spaceship/LunarBackground.gd

extends ColorRect

# Feeds the lunar shader the camera's position split into a CELL INDEX plus
# a fraction within that cell, rather than a raw world position. See the
# long note in Starfield.gd for why - identical reasoning, identical split.
#
# Forgejo #68: also bakes the crater field once, at load, instead of letting
# it be re-derived per pixel per frame. The field is a fixed function of
# world position, so every frame was recomputing an identical answer. It is
# rendered once into a square texture covering one period of the field (the
# field is periodic - see the cell wrap in lunar_surface.gdshader) and
# lunar_tiled.gdshader then scrolls that texture with repeat wrapping, which
# keeps the canvas infinite while the per-frame cost drops to one fetch.

# Single source of truth for the tile period, shared with BOTH shaders. The
# field repeats every TILE_CELLS * cell_size world units - at 32 cells of
# 220 that is 7040 units, several screens wide at default zoom.
const TILE_CELLS: float = 32.0
# Texture resolution for the bake, in pixels per cell. One cell is cell_size
# (220) world units, so this is a ~3.4x magnification at default zoom. That
# is affordable because the field is smooth by construction: the sharpest
# feature is the rim band, roughly 0.12 cells wide, which still lands across
# ~8 texels here and so survives bilinear magnification as a gradient rather
# than as steps.
const BAKE_PIXELS_PER_CELL: int = 64

# The palette, sampled from a NASA Artemis lunar photograph. These live on
# the node rather than as parameters of the drawing material because the
# shader that actually consumes them is the BAKE shader, which the scene
# never references directly - leaving them as orphan parameters on the tiled
# material would make them look live when they are only read once at load.
@export var base_color: Color = Color(0.75, 0.75, 0.76)
@export var crater_color: Color = Color(0.44, 0.44, 0.45)
@export var highlight_color: Color = Color(0.89, 0.89, 0.90)

@onready var camera: Camera2D = get_node("../../Camera2D")


func _ready() -> void:
	_bake_field()


# Asked of the shader itself rather than by comparing against a known path,
# so this keeps working whichever shader a surface profile swaps in: the
# question is only ever "does what is drawing this background actually
# sample a baked field".
func _material_wants_baked_field() -> bool:
	if material == null or material.shader == null:
		return false
	for uniform in material.shader.get_shader_uniform_list():
		if String(uniform.get("name", "")) == "field_texture":
			return true
	return false


func _process(_delta: float) -> void:
	var viewport_size: Vector2 = get_viewport_rect().size
	var cell_size: float = material.get_shader_parameter("cell_size")
	var p: Vector2 = camera.global_position / cell_size
	var cell: Vector2 = Vector2(floor(p.x), floor(p.y))

	# The tiled shader samples a repeating texture, so only the camera's
	# position WITHIN the tile matters. Wrapping here rather than in the
	# shader keeps the raw cell index - which reaches the thousands after a
	# while - out of a mobile float entirely: GDScript does this in double
	# precision. fposmod, not fmod, so flying into negative coordinates
	# wraps to the far side of the tile instead of mirroring.
	material.set_shader_parameter("camera_cell", Vector2(
		fposmod(cell.x, TILE_CELLS), fposmod(cell.y, TILE_CELLS)))
	material.set_shader_parameter("camera_frac", p - cell)
	material.set_shader_parameter("camera_zoom", 1.0 / camera.zoom.x)
	material.set_shader_parameter("viewport_size", viewport_size)
	material.set_shader_parameter("tile_cells", TILE_CELLS)


# Renders exactly one period of the procedural field into a texture and hands
# it to the tiled shader. Runs once, during the planet-arrival fade, where a
# single expensive frame is invisible.
func _bake_field() -> void:
	# Headless runs - every scene-based test - rasterize nothing, so a bake
	# would allocate a render target, await a draw that never produces
	# anything readable, and leave this coroutine suspended forever holding
	# the SubViewport alive. Nothing is lost by skipping: there is no display
	# for the background to appear on.
	if DisplayServer.get_name() == "headless":
		return

	# One frame of grace before committing to the bake. A surface profile can
	# replace this material's shader outright - a gas giant brings its own
	# background (#51) - and PlanetSurfaceController does that in _ready(),
	# alongside this one, with no ordering guarantee between them. Rendering
	# a 2048-square lunar tile for a shader that will never sample it would
	# waste exactly the frame the player is already waiting on to land.
	await get_tree().process_frame
	if not _material_wants_baked_field():
		return

	var procedural: Shader = load("res://lunar_surface.gdshader")
	if procedural == null:
		push_warning("lunar background: could not load the field shader to bake")
		return

	var side: int = int(TILE_CELLS) * BAKE_PIXELS_PER_CELL
	var bake_material := ShaderMaterial.new()
	bake_material.shader = procedural
	# Carry the look across from whatever the scene configured, so the tile
	# is baked with the colours actually set on this node rather than the
	# shader's own defaults.
	bake_material.set_shader_parameter("base_color", base_color)
	bake_material.set_shader_parameter("crater_color", crater_color)
	bake_material.set_shader_parameter("highlight_color", highlight_color)
	bake_material.set_shader_parameter("cell_size", material.get_shader_parameter("cell_size"))
	bake_material.set_shader_parameter("tile_cells", TILE_CELLS)
	# Non-zero switches the field shader into bake mode: UV maps straight
	# onto one tile and the per-screen-pixel dither is left out.
	bake_material.set_shader_parameter("bake_cells", TILE_CELLS)

	var sub_viewport := SubViewport.new()
	sub_viewport.size = Vector2i(side, side)
	sub_viewport.disable_3d = true
	sub_viewport.transparent_bg = false
	sub_viewport.render_target_update_mode = SubViewport.UPDATE_ONCE
	var bake_rect := ColorRect.new()
	bake_rect.material = bake_material
	bake_rect.size = Vector2(side, side)
	sub_viewport.add_child(bake_rect)
	add_child(sub_viewport)

	# One frame for the SubViewport to actually draw, then read it back.
	await RenderingServer.frame_post_draw
	var image: Image = sub_viewport.get_texture().get_image()
	sub_viewport.queue_free()

	# Headless runs (every scene-based test) render nothing at all, so there
	# is no image to read back. That is not a failure: the background simply
	# has nothing to show, and the rest of the scene is unaffected.
	if image == null or image.is_empty():
		return

	# Mipmaps are what keep a crater field from shimmering when the camera
	# zooms out and each screen pixel starts covering many texels.
	image.generate_mipmaps()
	var texture := ImageTexture.create_from_image(image)
	material.set_shader_parameter("field_texture", texture)