RTS Game-play Part 2: Resource Systems

November 19th, 2008, RTS Design

jeb <3 RTSThis is the second part of my RTS game-play article series, and we’re finally getting on with the more beefy stuff. This time I will discuss the subject of RTS games’ resource systems. The order of these blog posts may seem a little bit random… which is completely true! However, resource systems are quite independent from other game concepts, which makes it an easy place to begin. When I feel finished with all parts, I will post the whole document in a more organized manner. Until then, try to keep up with my floating idea blobs! :)

Resource Systems

Much of today’s resource systems can be tracked back to the games of the 90s. The basics of a resource system are resource collection and production. In Dune 2 (Westwood Studios, 1992) you have a single resource called melange (or simply “spice”), which is gathered from sand dunes and sold at refineries for credits. Credits are then used to buy both units and structures. Collecting spice is performed by slow-moving “harvesters.” The amount of credits you can have at the same time is limited, but can be increased by adding spice silos to your base. Dune 2’s resource system is also used in Westwood’s following game, Command & Conquer: Tiberian Dawn. The spice is replaced by tiberium, but the concept is the same.

Since then games have tried different number of resource types, different rate of expenses, and various forms of gathering them. Some of these variations will be covered here.

Continuous vs Discrete

One thing that is important is that the expense of resources in Dune 2 and Command & Conquer is that it’s continuous. What that means is that when buying structures or units, the credits count is continuously decreased until the purchase is completed. This has the effect that you can begin constructing something even though you don’t have enough credits, but it’s possible that you will have to wait for the next harvester to return before you get what you bought. In theory, especially in Command & Conquer 3: Tiberium Wars (EA Los Angeles, 2007) with its multiple build queues and upgrade options, you may end up buying several items of which none will be completed. It’s the player’s job to predict how much income that is arriving, and prioritize by holding or canceling production.

Total Annihilation (Cavedog Entertainment, 1997) and Supreme Commander (Gas Powered Games, 2007) use a full continuous resource system. Resources gathering is measured in “per second”, and so is resource expenses. If you have more income than expenses, the excess will be added to your stash, which is limited. The limit can be increased by adding silos and batteries, but generally it’s optimal to keep your resource stash close to zero at all time, and with a slightly negative income/expense ratio.

Blizzard Entertainment has used a discrete resource system in all their games. A discrete system is a system where resources are removed instantly, and it’s not possible to buy something that you can’t afford. This system is used in a lot of other games as well, such as Age of Empires (Ensemble Studios, 1997) and the spin-off series Empire Earth (Stainless Steel Studios, 2001).

The big difference between continuous and discrete systems is the implicit cost of time. With a continous system the player is allowed to save time by constructing something he can’t afford yet. For example, if the resources for a certain unit is gathered at twice the rate of the unit’s production time (and the player has two factories), the difference is the following:

  1. The player wants to buy 2 units. In the continous system the player begins construction of both units. In the discrete system the player is only able to begin construction of one of them.
  2. The player receives resources for the second unit. In the continous system, both units are at 50%, in the discrete system the player may begin construction of the second unit.
  3. Full construction time. In the continous system the player receives both units. In the discrete system the player receives the first unit, while the other is still at 50%.

In the long run both systems will yield the same amount of units, since they are both restricted by the rate of income. However, it does affect the player’s decision making. In a continuous system, if you need something you should begin constructing it immediately to save time. In discrete systems you are forced to prioritize more, and is also more intuitive since it’s how you pay for things in real life. Discrete systems also give the player direct feedback on the economy; the player will receive the “you need more resources” error message when he tries to build something, instead of receiving it a couple of moments later. C&C 3 helps the player by displaying a purchase’s cost with red digits if it’s higher than the current stash of credits.

Active vs Passive

As already mentioned, in Dune 2 and C&C resource collection is handled by harvesters that move between resource fields and refineries. This is “active resource collection,” because a game entity is involved in the process. This entity can be micro’ed by the player if necessary, but generally it takes care of itself as long as there are resources remaining to collect. Warcraft, Starcraft and Age of Empires are other examples of games that use active resource collection.

In passive resource collection, resources are added through other means than entities picking them up. Passive income is also usually infinite or near-infinite and will continue until the opponent destroys it. Passive income is used in Total Annihilation, Dawn of War (Relic Entertainment, 2004) and in some extent Command & Conquer 3 (tiberium spikes) and Warcraft 3 (Night Elves collecting wood, Blizzard Entertainment, 2002).

