Sunday, June 19, 2011

On inventory limits

I doubt Kerkerkruip will have an inventory limit. Given that almost all games which feature inventory also feature an inventory limit (the big exception being modern IF), it may be interesting to look at the phenomenon in a bit more detail.

Inventory limits come in several forms.
  1. You can have a straight-up maximum amount of items which the player can have in his inventory. Common in early IF, but pretty unusual in graphical games and even non-graphical rogue-likes, which generally go for the second option, or a combined second/third option. It is used for weapon possession in some shooters, like Borderlands, Left 4 Dead, Crysis 2 and Duke Nukem Forever. (Some of these claims based on reading reviews.)
  2. You have a limited amount of types of items you can carry, but can stack multiple tokens of the same type in one inventory slot. There may be a maximum amount of tokens such a slot can hold, and you may or may not be allowed to dedicate multiple slots to a single type of item. Examples are most rogue-likes and the classic Black Isle RPGs (Baldur's Gate, Planescape: Torment).
  3. You have a maximum amount of X you can carry, and different items (or item stacks) come with a different amount of X. This X is commonly either weight (many rogue-likes, The Witcher 2) or space (Diablo 2). This allows the designer to make some items easier to keep in your possession than others, which can be used for interesting tactical design. Most often, however, such a system seems to be designed for 'realism': as far as I can see there are no deep game design reasons why a suit of armour takes up more X than a ring in Diablo 2 and Dungeon Crawl. It just does because that is 'realistic'. (In case it wasn't clear, I have a deep disdain for any design argument that invokes realism. When I see someone complain that it isn't realistic if you can just shoot a bow without having to buy or find more arrows all the time, I want to punch that person.)
  4. Finally, there is a system where you can carry just a few kinds of items, and each kind has a specific maximum. This is almost universally used for ammunition in shooters, where you might be able to carry (say) up to 200 pistol bullets, 50 shotgun shell, 12 rockets, and 4 grenades. You cannot take more grenades by taking less shells.
Given that many games have inventory limits, what good do they do? Let's look at some of the bad reasons first. (I don't want to categorically claim that these reasons could never be good for any game, but it would have to be a rather special game.)
  1. Inventory limit puzzles! Right up there with mazes, everyone hates the kind of old IF puzzle where you can take only 3 items with you through the portal, and you get stuck if you take the wrong ones. Or if you can return through the portal, it just adds tedium as you move back and forth to get new items. You'd have to think of a brilliant puzzle to make this kind of thing work again.
  2. Realism. See above.
  3. Giving the player decisions, and then punishing unwise decisions through boredom. If the player can stash her loot away, can go get it when she needs it, and the only (or major) associated cost is player boredom -- you have done something wrong as a designer. To a certain extent, a game like Dungeon Crawl suffers from this: you can't escape to get to your stash at every point, and there is an objective cost (hunger), but in general dropping stuff in a stash and retrieving it when you need it works. And is boring.
  4. Making the optimal strategy really boring. In almost every game with shops that buy items, the optimal strategy is to pick up absolutely everything until your inventory is full, then returning to the shop and selling all items. Generally, this would mean that you are spending half the game travelling between the action and the shop. Nobody does this, instead only picking up the most valuable items; but the game does in fact reward you for doing the really boring thing. Not a good design idea.
  5. Getting people to buy another copy of your game. You can artificially increase your inventory limit when you play Diablo 2 on if you own a second copy of the game -- it allows you to log in as two players at once and swap items between characters. Really. Try getting a full set of set items without this trick.
 Now for some good reasons for having inventory limits.
  1. Adding interesting tactical/strategic decisions. Do you take the scroll of teleportation for an easy escape, or the axe of fire for when you meet a hydra? Is that ring of poison resistance really more important than the boots of stealthiness? And so on. In order for there to be real choice, it is essential that items you do not take become unavailable: they either disappear or the in-game cost of retrieving them becomes too great. (If this fails, you get option 3 above.) It is also essential that you have enough non-hierarchically related items to make the decisions interesting: if one item is always clearly better than another, there is no decision.
  2. Saving the player from becoming overwhelmed by choice. While having options is fun, having 328 options is not fun. If my inventory in Borderlands contains more than ten weapons, I'm starting to lose sight of what they all do, and I'll just stop using them -- there's no way I could go through all of them and choose the right one during combat. So thank god for an inventory limit that at least forces me to get rid of my weapons when I hit the 20 weapon mark. Otherwise, I would at some point face the daunting prospect of having to look through 1000 weapons and finding the best one...
  3. Making the player use items. In general, if you are not risking permanent death, it is strategically best to save your best items for later. So the player who is implementing the best strategy might be saving up all the cool stuff, never using any of it. But if she hits an inventory limit, using it suddenly becomes the best choice -- and this often adds to the fun of the game. For instance, the best 'normal' weapon in Half-life 2 is the Overwatch machine gun. This gun has a ridiculously low ammo limit of 90 bullets. This means that once you hit 90 bullets, and you see some Overwatch guys (who will drop ammo for it), you ought to use this highly satisfying gun. If there were no inventory limit, you might try to do everything with your pistol, stockpiling thousands of machine gun bullets for who knows what bad-ass enemies the rest of the game might bring.
All of these good reasons, especially the latter two, are more applicable to long games than to very short games. In Desktop Dungeons or Kerkerkruip, there is no temptation to save your items for a later threat, because the final threat is right there, in your face. There is no possibility of being overwhelmed by choice, because you'll never have more than ten items. And given the small world of the game, it is not easy to see how you could implement a system where items become permanently unavailable if you don't pick them up. All right, I can think of some systems. But they wouldn't be very natural, and you'd only want to implement them if you were certain they would result in good tactical choices. In a game with few items, it is doubtful whether further restricting the amount of items you can use is the best source of more tactical decisions.

