SurfaceGui space -> world space

guiPointToWorld.lua

-- Vector3 guiPointToWorld(SurfaceGui gui, Vector2 point)
-- SurfaceGui space -> world space
local function guiPointToWorld(gui, point)
	local adornee = assert(gui.Adornee)

	local normalizedPoint = point/gui.AbsoluteSize
	local dir = Vector3.FromNormalId(gui.Face)

	local projY = math.abs(dir.z + dir.x)
	local projZ = dir.x + math.abs(dir.y)

	local origin = Vector3.new(
		dir.x - dir.y - dir.z,
		dir.y + projY,
		dir.z + projZ)*0.5
	
	local surfX = Vector3.new(dir.z, 0, -projZ)*normalizedPoint.x
	local surfY = Vector3.new(dir.y, -projY, 0)*normalizedPoint.y

	local pos = origin + surfX + surfY

	return adornee.CFrame*(adornee.Size*pos)
end