Bullosseum/blood_decals.gd

extends MultiMeshInstance3D
## A whole level's worth of persistent blood splats rendered in one draw call. Each stain
## is a flat quad laid on the surface it hit — floor or a nearby wall — picked from a
## procedurally-baked atlas of splat shapes and jittered per instance (random shape, yaw,
## size, brightness) so no two read alike.
##
## The pool is a fixed ring buffer: new splats overwrite the oldest, so the instance count
## and fill cost stay bounded no matter how long a fight runs — the property that keeps it
## cheap on the web/mobile target. The game shell rebuilds this node into LevelRoot on every
## level load, so the stains die with the level (exactly one level's worth, never leaked
## into the next). Fire splats through the `Gore` autoload, not this node directly.

const PhysicsLayers = preload("res://physics_layers.gd")
const SHADER_PATH := "res://blood_decals.gdshader"
const GROUP := &"blood_decals"

# Atlas: ATLAS_CELLS distinct splat shapes packed into one row, CELL_PX square each.
const ATLAS_CELLS := 8
const CELL_PX := 64

var _rng := RandomNumberGenerator.new()
var _mm: MultiMesh
var _cap: int = 0
var _next: int = 0  # ring-buffer write cursor
var _filled: int = 0  # how many ring-buffer slots have ever been stamped, capped at _cap


func _ready() -> void:
	add_to_group(GROUP)
	_rng.randomize()
	_cap = maxi(int(DP.f("blood_cap")), 0)
	_build()


func _build() -> void:
	var quad := QuadMesh.new()
	quad.size = Vector2.ONE  # unit quad; the instance basis carries the real size

	var mat := ShaderMaterial.new()
	mat.shader = load(SHADER_PATH) as Shader
	mat.set_shader_parameter("atlas", _bake_atlas())
	mat.set_shader_parameter("cells", ATLAS_CELLS)
	material_override = mat

	_mm = MultiMesh.new()
	_mm.transform_format = MultiMesh.TRANSFORM_3D
	_mm.use_custom_data = true
	_mm.mesh = quad
	_mm.instance_count = _cap
	# Godot renders only the first `visible_instance_count` instances, so unstained slots simply
	# aren't drawn — no need to park them off-map. (Parking them via set_instance_transform doesn't
	# work anyway: MultiMesh's per-instance buffer lives server-side and isn't reliably readable
	# back through get_instance_transform, so a "hide by moving far away" scheme can't even be
	# verified, let alone trusted.) Starts at 0 and grows as splats land.
	_mm.visible_instance_count = 0
	multimesh = _mm


## Stain the world at `world_pos`: one splat cluster on the floor beneath it, plus a splat
## on any wall within `blood_wall_reach`. `dir` is the spray heading (used to seed the wall
## fan); `size` is the base quad size in metres.
func splat(world_pos: Vector3, dir: Vector3 = Vector3.ZERO, size: float = 0.6) -> void:
	if not is_instance_valid(_mm) or _cap <= 0:
		return
	var space := get_world_3d().direct_space_state
	if space == null:
		return

	# Floor directly under the hit.
	var floor_hit := _ray(space, world_pos + Vector3.UP * 0.5, world_pos + Vector3.DOWN * 4.0)
	if not floor_hit.is_empty():
		_stamp_cluster(floor_hit.position, floor_hit.normal, size)

	# Walls near the hit: a fan of outward rays, seeded on the spray heading. Only surfaces
	# within reach catch blood, so an open-arena hit stains nothing but the floor.
	var rays := maxi(int(DP.f("blood_wall_rays")), 0)
	if rays <= 0:
		return
	var reach := DP.f("blood_wall_reach")
	var seed_dir := Vector3(dir.x, 0.0, dir.z)
	seed_dir = seed_dir.normalized() if seed_dir.length_squared() > 0.01 else Vector3.FORWARD
	var origin := world_pos + Vector3.UP * 0.6
	for k in rays:
		var ang := TAU * (float(k) + 0.5) / float(rays)
		var out := seed_dir.rotated(Vector3.UP, ang)
		var wall_hit := _ray(space, origin, origin + out * reach)
		# Only stain near-vertical surfaces here; the floor is already handled above.
		if not wall_hit.is_empty() and absf((wall_hit.normal as Vector3).y) < 0.6:
			_stamp(wall_hit.position, wall_hit.normal, size * 0.85)


func _ray(space: PhysicsDirectSpaceState3D, from: Vector3, to: Vector3) -> Dictionary:
	var q := PhysicsRayQueryParameters3D.create(
		from, to, PhysicsLayers.WORLD | PhysicsLayers.CORPSE
	)
	q.collide_with_bodies = true
	return space.intersect_ray(q)


