Thinking spatially: inventory systems


Three robots with big space on the left on grey background

Whether pack rats or not, players typically need to carry and keep track of a vast number of items as they travel through a game world. The system for storing player items is known as an inventory, presumably named after the shopkeeping inventory. But is its purpose really just to catalogue? Or are the choices you make when designing your game’s item management system far more important to the feel of your overall game? And what challenges and opportunities come from building a game’s inventory system on futuristic technology like SpatialOS?

To find an answer, we need to go back to the beginning…

Hamurabi by Doug Dymant

Inventing the inventory

The first inventory system in a game is lost in the mists of time – we’d guess Babylonian civilisation management game Hamurabi) (1968) or possibly The Oregon Trail (1971). In both of these, the inventory is a visual or verbal representation of the way that your character has access to ‘stuff’, like grain, spare wheels, raw materials, weapons, armour or quest items.

The most common early form of inventory was a grid of squares. Each square – often called a “slot” – contains an icon for the item it contains. Highlighting a slot allows you to identify what’s in it, read that item’s description, or choose another action. This could be equipping it to your character, using it on another object in the game world, dropping it, or, in some games, combining it to form a new object.

This basic kind of inventory initially entered gaming through pen-and-paper role-playing games like Dungeons & Dragons. In appearance and utility, the inventory archetype mirrors the templates provided the paper lists of equipments that players and Dungeon Masters personally designed and maintained.

DOOM by id Software

Becoming more than a menu

But as gaming spawned new genres, the inventory needed to find new forms. For one, first-person shooters became popular. The early 2.5D games like Doom and Duke Nukem 3D varied their gameplay by letting players use a huge array of different weapons. After all, jumping and crouching mechanics, let alone things like destructible terrain, were in their infancy. Without weapon-switching, there wasn’t much in the FPS designer’s toolbox to shake things up. Instead, their approach centred around a  “run-and-gun” style of play, so the new form of inventory would have to be quicker than the RPG menu approach.

The innovation was a hotkey system where weapons were bound to keys on the keyboard. A simple tap of the finger would bring up your chosen weapon almost instantly. Sometimes this was displayed in a UI hotbar too. Later games even used the mouse wheel as a scrolling weapon selector. Thus, the speed and flow of these games was not interrupted by having to pause to change weapons.

As you can see, the rhythm or beat of a game is directly influenced by its designer’s choice of an inventory system. So at what point should you start thinking about your inventory? It seems pretty much from the outset.

Skyrim - selecting weapons

A mirror of game design

A well-designed inventory should mirror the kind of game you are making. It needs to aid the player in defeating whatever challenges they choose, and it needs to help build and characterise your game world.

Think about a fantasy role-playing game like an Elder Scrolls title. It’s all about going on epic quests to loot huge dungeons filled with countless dangers. To do this, your character must be able to carry, understand and access the right equipment. The game’s inventory provides this solution. It is designed to show as clearly as possible what your character is carrying on their body (armour, weapons), as well as what you have in reserve (health-restoring potions, a secondary weapon). It lets you read item properties, and make informed decisions, like swapping out your current armour for the least flammable set if you anticipate a fight with a dragon.

Additionally, a “hard-fantasy” game might be concerned with a degree of realism and physicality. You may wish your character to only be able to carry a realistic number of items for their build and strength. The inventory system, therefore, needs to take into account even more levels of detail like item weight and size. Then it has to work in harmony with other game systems to perform the necessary calculations that directly affect your character’s movement speed and agility statistics. At this point, the line between gameplay mechanic and inventory starts to blur, further evidencing just how crucial your design choices are.

What about the aforementioned dungeon looting? This introduces concepts like object inventories, such as a treasure chest that contains its own roster of items. The player needs to be able to interact with this to pick their loot, so you’ll need to provide a way of visualising this in the UI. It may work the other way round too, and you may need game objects to store items players put in them, maybe for safe keeping. Due to the high memory requirements of this kind of function and the time taken to handcraft areas, designers often use techniques like procedural generation to populate treasure chests or items that NPCs drop when they’re killed.

dark souls inventory

Selecting your baggage

With so much information to convey, a highly complex inventory might actually comprise several screens. And whilst this might ideally mirror the things you want players to be able to do, you should still think about what this does for its pace and feel of your game. In Elder Scrolls-style games, where the focus is ultimately on role-playing whatever kind of fantasy character you like, a multi-screen inventory is a sensible fit. It might need screens for potion mixing, weapon comparison, skill levelling, a map, and all sorts of other diverse activities. It also needs to pause the game to give players a chance to take the information in.

But in the RPGs System Shock and Dark Souls, both far more combat-oriented and unforgiving in design, a “pausable” inventory would damage the game in ways that might not seem obvious at first. If you could opt out of a tricky situation in the game’s hostile world by hitting the pause button, the fear factor would evaporate too. Instead, the designers realised it was far more effective if accessing your items still left you at risk of attack. After all, fumbling in your pockets for your front-door keys isn’t going to prevent you from being savaged by the neighbour’s dog. As such, these games are tense adventure game where the few safe areas feel like a genuine sanctuary.

This choice better suited the game’s design and also encouraged the kind of gameplay the designers intended. Players of Dark Souls need to ensure their quick-select slots have the right items to hand because they can’t rely on the game giving them the chance to scroll through all the other things they are carrying to find the right thing. A player’s expertise must include caution and item selection before proceeding into new areas.

It seems that your inventory should not only sing in harmony with your game’s overall design, but can also be a band leader, encouraging player behaviours and personalities.

