(uv * Vector2(mask_image.get_size()))
	var rect := Rect2i(px_center.x - radius, px_center.y - radius, 2 * radius, 2 * radius)
	mask_image.fill_rect(rect, MASK_COLOR)
	# TODO: can we call this once after all painting in a single frame is finished?
	_update_gunk_material()