In both systems the player will have to practice to understand how quickly resources are gathered, how many harvesters that are needed or how many passive resource points that is required for a certain technology build. Since passive income usually is connected to positions on the map, such resource systems will put more pressure on the players to be offensive and multi-tasking, forcing them to attempt to gain control of locations that may be difficult to defend. Most people will find RTS games with passive income to be “hectic” on a macromanagement level, because you need to get as many passive positions as possible, and when there are no more spaces you need to take your opponent’s positions instead. The game will consist of several small battles until either player gets the upper hand and can make a move on the main base.

Active income is more focusing on defending a handful of locations. As long as there are resources remaining to collect the player is allowed to sit tight and make plans for the next move. Active income is also limited by the number of harvesters, and not by the number of resource locations available, which gives the player a more comfortable situation of choosing their own economy. Obviously there is a need for expanding to new locations and preventing your opponent from doing the same. However, this is a decision that the player makes, and is not forced upon him by the way the resource system is made.

Resource Types

Most RTS games have several resource types. For example, Starcraft has minerals and vespene gas, and Age of Empires has four (food, wood, rock and gold). Resource types are used to balance units in the game, and offer the players methods of choosing different strategies. Resource types can be available in different amounts (vespene gas is collected slower than minerals), have relations to each other (wood creates food in late-game Age of Empires) or be gathered differently (requisition and power in Dawn of War).

The difference between having one and having several resource types is not how you gather them, but the option not to gather. Games that don’t utilize this option are less enjoyable and force players to do more “work”. To get a good example of this, check the Analyzing Supreme Commander section below. Not gathering allows players to focus on certain strategies and hopefully get an advantage over an opponent who wants it all. For instance, the stone resource in Age of Empires is mainly used for defensive structures. If you aren’t interested in defense, you can use those workers on gathering more gold instead (which allows for more technology and offensive units).

While adding more resource types to a game adds strategy options, it also adds confusion. Empire Earth 2 has ten resource types! Food, wood, stone, gold, tin, iron, salpeter, oil, uranium and technology points. Luckily, not all of these resources are available at the same time, but it’s still way too many. Mad Doc Software added these for historical accuracy, but it neither adds strategy nor entertainment.

There are no games that I know of which only has one resource and a discrete payment system, so it’s hard to analyze how that would play. Command & Conquer has a single resource type, but since the payment system is continous it wouldn’t make much of a difference if there were two. You would still buy what you wanted, rather than what you could afford.

Other Resource Systems

It’s not easy to invent fresh resource systems on-the-fly, but there is obviously room for other ideas than those used so far. In many cases, resources are about gathering something so that a number in the user interface is increased (the stash). It’s easier to invent new ways of gathering than new ways of increasing the number.

One way to make it more different is to avoid using a stash value. One example of this is our game Harvest: Massive Encounter (Oxeye Game Studio, 2008), where one of the resources is energy flying around in a network of energy links. The energy is a part of the game rather than a part of the user interface, and the player will have to make sure that the energy is at the right place and not just that there is enough of it. However, as you know, Harvest: Massive Encounter is a different kind of RTS than the other games in this article, but it’s a good example of how resource systems can be made differently.

13 Responses to “RTS Game-play Part 2: Resource Systems”

  1. Urre :

    Interesting read. Seeing as the article was about resource systems, I guess it’s not a shocker, but what about RTS games without resource systems? One particular game pops into mind is Z, by Bitmap Brothers. There are other obvious examples, but I like this one in particular, as it’s quite a thing of its own.

    In Z you capture zones. This can be seen in other RTS games, but the big difference here is that capturing a zone doesn’t mean increased income rate. What it means, is decreased build-time. Each zone you own, shaves off a certain percentage of the build time in all your factories. This combined with the fact that you can only aquire new factories by taking over zones, in which case you also get whatever the factory was constructing at the moment of takeover. Makes for very fun action, capturing a zone mere seconds before it completes the build, and get some free new units too!

    One could argue that this also is a form of resource system, which I guess is fine. My point, however, is that the capturing of these zones is very intuitive and generally just part of the ongoing battle, rather than an entirely separate action in itself, which distracts from the war aspect of the game. It has always been the most frustrating part for me when playing most RTS games. Either I’m neglecting my base, stacking up on loads of resources and not building new units, or I’m neglecting the war, resulting in a much stronger opponent.

    I’m currently in the process of making an RTS with a derivate of this system, which is why I’m so fascinated by it. Or perhaps I’m making the game, due to the fascinating system :)

  2. admin :

    Oh, that’s very clever! Thanks for pointing it out. I have only played Z once, and that was a loooooong time ago. I’ll remember to include this in the finished article.


