I've been struggling to talk about density of space in games. Here's my latest attempt!
There are a lot of ways to measure 'density' of space. How far you can see, how quickly and freely you can move, how many enemies are around, how many resources, how many NPCs.
But I find these are all proportional. If you can see further, then you can move more quickly/freely and there are fewer encounters per square meter. Similarly, if you are stuck in a cramped little maze, you can't see very far, you don't move very freely, and there are more encounters per square meter.
These things vary over the course of a level. That's the heart of good level design. Here's a huge area with a ravine you can't cross, a monster closet, an area with a bunch of resources clumped up. However, if you average all the variations together, you'll find that the level revolves around a specific density, and all the different elements that can be used to measure density all reflect that choice.
Because of that, we can really use any of these things as a measure of density. We don't have to painstakingly consider them separately.
I'm going to use how the player sees as my measure of density. The more space the player sees at once, the sparser the space is.
Of course, this isn't just camera-view space - it includes HUD popups and overlay maps. If you have a map that's easy to reference, or floating decals showing points of interest outside of your view, then you can see a lot further than your camera view indicates. That means your space is sparser, which means you'll move faster/more freely, and you'll have fewer resources/encounters per meter.
With those assumptions in mind, let's get a bit aggressive with the concept now that it's been laid out.
How Dense is Dense?
When people think about spatial density, they normally think in terms of something like "rooftops are sparse, vents are dense".
They aren't wrong. However, in most games those are outliers, not part of the core experience. You can tell, because if you pretend that vents or rooftops are the point of the game, you can easily see that there would be different mechanics in place to make the game more fun. In most games, rooftops and vents are changes of pace, not distinct levels.
You can see the core gameplay break down in these places. In fact, in most modern games, the vent sequence is guaranteed to not have any enemies in it, since you're too cramped to even fight. Space is too dense to maneuver.
That said, you can get a lot more sparse than rooftops. What about the boat sequences in Windwaker? What about the "lost in the desert" sequences in Quest For Glory 2? You can get very sparse!
The thing I noticed is that you can't really get very sparse or cramped before violent gameplay disintegrates. Violent gameplay is about removing enemies from play, and that fundamentally doesn't work if there are too few enemies. As you get sparse, the gameplay becomes about getting to the few things you think you see, rather than removing them from play.
You can get very dense and still have violent gameplay - old robotron games show that - but there's still a limit. As your movement gets more and more constrained, maneuvering gets tighter and tighter. As density goes up, violent gameplay slowly changes from violence into racing with obstacles. A good example of this is Tempest 2000, which is technically a "violent" game but it's mostly about desperately trying to wiggle from side to side in the tight confines given you.
However, other kinds of gameplay work fine in dense and sparse spaces.
Most games have one core kind of play, and it revolves around a specific kind of density. Density fluctuates from level to level, moment to moment, but the gameplay is intended to work at a specific density and that's the density their levels keep flowing back towards. In contrast, consider Skyrim. It has several largely unrelated kinds of play that allow it to support dense and sparse spaces natively, rather than as exceptions.
Skyrim has three kinds of gameplay that live side by side. It has the violent medium-density-space gameplay - raiding dungeons, clearing bandit camps, fighting dragons. But it also has the low-density gameplay of exploring the world and the high-density gameplay of stealing things from townsfolk.
Each type of space has a different kind of fundamental gameplay, even though they all use the same user interface.
This isn't to say Skyrim is very good at it. Skyrim is notable mostly for this existing at all, not for doing it well. The different kinds of play do not offer meaningfully intertwined reward systems, a heroic character will never see the denser half of the game, and the play balance is completely awful unless you mod the hell out of your game. But it exists!
I think this is the future of open world games.
I look at games like Saints Row or older GTA games (haven't played the new one), and I see games struggling to have multiple densities. You can be on foot, you can be in a car, you can be in a jet. These are obviously trying to see space from different perspectives, trying to change the density of the play. Unfortunately, they don't quite have it figured out. They don't seem to be sure what they actually want.
Being in a jet does change how you see the world, but it doesn't change how the world interacts with you. This means the new sparseness is not a native density, but an exception: it's typical space smeared awkwardly into a jet's visor, rather than a different kind of play supporting that new sparseness at a deep level.
The spaces don't need to clash and feel disconnected, like with Skyrim. But they do have to change.
Let's Talk Gameplay
If we wanted to come up with an open-world game that supports a wide variety of densities, we would need to consider how to link them together.
Let's start stock, with a typical open-world game setting. Rather than fantasy, let's set it in modern times - we'll call this a GTA-like rather than an RPG.
The medium-density setting of the urban environment is what most players are used to. In this environment they run around, jump cars off riverbanks, shoot cops, etc. Basically every GTA clone ever made.
We also need a lower- and higher-density way to play. And, importantly, they have to reflect on each other.
Let's say the high-density space would be places you actually go into. Shops, homes, and the like. In these spaces there wouldn't be very much call for violence - you could shoot a shopkeep and go on a rampage, but there's not any significant reward for it. Instead, these tight spaces are about arranging people and things.
It's about stocking a friend up with beer he likes by putting it on his shelf, or about hooking up some people that have never met each other by introducing them and kicking off a conversation, or its about sabotaging the air conditioning so it gets abysmally hot. It's about the things in this dense space and your inventory, and using them to affect each other. As a key part of this: these aren't quests. It's all open world. Sabotaging an air conditioner always has the same kind of effect everywhere. Putting something on someone's shelf - or taking something from their fridge - registers in the same way even if people have different responses.
Some possible rewards are obvious - you get resources if you steal them, and havok can be its own reward. However, we want our rewards to echo properly, they need to affect the player in other spaces. In this case, a good reward might be whether or not someone will follow you as an NPC if you phone them, and how powerful they will be when they do.
Low-density, we have a lot of options. We could make some kind of wilderness/outskirts thing. But this is a techie wonderland, so I have two ideas.
The first is drone flying. You can use a drone to wander the city. Unlike wandering as a human, the drone cannot stop and chat - it doesn't even go down to street level unless you want to crash it. Instead, the drone scans for specific people whose phones you've bugged, open wifi networks, and so on. The city is a glowing map of flowing data, allowing you to search for points of interest, tidbits of data to harvest, and track individuals. The city is also large - real city size.
The second option is exactly the opposite: drone evasion. After curfew, drones watch the city. You can't use vehicles - they'll be spotted immediately. Instead, you have to move on foot. Everyone's gone home, the city is much less dense. It's just you and the glowing patterns of light that indicate where the drones can see. Change your gear to change how long it takes a drone to realize you're a person, or how hard it is for street-level cameras to identify you.
These kinds of play can all feed back on each other in relatively interesting ways. Cause a ruckus during the day in medium-density space, and that area will be crawling with cops and drones at night - leaving fewer in the rest of the city. Sneak somewhere at night to hack into security and change what you can see during a drone expedition. Do a good drone expedition to find a weak point in the mafia's defenses.
Normally this would all be scripted missions, but if the play exists, these can be emergent situations. Scripted missions are mostly useful for hiding parts of the game that don't flow well, don't emerge naturally. If our spaces blend together well and support each other, we don't need nearly as much scripting.
This is a pretty conservative kind of design, too. There are a lot of much more out-there designs where the spaces get a whole lot more dense and less dense. A game on a starship where you get stuck in your crew's dreams at night, for example. A camping game where you spend the day hiking and the night trapped in a two-person tent with three other people.
There's lots of options. I just wanted to show you that density goes a whole lot further than most people might think.