trove.lua

-- Renders the sky on the faces of parts tagged `RenderSky:(side)`.

local Lighting = game:GetService("Lighting")
local RunService = game:GetService("RunService")
local CollectionService = game:GetService("CollectionService")

local sky = Lighting:WaitForChild("sky")
local playerGui = game:GetService'Players'.LocalPlayer:WaitForChild'PlayerGui'

local VEC_XZ = Vector3.new(1, 0, 1)
local VEC_YZ = Vector3.new(0, 1, 1)

local SKYBOX_MAP = {
	["Bk"] = CFrame.fromEulerAnglesYXZ(0, math.rad(180), 0),
	["Dn"] = CFrame.fromEulerAnglesYXZ(-math.rad(90), math.rad(90), 0),
	["Ft"] = CFrame.fromEulerAnglesYXZ(0, 0, 0),
	["Lf"] = CFrame.fromEulerAnglesYXZ(0, -math.rad(90), 0),
	["Rt"] = CFrame.fromEulerAnglesYXZ(0, math.rad(90), 0),
	["Up"] = CFrame.fromEulerAnglesYXZ(math.rad(90), math.rad(90), 0),
}

-- Builds the six-sided skybox model.
local function sky_sphere(sky)
	local model = Instance.new("Model")
	model.Name = "SkyboxModel"
	for prop, rotationCF in SKYBOX_MAP do
		local side = Instance.new("Part")
		side.Anchored = true
		side.Name = prop
		side.CFrame = rotationCF
		side.Size = Vector3.one
		side.Parent = model
		local mesh = Instance.new("SpecialMesh")
		mesh.MeshId = "rbxassetid://3083991485"
		mesh.Offset = Vector3.new(0, 0, -0.683)
		mesh.Parent = side
		local decal = Instance.new("Decal")
		decal.Texture = sky["Skybox" .. prop]
		decal.Parent = side
	end
	return model
end

local function window_new(part, face)
	local self = {}

	local surfaceGui = Instance.new("SurfaceGui")
	surfaceGui.LightInfluence = 0
	surfaceGui.Name = "SkyboxViewportWindow"
	surfaceGui.Face = face
	surfaceGui.Adornee = part
	surfaceGui.ClipsDescendants = true
	surfaceGui.SizingMode = Enum.SurfaceGuiSizingMode.FixedSize
	surfaceGui.ZOffset = 2
	surfaceGui.Parent = playerGui

	local camera = Instance.new("Camera")
	camera.Name = "SkyboxCamera"
	camera.Parent = surfaceGui

	local viewportFrame = Instance.new("ViewportFrame")
	viewportFrame.BackgroundTransparency = 1
	viewportFrame.Size = UDim2.fromScale(1, 1)
	viewportFrame.Name = "Skybox"
	viewportFrame.ZIndex = 1
	viewportFrame.Ambient = Color3.new(0.75, 0.75, 0.75)
	viewportFrame.LightColor = Color3.new(1, 1, 1)
	viewportFrame.LightDirection = -Lighting:GetSunDirection()
	viewportFrame.CurrentCamera = camera
	viewportFrame.Parent = surfaceGui

	self.surfaceGui = surfaceGui
	self.skyboxCamera = camera
	self.skyboxViewportFrame = viewportFrame

	return self
end

-- Called exactly once per window.
local function window_set_sky(self, sky)
	local skyCopy = sky:Clone()
	skyCopy.Name = "Sky"
	skyCopy.Parent = self.skyboxViewportFrame

	local skyModel = sky_sphere(sky)
	skyModel:ScaleTo(10)
	skyModel.Parent = self.skyboxViewportFrame
end

-- Computes the world-space CFrame and size of the adornee's face.
local function window_get_surface(self)
	local surfacePart = self.surfaceGui.Adornee
	local v = -Vector3.FromNormalId(self.surfaceGui.Face)
	local u = Vector3.new(v.Y, math.abs(v.X + v.Z), 0)
	local relativeCF = CFrame.fromMatrix(Vector3.zero, u:Cross(v), u, v)
	local worldCF = surfacePart.CFrame * CFrame.new(-v * surfacePart.Size / 2) * relativeCF
	local size = Vector3.new(
		math.abs(relativeCF.XVector:Dot(surfacePart.Size)),
		math.abs(relativeCF.YVector:Dot(surfacePart.Size)),
		math.abs(relativeCF.ZVector:Dot(surfacePart.Size))
	)
	return worldCF, size
end

