minetest_inventory_icon/init.lua

inventory_icon = {}
inventory_icon.hudids = {}

function inventory_icon.get_inventory_state(player) 
	local inv = player:get_inventory()
	local size = inv:get_size("main")
	local occupied = 0
	for i=1,size do
		local stack = inv:get_stack("main", i)
		if not stack:is_empty() then
			occupied = occupied + 1
		end
	end
	return occupied, size
end

minetest.register_on_joinplayer(function(player)
	local name = player:get_player_name()
	inventory_icon.hudids[name] = {}
	local occupied, size = inventory_icon.get_inventory_state(player)
	local icon, color
	if occupied >= size then
		icon = "inventory_icon_backpack_full.png"
		color = "0xFF0000"
	else
		icon = "inventory_icon_backpack_free.png"
		color = "0xFFFFFF"
	end
	inventory_icon.hudids[name].icon = player:hud_add({
		hud_elem_type = "image",
		position = {x=1,y=1},
		scale = {x=1,y=1},
		offset = {x=-32,y=-32},
		text = icon,
	})
	inventory_icon.hudids[name].text = player:hud_add({
		hud_elem_type = "text",
		position = {x=1,y=1},
		scale = {x=1,y=1},
		offset = {x=-36,y=-20},
		alignment = {x=0,y=0},
		number = color,
		text = string.format("%d/%d", occupied, size)
	})
end)

minetest.register_on_leaveplayer(function(player)
	inventory_icon.hudids[player:get_player_name()] = nil
end)

inventory_icon.timer = 0
minetest.register_globalstep(function(dtime)
	inventory_icon.timer = inventory_icon.timer + dtime
	if inventory_icon.timer > 1 then
		for playername,hudids in pairs(inventory_icon.hudids) do
			local player = minetest.get_player_by_name(playername)
			local occupied, size = inventory_icon.get_inventory_state(player)
			local icon, color
			if occupied >= size then
				icon = "inventory_icon_backpack_full.png"
				color = "0xFF0000"
			else
				icon = "inventory_icon_backpack_free.png"
				color = "0xFFFFFF"
			end
			player:hud_change(hudids.icon, "text", icon)
			player:hud_change(hudids.text, "text", string.format("%d/%d", occupied, size))
			player:hud_change(hudids.text, "number", color)
		end
	end
end)