init.lua

-- See https://github.com/luanti-org/luanti/issues/15692
-- Identifies possibly affected bones and prints scales to be used for the described "negative scale" workaround

assert(modlib.version >= 103)

local media_paths = modlib.minetest.media.paths

local function nodes(filepath)
	local stream = assert(io.open(filepath, "rb"))
	local model = assert(modlib.b3d.read(stream))
	stream:close()

	return coroutine.wrap(function()
		local function visit(node)
			coroutine.yield(node)
			for _, child in ipairs(node.children) do
				visit(child)
			end
		end
		visit(model.node)
	end)
end

local function isclose(a, b)
	return math.abs(a - b) < 1e-3
end

for name, path in pairs(media_paths) do
	if name:match"%.b3d$" then
		local first = true
		local _, it = xpcall(function() return nodes(path) end, function(err)
			core.log("warning", ("Failed to read model %s: %s"):format(name, err))
			return function() end
		end)
		for node in it do
			local rot_quat = node.rotation
			local rot_mat = modlib.matrix4.rotation(rot_quat)
			local rxx, ryy, rzz = rot_mat[1][1], rot_mat[2][2], rot_mat[3][3]
			local function abs1(x) return isclose(math.abs(x), 1) end
			local is_scaling = abs1(rxx) and abs1(ryy) and abs1(rzz)
			local flips_axis = isclose(rxx, -1) or isclose(ryy, -1) or isclose(rzz, -1)
			if is_scaling and flips_axis then
				if first then
					print(("Possibly affected bones in `%s`:"):format(name))
				end
				first = false
				local sx, sy, sz = unpack(node.scale)
				-- Suggest scales for the negative scale workaround
				print(("Bone %s should get a scale of (%.01f, %.01f %.01f)"):format(
						node.name, sx * rxx, sy * ryy, sz * rzz))
			end
		end
	end
end