  3. Daniel Bernhoff :

    One of the things I find most interesting about strategy games is how resources, terrain and enemy contact create an interactive chain reaction where one thing emerges from the other. In Warcraft you cut down the trees to get lumber, and in this you also create new passages for your troops. In your own Harvest, you expand to extract more minerals, which in turn drives you to expand your base even more. In Total Annihilation, I’m told, the attacking enemy may be destroyed by your automatic defense turrets leaving heaps of scrap metal which you either can leave as an obstacle for other attacking enemies or harvest for resources.

    I agree Harvest makes a great example of how resources can be represented in the games in other ways than a stash value in the interface. I believe it is also an example of how the line between resources and units may be thin or blurred. With another (though probably less fitting) graphical representation, the energy clusters could instead be autonomously working drones. If so, would we consider them units instead? Would they stop being a resource if we had some kind of limited direct control over them?

    I don’t completely agree that more resources necessarily adds strategy options while also adding confusion. What if each resource could be spent in one or a few possible ways, and your abilbity to harvest them depends on your tech level or on spending other resources in trade? The early resources may be phased out because the source dries up or because you reach a limit for how much of it you can spend in a meaningful way. This all would drive you to adapt your strategies in an inevitable but still dynamic way, while keeping your options down to a number you could handle and appreciate.

    We should talk more about this over a beer some day!

  4. orson :

    Hey, good read!

    As Urre says, Z has an especial system of rules that we can consider as some form of resource. I remember another kind-of-old RTS, 7th Legion, in which you’re rewarded credits for each enemy unit you destroy. Als the game featured special powers in the form of “cards” that were given randomly a bit like what happens with darwinia/multinia.

    While this systems are indeed interesting, I also think it would be wise to analyze what design implications come along with them, and how affects gameplay an the type of game you want to create, or end up creating at least.
    Just as you analyzed the active/passive resources, and continue/discrete resources, the game rules in Z & 7th Legion have a general inmediacy that -at least in my opinion- favors rushing. Just like the games of massive entertainment adopt some kind of puzzle-like gameplay -haven’t played World in Conflict, but for what I’ve read expands on the principles of the ground control series (btw i think you can still DL the first game from fileplaent for free if you want to try it) and that is: given an scenario, objectives & a couple of units, you have to ’solve’ the level. Yes, there are several strategies & tactics you can use (purchasing special weapons, unit types, or more dropships) but you have a game that plays waaay more slowly that the Z series in comparison.

    All in all, both games can be considered RTS; and more or less any strategy game posses one kind of resource to apply in some sort of way, even in the classic turn-based ‘wargames’ which also offers the puzzle-like gameplay: isn’t action points, time managment and turns actiong like some kind of currency that effectively alters the way the game is designed & played?

    They are questions related to creating, designing a game, and they came with no clear answers.
    which are the game’s rules? are the game’s rules creating waht type of experience? does any of that rules covers a resource system? how does the resources work?
    Finally, I’ve found that paying attention to how some board games are designed can also give pretty important clues on how to understand a good set of rules which can then be somewhat applied to computer games.

    PS: sorry for the bad english if applies! kinda lost some practice with the language ;-p cheers!

  5. Alevice :

    Nice series of articlea. I really love RTS, moreso analyzing them. I think you nailed perfectly teh most common resource systems, but I feel you blurred objectivity by throwing a little ‘undesirable’ conclusions concerning the approaches – you favored discrete and active recollection by showing out the negative points of the respective others, and regardless of my agreement on your opinions, are not in my opinion apropiate for the analysis of them.

    That said, I disagree active systems are inherently better for the player, especially on the statement that active income is limited by the number of harvesters, because more often than not the game designers limit the number of harvesters capable of recollecting the resources at a given point.

    As an example, Vespene Gas in starcraft can only have a single unit harvesting it at the given point, and the optimal number of harvesters assigned to a Vespene Factory, assuming the drop point is the closest possible, is just three. Any more units will just make a larger queue, but not faster recollection.

    This system has also the disadvantage that you have to invest into harvesters, and given how often those harvesters also act as the builders for structures, you constantly have to pressure yourself to decide wether to recollect or build, and in faster paced games, that decision is horribly crucial to just ignore it.

    Also, the number of resources to collect don’t necessarilly make a game more confusing. Prime example would be Rise of Nations, where the emerging economy truly adds for a more strategic planning for the player, and tie quite well with how the economy in the world progressed.

