Dev Diary: Balancing Path of Exile

Path of Exile contains a lot of numbers. A relatively complex item or skill probably has twenty or more specific values behind the scenes representing damage values, ranges, durations and other properties. Balancing these numbers is an extremely difficult challenge but is necessary for the game to feel fair and rewarding for all players. We’ve spent a lot of time developing a system that allows us to easily balance these gameplay values.

As more advanced combat calculations were added, the balance requirements became complicated. By mid 2008 we had various types of elemental damages, accuracy calculations, damage mitigation and critical hits. Items had mods that added specific ranges of these values. Because we were still picking these values by hand, substantial time was spent changing existing numbers to be fair as calculations changed. When we added a chilling effect due to cold damage, we had to manually go through every instance of cold damage on a mod, weapon or monster and reduce the values so that it was now fair.

Our Custom Balance Tool

In early 2009, we decided to build a dedicated tool for game metadata editing. On the surface it looks similar to a spreadsheet, but has substantial scripting capabilities and can export directly to the binary format that our game loads. In addition to handling the balance data, it also lets us edit and localise all of the text the game uses.

With this tool complete, we took a second look at what we were trying to achieve with the balance. It became apparent that the difficulty wasn’t just in picking values that were fair relative to each other, but also resolving questions like “how large should the effect of an item’s mod be relative to a passive skill?”

This lets us calculate, for example:

• A skill gem balanced for player level 2 is worth 15 effectiveness. This means an early fireball inflicts 11-16 fire damage. A level 50 skill gem on the other hand is worth 106 effectiveness. A fireball appropriate for that level would do 78-113 fire damage.
• A level 10 base item is worth 28 effectiveness. If all of that effectiveness is spent on damage, then it represents a median 27 physical damage per second. For a 90% strength, 10% dexterity one-hand club, that’s 18-42 damage at 0.9 attacks per second.
• A level 6 passive skill is worth 10 effectiveness. An example skill balanced for this level/effectiveness is “+7% damage with swords and +4% attack speed with swords”.
• A level 1 item mod is worth 15 effectiveness. A level 11 item mod is worth 18 effectiveness. For the early increased life mods, that’s 37-45 increased life at level 1, or 45-54 increased life at level 11.

The great thing about this effectiveness system is that it’s so easy to tweak the balance of the game. For example:

• If we feel that fire damage is overpowered, we can change how much effectiveness it costs, and all the fire damage in the game is reduced at once.
• If players find a fight involving zombies difficult, we can either tweak those specific areas’ zombies, all zombies in the game, or all monsters, depending on the specific complaint.
• As specific mods become popular or unpopular among alpha testers, we slightly tweak how much effectiveness they cost until the fair values are determined.
• When we add a new skill or support gem, we pick what stats it affects and the system automatically spends the right amount of effectiveness on those stats. It’s rare to have to tweak this much.

We’re very pleased with the balance system that we’ve come up with for Path of Exile. There’s still manual supervision required, but it’ll be a great help going forward as we continue to add content to the game and get feedback from our expanding pool of alpha (and soon, beta) testers.