Sunday, 23 January 2011

Designing a Magic System Redux - Part Four (Pick any two)

You are encouraged to read parts one, two and three of this article series, and are strongly encouraged to read the original Designing a Magic System series of articles if you have not already done so. It turns out I had this article sitting in draft for about six months, so some of the Team Fortress 2 figures are a little out of date.

One thing I've tried to play with in Unangband is alternative character development models to the standard RPG tropes: experience and levels. Not that you'd ever guess by playing the game which appears hugely focused on level increase through experience gained from killing monsters. But a high level character in any Angband variant is incredibly fragile without the equipment that they will have gained along the way, which makes Angband much more about the correct selection and management of discovered items in the inventory, than it is about level gain or monster slaying.

I've discussed previously the differences between skills and classes, and some of the general disadvantages of each approach. I've also gone into detail about how to design a magic system, and how it may be worthwhile looking at differing systems of progression, without going into detail about the implementation details.

I want to talk about these details here - taking two specific progression designs from Unangband and give you the reasoning behind each of them; with specific regard to making the choices interesting.

The weakness of a lot of progression systems is that the choices are uninteresting. Take how most RPG skill systems work: where you buy skills from one or more talent trees using a pool of points which increases the longer you play the game. The choices in most of these systems are not exclusive, and instead merely become an ordering decision, that is, if you buy skill A, you can always buy skill B later on, and there is no difference ultimately in whether you choose A first or B first, provided you get both.

This problem is exacerbated in when the cost of skills deeper in the tree escalates - the Civilisation series of games being a good example - because you can skip buying one of the more expensive skills to effectively 'catch up' on all the cheaper skills you elected not to purchase previously.

The effect in both instances is that the characters end up being homogeneous midway through their progression - and only start to differentiate themselves at the higher level skills, instead of through out the talent tree.

You may argue that a particular system has skills which work in synergy, so that it is always worth picking skill ABCD in sequence, instead of EFGH, because the skills combined are worth more than e.g. ABGH. But in that instance, you're actually presenting less choice to the player, because ABGH users will be beaten by ABCD or EFGH users, so that despite having 8 skills in this example, there are only two real choices.

You may also argue that a particular system has pre-requisites so that to get skill C requires that you have learned skills A and B, E requires A and C, and so on, so that the skills at the tips of the tree are entirely dependent on which skills you have chosen earlier. But by using pre-requisites, you're again restricting the range of possible choices to only be meaningful at the tips of the talent tree, as opposed to choices further up the trunk.

There are several ways to avoid this. The first is by either using slots or sockets, so that the player can only choose a set number of skills to equip out of the possible talent trees. The inventory system in Angband is an example of using 22 slots to carry all possible discovered items, and the equipment system an example of using sockets to carry a single weapon, shield, amulet, light source, 2 rings and so on. A particular socket may hold weak, middle or powerful skills, so that you are forced to pick only one of the set of possible skills of each grade.

Take Team Fortress 2 as an example. At the time of writing, the sniper has 2 possible primary weapons, 3 secondary weapons, and 2 melee weapons. Using only 7 weapons, we have 12 possible sniper combinations because of the division of weapons into these 3 exclusive buckets. And if we could choose 3 items to carry, from 7 items total, there would be (7x6x5)/3! = 35 choices in all. Whereas a talent tree of 7 skills from, for example, 100 Rogues, would have at most 3 branches of up to 3 depth. Assuming a similar 3 selection limit, and approximating the possible choices by using a combinations with repetition counting approach, there can only be (5x4x3)/3! = 10 choices.

In general, if you have lots of skills and don't allow the player a large number of selections, you are better choosing a slot or socket approach. If you do allow a large number of selections, relative to the total number of skills available, the analysis of choices available in the talent tree by using a combinations with repetition approach breaks down, and you are instead limited by the depth of each talent tree which at best behaves like the slot approach. The socket approach has degenerate cases where the talent tree behaves better, but in general is far easier to design.

The second way to avoid this is with a sliding window approach, which is what I use in Unangband for developing abilities for familiars. A sliding window allows you at best only a subset of choices at any particular time, and periodically the window slides, so that early choices are no longer available, and later choices become available. With familiars, you can choose a new ability every two levels, and every four levels the window slides 10 abilities further on into the list of possible abilities. Since the window is also 10 abilities big (but in general does not have to be the same size as the slide distance), effectively you can choose 2 abilities out of 10 different abilities every four levels. This gives you approximately (10*9)^12 choices over the course of the game, at the cost of designing 120 different familiar abilities and putting them in an approximate order of power.

(I would hate to have to design talent trees containing 120 skills to give the same variety of choice.)

I could, of course, provide far more choices by allowing the player to pick from any of the 120 familiar abilities every time they advanced two levels (the slot approach), but aside from the game balance issues of allowing some high level monster abilities from the start, there is the real consideration that the human mind is limited in its ability to comprehend more than a limited set of choices, and breaks down its rational decision making process if presented with too much choice at once.

This limit also suggests an upper bound for the number of available talent trees to choose from, as well as the number of slots or sockets and the number of choices to be presented per socket. A lot of this is avoided in Angband because you only encounter so many items at once, and must discard choices that you've no room for in your inventory or equipment.

1 comment:

Timothy said...

Titan Quest's pick 2 from 8 trees meant 64 classes from 8 trees... Meaningful choices in the late game were a bit limited though