Adjusts Roblox Rthro avatars so that their height scales match a 5 stud high R15 rig

RthroScaleFix.lua

local CLASSIC_HEIGHT = 5

local function getObjectValue(parent: Instance, name: string, default)
	local found = parent:FindFirstChild(name)
	if found then
		return found.Value
	end
	return default
end

local function matchHeadToMesh(head, headMesh)
	for _, child: Instance in pairs(head:GetChildren()) do
		if child:IsA("Attachment") and not headMesh:FindFirstChild(child.Name) then
			local Attacthment: Attacthment = child
			local vec3Copy = Instance.new("Vector3Value")
			vec3Copy.Name = Attacthment.Name
			vec3Copy.Value = Attacthment.Position
			vec3Copy.Parent = headMesh
		end
	end

	local partScaleType = head:FindFirstChild("AvatarPartScaleType")
	if partScaleType and not headMesh:FindFirstChild("AvatarPartScaleType") then
		partScaleType:Clone().Parent = headMesh
	end
end

local function getMaxHeight(humanoid: Humanoid)
	local humanoidRootPart = humanoid.RootPart
	local character = humanoid.Parent

	local upperTorso = character.UpperTorso
	local head = character.Head

	local root = character.LowerTorso.Root
	local waist = upperTorso.Waist
	local neck = head.Neck

	-- part0 * c0 == part1 * c1
	local lowerTorsoCF = root.C0 * root.C1:Inverse()
	local upperTorsoCF = lowerTorsoCF * waist.C0 * waist.C1:Inverse()
	local headCF = upperTorsoCF * neck.C0 * neck.C1:Inverse()

	local upperTorsoTop = upperTorsoCF.Y + upperTorso.Size.Y / 2
	local headTop = headCF.Y + head.Size.Y / 2

	-- Sometimes the upper torso is higher than the head.
	-- For example: https://www.roblox.com/bundles/429/Magma-Fiend
	return math.max(upperTorsoTop, headTop) + humanoidRootPart.Size.Y / 2 + humanoid.HipHeight
end

local function rthroScaleFix(character: Model)
	local humanoid = character.Humanoid
	local humanoidRootPart = humanoid.RootPart

	local floorCF = humanoidRootPart.CFrame * CFrame.new(0, -(humanoidRootPart.Size.Y / 2 + humanoid.HipHeight), 0)

	local height = getMaxHeight(humanoid)
	local scale = (CLASSIC_HEIGHT / height) * getObjectValue(humanoid, "BodyHeightScale", 1)

	local head = character.Head
	local headMesh = head:FindFirstChildWhichIsA("SpecialMesh")
	local isFileMesh = (headMesh.MeshType == Enum.MeshType.FileMesh)

	local accessories = {}
	for _, child: Instance in pairs(character:GetChildren()) do
		if child:IsA("Accessory") then
			local Accessory: Accessory = child
			local handle = Accessory:FindFirstChildWhichIsA("BasePart")
			local weld = handle:FindFirstChild("AccessoryWeld")
			weld:Destroy()
			Accessory.Parent = nil
			accessories[Accessory] = true
		end
	end

	matchHeadToMesh(head, headMesh)

	for _, child: Instance in pairs(character:GetDescendants()) do
		if child:IsA("Motor6D") then
			local Motor6D: Motor6D= child
			local p0 = Motor6D.C0.Position
			local p1 = Motor6D.C1.Position
			Motor6D.C0 = (Motor6D.C0 - p0) + p0 * scale
			Motor6D.C1 = (Motor6D.C1 - p1) + p1 * scale
		elseif child:IsA("Attachment") then
			local Attachment: Attachment = child
			Attachment.Position = Attachment.Position * scale
			Attachment.OriginalPosition.Value = Attachment.OriginalPosition.Value * scale
		elseif child.Name == "OriginalSize" then
			local parent = child.Parent

			if parent:IsA("BasePart") then
				parent.Size = parent.Size * scale
				child.Value = child.Value * scale
			elseif parent == headMesh then
				for _, v3 in pairs(parent:GetChildren()) do
					if v3:IsA("Vector3Value") and v3 ~= child then
						v3.Value = v3.Value * scale
					end
				end

				if isFileMesh then
					parent.Scale = parent.Scale * scale
					child.Value = child.Value * scale
				end
			end
		end
	end

	for accessory: Accessory, _ in pairs(accessories) do
		local handle: BasePart = accessory:FindFirstChildWhichIsA("BasePart")
		handle.OriginalSize.Value = handle.OriginalSize.Value * scale
		humanoid:AddAccessory(accessory)
	end

	humanoid.HipHeight = humanoid.HipHeight * scale
	humanoidRootPart.CFrame = floorCF * CFrame.new(0, humanoidRootPart.Size.Y / 2 + humanoid.HipHeight, 0)
end

return rthroScaleFix