# A main splat plus a scatter of smaller droplets around it, all lying on the same surface.
func _stamp_cluster(pos: Vector3, normal: Vector3, size: float) -> void:
	_stamp(pos, normal, size)
	var n := normal.normalized()
	if n.length_squared() < 0.5:
		n = Vector3.UP
	var up := Vector3.UP if absf(n.dot(Vector3.UP)) < 0.99 else Vector3.FORWARD
	var tx := up.cross(n).normalized()
	var ty := n.cross(tx).normalized()
	var count := maxi(int(DP.f("blood_satellites")), 0)
	for s in count:
		var rad := size * _rng.randf_range(0.4, 1.2)
		var a := _rng.randf() * TAU
		var off := tx * (cos(a) * rad) + ty * (sin(a) * rad)
		_stamp(pos + off, n, size * _rng.randf_range(0.25, 0.5))


# Write one splat into the ring buffer, oriented flat on the surface and lifted a hair along
# its normal (layered by write index) so coplanar quads don't z-fight the surface or each other.
func _stamp(pos: Vector3, normal: Vector3, size: float) -> void:
	var n := normal.normalized()
	if n.length_squared() < 0.5:
		n = Vector3.UP
	var i := _next
	_next = (_next + 1) % _cap
	var eps := 0.015 + float(i) * 0.00015
	var yaw := _rng.randf() * TAU
	_mm.set_instance_transform(i, Transform3D(_surface_basis(n, yaw, size), pos + n * eps))
	var cell := float(_rng.randi_range(0, ATLAS_CELLS - 1))
	var shade := _rng.randf_range(0.6, 1.0)
	_mm.set_instance_custom_data(i, Color(cell, shade, 0.0, 0.0))
	_filled = mini(_filled + 1, _cap)
	_mm.visible_instance_count = _filled


# Basis for a QuadMesh (face along local +Z) lying flat on a surface with the given normal,
# spun by `yaw` about that normal and uniformly scaled to `size`.
func _surface_basis(n: Vector3, yaw: float, size: float) -> Basis:
	var up := Vector3.UP if absf(n.dot(Vector3.UP)) < 0.99 else Vector3.FORWARD
	var x := up.cross(n).normalized()
	var y := n.cross(x).normalized()
	var c := cos(yaw)
	var s := sin(yaw)
	var b := Basis()
	b.x = (x * c + y * s) * size
	b.y = (y * c - x * s) * size
	b.z = n
	return b


# ── Atlas baking ──────────────────────────────────────────────────────────────
# Draw ATLAS_CELLS irregular blood shapes into one row. Each shape is a metaball field —
# a big central blob, a few overlapping lobes and some flung droplets — thresholded to an
# organic silhouette. Baked once per pool (once per level load, ~ms) with a fixed seed so
# the shape set is deterministic and every level's stains match.

func _bake_atlas() -> ImageTexture:
	var img := Image.create(ATLAS_CELLS * CELL_PX, CELL_PX, false, Image.FORMAT_RGBA8)
	img.fill(Color(0.0, 0.0, 0.0, 0.0))
	var rng := RandomNumberGenerator.new()
	rng.seed = hash("bullosseum-blood")
	for c in ATLAS_CELLS:
		_draw_splat(img, c * CELL_PX, rng)
	return ImageTexture.create_from_image(img)


func _draw_splat(img: Image, ox: int, rng: RandomNumberGenerator) -> void:
	var px := float(CELL_PX)
	var mid := px * 0.5
	# blobs are (centre_x, centre_y, radius) — a central mass, lobes, then far droplets.
	var blobs: Array[Vector3] = []
	blobs.append(Vector3(mid, mid, px * rng.randf_range(0.22, 0.30)))
	for i in rng.randi_range(3, 6):
		var a := rng.randf() * TAU
		var d := px * rng.randf_range(0.10, 0.34)
		blobs.append(Vector3(mid + cos(a) * d, mid + sin(a) * d, px * rng.randf_range(0.06, 0.16)))
	for i in rng.randi_range(2, 5):
		var a := rng.randf() * TAU
		var d := px * rng.randf_range(0.30, 0.46)
		blobs.append(Vector3(mid + cos(a) * d, mid + sin(a) * d, px * rng.randf_range(0.02, 0.05)))

	for y in CELL_PX:
		for x in CELL_PX:
			var field := 0.0
			for b in blobs:
				var dx := float(x) - b.x
				var dy := float(y) - b.y
				field += (b.z * b.z) / (dx * dx + dy * dy + 1.0)
			var a := smoothstep(0.75, 1.15, field)
			if a <= 0.004:
				continue
			# Denser field (splat interior) reads a touch richer/darker than the thin edges.
			var t := clampf(field * 0.5, 0.0, 1.0)
			var col := Color(0.62 - 0.16 * t, 0.05 - 0.03 * t, 0.04 - 0.02 * t, a)
			img.set_pixel(ox + x, y, col)