Resident Evil 4 by Capcom

The inventory system as an element of gameplay

This is a more nuanced point that is harder to articulate. So by way of an example, let’s look at Resident Evil 4 – a game frequently lauded for how it handles inventory design. Taking its cue from the System Shock series, the game designers provide an interesting minigame-like approach centred around a “briefcase”. If you pick up a new weapon, you have to rotate it so that it will fit in the limited briefcase slots you have at your disposal. It becomes quite fun in its own right to combine items or position them carefully around the more oddly shaped weapons to achieve the best possible loadout. More than this, it places constraints on the player which, in turn, ramp up the drama of the game. At no point can you really become overpowered – you simply can’t load up with all the unbalanced weapons like the one-hit-kill, boss-skipping rocket launchers because you probably don’t have space.

Thus, the game’s pace, as well as difficulty (especially when you’re running low on ammo) is dictated by its inventory. In one particularly tense scene, the player is besieged by zombies in a two-storey house which must be defended for several minutes from wave after wave of enemies. Poor choices, like entering this area with several guns with limited room for ammo, instead of one gun and lots of ammo, drastically change the experience.

A side benefit is that the game even becomes significantly more replayable. In future playthroughs, when you know what to expect from each area, you can choose to take a new weapon that might help or hinder you. Grenade-only playthrough, anyone?

With all of these inventory constraints and gameplay outgrowths, in Resident Evil 4’s combat, ballistic physics programmer takes a back seat to item management.

world of warcraft inventory 2

Designing game inventories for MMOs and more modern games

The importance of designing your inventory system as an extension of gameplay is felt in all genres, including modern MMOs.  For instance, when designing a massively multiplayer, class-based shooter, you could use inventories to reinforce the differences between character classes. It would make sense for a character called, say, an “engineer” or “courier” to have more inventory slots than a “scout” or ‘“sniper”. This increases the value of keeping support troops alive on your team as they can run and drop important equipment between frontlines. Essentially you’re influencing the balance of your game via inventory design.

But beyond the metagame that you can control, you may be also surprised by how the inventory you make creates emergent player personalities and behaviours that you perhaps didn’t account for and might need to rebalance in a future patch. Players may find that the option to drop items on the floor means that they can create map-breaking staircases that let them boost their way over scenery that you hadn’t intended to be scalable. This could give them advantages over a rival team and you might wish to rethink providing this option. (This is exactly what happened with rocket-jumping in the early days of Quake multiplayer, after all).

In a different kind of massively multiplayer game, say, a survival game, character inventory size can be used to control player progression too. As is the case with many of these games, salvaging items and using them to craft new objects presents the main form of progression and survival. You could aim to limit the number of items a character can hold to limit the pace at which they can power up. This might feed your monetisation and game economy strategies, for instance.

Worlds Adrift by Bossa Studios screenshot

Future inventories and SpatialOS

In what ways will inventory design be affected by even further changes and trends in the gaming landscape? It’s difficult to say because new technologies are only just emerging. Virtual Reality and Augmented Reality obviously present their own challenges, for example how a pop-up inventory can interfere with the player’s visual field.

In terms of our own technology, SpatialOS, we can at least anticipate how our persistent game worlds – where all things inside the world are finite and persist between play sessions – will demand new things of inventory design.

In a game with finite resources, like a SpatialOS-powered survival MMO, do you opt for players to drop all the items they are carrying when they are killed by other players? Should you even include those things that they have invested huge amounts of time to earn? Then, should other players be able to pick them up? Implementing this system would make your game a harsh but meaningful one, filled with the thrill of risk/reward consequences. On the other hand, you may create a game that is very static and twitchy instead, one where any sort of risk to your character could involve a total loss of progress.

The Trail by 22Cans screenshot

One thing that Spatial games could do is make player inventories fully present in the game world, rather than essentially running locally to the player. Designers could follow EA’s Ultima 7, which was designed so that objects sat loosely in bags and got physically jumbled around by your character’s movement. Peter Molyneux’s innovative The Trail built on that, by representing everything a player carried on their avatar, giving enemies and friends detailed knowledge about their capabilities and resources and allowing players to drop items from over-filled bags. Or designers could follow 2008’s Alone in the Dark, where players have to actually sort through their possessions in real time to find a particular object.

In a game designed like this, if you see an armoured player weighed down with ore, you can guess that she’ll be a slow, hard target – but see one lightly-equipped with medical tech and you’ll guess he’ll be an easy target, but able to support his friends. Worlds Adrift does something similar with the direct visual representation of a ship’s design, systems and materials, which signal to knowledgeable players how effective the ship will be at different tasks. Dungeon Siege even suggested that the player uses friendly NPCs such as pack mules, which have your possessions and which themselves could be stolen – or, riffing on Terry Pratchett’s Luggage, they could steal themselves.

An alternative mechanic would be to have a more complex environmental degradation system – where objects, including backpacks, wear down from use and weather effects, or have varied fragility levels. Imagine a quest to steal a delicate ice statue from a cave full of aggressive Yeti – you might be able to defeat them in combat, but would the bowl survive either the fight or the heat of the midday sun?

Finally, you could go quite the opposite way, and have no inventory whatsoever – but take a leaf out of DOTA 2’s book and have objects delivered to the player by courier every time they’re needed. The further a player gets from a supply base or shop, the longer it takes an object to arrive.

It all comes down to your design, and how seriously you’re thinking about inventory when designing gameplay and building a world.

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