-- Per-frame render; updates `skyboxCamera` from `camera`.
local function window_render(self, camera)
	local surfaceCF, surfaceSize = window_get_surface(self)
	local cameraCF = camera.CFrame
	local cameraPos = cameraCF.Position
	local dv = surfaceCF:VectorToObjectSpace(surfaceCF.Position - cameraPos)
	local dvXZ = (dv * VEC_XZ).Unit
	local dvYZ = dv * VEC_YZ
	local dvZ = -dvXZ.Z
	local camXZ = (surfaceCF:VectorToObjectSpace(cameraCF.LookVector) * VEC_XZ).Unit
	local scale = camXZ:Dot(dvXZ) / dvZ
	local tanArcCos = math.sqrt(1 - dvZ * dvZ) / dvZ

	local xCross = surfaceCF.YVector:Cross(cameraCF.ZVector)
	local xVec = xCross:Dot(xCross) > 0 and xCross.Unit or cameraCF.XVector
	local levelCF = CFrame.fromMatrix(cameraPos, xVec, surfaceCF.YVector)

	local topC = surfaceCF * Vector3.new(0, surfaceSize.Y / 2, 0)
	local botC = surfaceCF * Vector3.new(0, -surfaceSize.Y / 2, 0)
	local camTop = levelCF:PointToObjectSpace(topC)
	local camBot = levelCF:PointToObjectSpace(botC)

	local topD = (camTop * VEC_YZ).Unit
	local botD = (camBot * VEC_YZ).Unit
	local fovH = 2 * math.tan(math.rad(camera.FieldOfView / 2))
	local surfFovH = math.tan(math.sign(topD.Y) * math.acos(-topD.Z))
		- math.tan(math.sign(botD.Y) * math.acos(-botD.Z))
	local ratio = surfFovH / fovH

	local w, h = 1, 1
	if self.surfaceGui.SizingMode == Enum.SurfaceGuiSizingMode.FixedSize then
		local canvas = self.surfaceGui.CanvasSize
		h = (canvas.Y / canvas.X) * (surfaceSize.X / surfaceSize.Y)
	end

	local dx = math.sign(dv.X * dv.Z) * tanArcCos
	local dy = dvYZ.Y / dvYZ.Z * h
	local dz = math.abs(scale * ratio * h)
	local m = math.max(w, h, math.abs(dx), math.abs(dy), dz)

	local renderRot = surfaceCF.Rotation
		* CFrame.fromEulerAnglesYXZ(0, math.pi, 0)
		* CFrame.new(0, 0, 0, w / m, 0, 0, 0, h / m, 0, dx / m, dy / m, dz / m)

	self.skyboxCamera.FieldOfView = camera.FieldOfView
	self.skyboxCamera.CFrame = (renderRot + cameraPos).Rotation
end

-- Part -> { [face: EnumItem (NormalId)] = Window }
local windows = {}

local function createwindow(part, face)
	local partWindows = windows[part]
	if partWindows == nil then
		partWindows = {}
		windows[part] = partWindows
	end

	local window = window_new(part, face)
	partWindows[face] = window
	window_set_sky(window, sky)

	-- The game leaves the SurfaceGui in PlayerGui when the part is destroyed;
	-- destroying it also removes its descendants (camera, viewport, sky).
	part.Destroying:Connect(function()
		partWindows[face] = nil
		window.surfaceGui:Destroy()
	end)
end

local sides = { "Front", "Back", "Left", "Right", "Top", "Bottom" }

for _, side in sides do
	local tag = "RenderSky:" .. side
	local face = Enum.NormalId[side]
	for _, part in CollectionService:GetTagged(tag) do
		createwindow(part, face)
	end
	CollectionService:GetInstanceAddedSignal(tag):Connect(function(part)
		createwindow(part, face)
	end)
end

RunService.RenderStepped:Connect(function()
	-- workspace.CurrentCamera can be nil between frames.
	local camera = workspace.CurrentCamera
	if camera == nil then
		return
	end
	for _, partWindows in windows do
		for _, window in partWindows do
			window_render(window, camera)
		end
	end
end)