It should also be obvious that none of the three reasons applies to either puzzle-based or puzzleless modern IF. They only apply to games with repeatable actions and underlying game mechanisms. So it is clear why modern IF has moved away from the inventory limit.


  1. Another exception is flash-based point-and-click/escape-the-room games, where you generally either have unlimited space or exactly enough space for every item in the game (and items may wind up in predesignated spaces -- the screwdriver always goes in slot five, no matter when you pick it up). Of course the expectation is that you will pick up everything you find. In most of the ones I've seen, there's no way to drop anything short of using it, though in many games objects simply disappear from your inventory when you've done everything you can with them. (The IF games that parody these make fun of the disappearing key, but I think it's an excellent way of giving you feedback.)

    Of course these games have basically the same puzzle structures as IF, so it's no surprise that they treat inventory similarly.

    Very nice analysis of the bad reasons for inventory limits. Nethack sometimes does well with strategy 1, particularly with very heavy items such as armor (and the knight's lance), and there's some element of strategy involved because you can't always escape back to your stash. But my remarkably successful game of SLASH'EM has been on hiatus for almost two years because it degenerated into a lot of stash-shifting. (Also, the best melee weapon for me now is a battle-axe, which with my unihorn and longbow and luckstone and healthstones is too damn heavy for me to carry the rest of my crap; this is good reason #1 but is still annoying.)

    Did you play the demo of full-fledged Desktop Dungeons? The freeware does have a limit for spells, which forces some interesting choices (especially because you need an open slot to convert glyphs), but I thought the demo went too far in putting potions and store items into an inventory limit too. For one thing, store items are already limited by the amount of gold you have, which forces strategic decisions in between games; for another, now the only reason to pick up potions before you need to drink them is a horrible interface botch where you can't use spells or items when you're standing on an item (or altar or stairway) -- though I'm assuming they'll fix that.

  2. Parasite Eve is an interesting case study. For the most part, its limited inventory is of the tactical/strategic decisions variety, and it has a unique twist: you can choose to upgrade your inventory with bonus points (which could otherwise be used to upgrade other things).

    However, the game has one feature which takes the "boring optimal strategy" to the MAX. In certain areas, monsters drop "Junk". You can take Junk back to your base and stache it, and if you bring back enough junk (some ungodly amount), you get the best weapon in the game. Nobody really does this, though... you'd have to have absolutely no life

  3. I'm someone who released a game with not only an inventory limit but also a puzzle where you have to deal with a portal that limits what you take through it. And that was in 2006! Any game element has to pass the "why is this in there?" test, and different games will answer that question differently. You dismiss "for realism", as do I -- but only when considered in isolation. I can imagine a game where the whole point is a realistic simulation of a scenario. In my case one of my game's big themes was that you had very limited ways to manipulate the world around you and had to overcome those limitations to meet your goals. For that game, the inventory limit meshed with and furthered the theme.

    Early IF spent a lot of time using drop-in puzzle elements without thinking about why they were there. Colossal Cave had maze-like rooms with non-matching directions (so that you would leave a room to the east but then return to it by going south) because that's what caves are like and Woods was creating a cave simulator. IF authors who followed Crowther and Woods did the same because, well, that's what was in Colossal Cave, leading to spaceships with mazes and college dorms with mazes. Andrew Plotkin then took the tired trope of caves and mazes and created "Hunter, in Darkness" in which these elements are central to what the game is about. I think inventory limits can be similarly redeemed.

  4. "I'm someone who released a game with not only an inventory limit but also a puzzle where you have to deal with a portal that limits what you take through it."

    I was hoping this was going to turn out to be a description of the door from the garage in "Common Ground."

  5. Oh, hah. I hadn't thought of that scene in Common Ground in quite that way, but yeah, it's definitely related since I made players repeat certain commands to get a feeling of tedium and frustration across.

    For anyone who hasn't played it, there's a bit in Common Ground where you're trying to get from your car into your house with a load of groceries. You have to close your car door (twice!) and unlock the house door. When you go to unlock the house door you drop your keys and have to retrieve them. I break a number of IF rules, like making you have to UNLOCK DOOR instead of the game auto-unlocking the door for you, but elide commands that don't add to the tedium. I remember it being a pain to tweak disambiguation and auto-commands until it gave the feel I was after. I wasn't sure how well that scene would work, but Iain Merrick's review mentioned it specifically (, which pleased me.

  6. Matt, I did not play the demo of Desktop Dungeons -- it seems to have been online only for a short time. It doesn't sound like a game that needs an inventory limit, though. (Well, yes, for the spells perhaps. Though there already is a reason to destroy spells, as this gives you a bonus that can be used immediately -- unless you are one of the races which turns spells into potions.)

    Xamuel: that sounds like a good implementation. (I cannot play the game, since it seems to be console-only.)

    Stephen: absolutely, it is of course possible to make an inventory limit puzzle work. Let's just say that it requires a little more thought than it was usually given in the early era of IF. :)

  7. In MegaZeux, you have a limit of sixteen keys. I can make up puzzles that would be impossible to complete if this limit were not in place!

    Or, in a text adventure game, that you have an inventory limit. But, one room types in its own commands for you (you can type in your own command after each of its automatic commands). If the first command is PICK UP THE BOMB and it will cause it to explode, then how can you enter that room without explodiing? O, what you need, is carry too many things now you cannot pick up the bomb. But, what happened after some wizard made you have infinite carrying capacity? Well, then, what you should do is get your own defused bomb and throw it into the room with the other bomb before entering. Now the command PICK UP THE BOMB is ambiguous.