    The closest I can think of for a single resource system with a discrete investment is Dawn of War with the Necrons – unlike the other players, they just need to recollect Power (which is generated atuomatically by creating a Power Generator) to invest in your army and your technologies, although they compensated this by giving them horrible build times to their units unless they capture locations (that other factions utilize for recollecting the Requisition resource), which speed their build times. Sadly, the system is quite innefficient, as Power recollection by means of its own consumption kind of complicates the management (which you could argue are compensated by the fact that Necrons have a very linear tech tree).

    Despite my so-called counter-arguments, I admit that the economy model of most RTS is a little too flawed and quite abstracted out to simplistic numbers in the long term. I have not played Harvest yet, but what i have read about the economy model strikes me as very interesting, and even closer to reality. I really wish the economy models of certain city building games inflenced a little more RTS game designers.

  6. admin :

    Very good points! I haven’t played Rise of Nation, but been hearing a lot about it lately (maybe because of this series, hehe). I’ll check it out when I come back from Japan. :)

    About the Vespene Gas thing… I haven’t played the SC2 alpha, but I’ve been reading that the vespene geysers “shuts down” when you’ve collected 300 gas. They turn on again after a certain delay, but the whole thing is just silly!


  7. JademusSreg :

    I would like to see this article expanded; it flirts with but does not confront the fundamental resource in Real Time Strategy: time.

    Also, you misspelled “saltpeter”.

  8. Damocles :

    Dune 2 actually has 2 ressources, the spice, but also
    the electricity as a static, expandable ressource.
    This is comparable to “supply depots” etc in Starcraft.

    The player must decide how much to invest into each ressource.

  9. Marco Afonso :


    This blog is like a big meal for my starving and thirsty RTS game theory :)

    I almost cry by remembering, not Dune 2, but Nether Earth: this was the real first RTS game in computer history.

    Have a look at it

    My favorite unique features of this game was:
    1. capture factories to increase resources
    2. factories could be destroyed (resource sources can be destroyed)
    3. give orders robots: search & destroy, destroy enemy factories, capture enemy factories, etc… (AI?!?)

    BTW, i’m developing a web-based RTS. If you want to share any ideas contact me at taviroquai@gmail.com

  10. Comrade Da :

    I’d probably define resources differently:

    Resources are divided up by how they fit into three categories:
    Destructive/non-destructive: A destructive resource damages terrain when it is collected. Destructive resources are limited by the amount of the resource can fit on the map. An example of a destructive resource is wood from Age of Empires, or Tiberium from C&C. An example of a non-destructive resource is ammo from Company of Heroes.
    Cumulative/non-cumulative: A cumulative resource is a resource which you get more of if you don’t spend it. An example of a cumulative resource is money from C&C Red Alert 2. An example of a non-cumulative resource is any population cap, or power from the C&C series of games.

    Erm… My 2 cents.

  11. typedef struct :

    Interesting, I’d never really thought about the implications continuous vs discrete spending.

    I don’t completely agree with the “active vs passive” distinction. “Active” resources are not purely based on the number of harvesters, but also harvester efficiency. As you add more harvesters to a location, each becomes less efficient. This (in addition to resources running out) does force you to expand to other areas. It adds the strategic decision: “do I stay here where it’s safe and try to build up enough to defeat my opponent, or do I risk defeat to expand quickly to get more efficiency from my harvesters, and thus more resources.”

    Also, a note of Empire Earth’s 10 resource types. You mention that they aren’t all available at the same time, but then dismiss them for not adding strategy. They in fact do, by making the map more dynamic. The areas you want to defend early on (say, gold mines) aren’t the same areas you need to control later (eg oil). So, the map changes throughout the game, and you need to adjust your strategy in turn. (I still agree that the added complexity isn’t worth it in this case). Destructible rocks in Starcraft 2 are another example of making the map more dynamic.

  12. Tim Zook :

    What happened to the “Analyzing Supreme Commander” mentioned in the article?

  13. Doc :

    Another interesting exception is Blood & Magic:
    “The basic units of the game are the basal golems, which are created at the Bloodforge. They provide mana for the production of new units and can be turned into buildings or monsters. When four are placed on a foundation, they may transform into a mystical site dedicated to the kind of magic the player’s choosing. Placing a basal golem adjacent to a friendly mystical site will allow them to change into a different creature, based on the type of mystical site. The player automatically is only able to use the weakest transformation, and can unlock more by researching. Researching costs experience, which is gained by creating or transforming basal golems, creating or destroying structures, casting spells and slaying enemies.”
    (copy from Wikipedia)

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