Game difficulty (Part 5)

After having discussed the grave shortcomings of rubberband AI in driving games, it's important to look at a game that does not only gets difficulty levels right, but also combines static and dynamic difficulty. For that, let's look at the Left 4 Dead games. These games are having an excellent difficult grading to allow for a wide variety of player skill levels, and then also make the difficulty match the overall atmosphere of the game.

In case you've been living under a rock, for the past couple of years, the premise behind Left 4 Dead is that you are part of a group of four survivors of a zombie apocalypse.  You have to make your way through hordes of zombies to an evacuation point where the military can rescue you. As you progress, you facing varying challenges, dispatching hordes of regular zombies, as well as facing off against zombies with unique powers that can temporarily incapacitate you (temporary provided your teammates save you!), have you swamped by regular zombies, or simply are the zombie equivalent of The Incredible Hulk.

The tank: bad news for anyone's day

In terms of difficulty, the player(s) select a base difficulty level as a rough measure of their skill. This determines basic things like the weapons given to the players and the amount of damage the zombies do per hit, the frequency with which "special" zombies appear and such things as mentioned in my second post in this series. However, where Left 4 Dead really comes into its own is how it dynamically adjusts the difficulty based on the performance of the players using its "AI director".

If the players are getting through the game far too comfortably, the director will increase the challenge of the game slightly.  Throwing more regular zombies at the player, or perhaps an additional special zombie that might incapacitate a player at an inopportune moment. The aim here is not to kill off a party member, but merely increase the level of danger if the players are comfortably surviving the zombie apocalypse with seemingly no effort. This can also occur if the players dawdle in a particular area for too long, which is used as a mechanic to make sure the players keep moving towards their destination rather than trying to exhaustively search everywhere in a level. Conversely, if the players are struggling and one or two of them have been killed, the director may ease up slightly, potentially placing some painkillers (which provide health) nearby. The game also offers the ability to "find" survivors so the dead players can rejoin the group. This is typically quite a challenge, but successfully rescuing one or more team members and salvaging the team from complete annihilation provides a great sense of achievement when it occurs.

Guns and ammo: Just what the director ordered

The reason Left 4 Dead's difficulty is so success is that it helps to reinforce the game's setting and atmosphere. The game relies heavily on teamwork and cooperation, and with the difficulty such that even highly skilled players will have great difficulty completing the game on the hardest difficulty should they try to play lone wolf. The game ensures that the levels are never a complete cakewalk for the players, as this would defeat the atmosphere portrated by the game and its never-ending zombie hordes. Even should a very skilled player start on the easiest difficulty, the game will still pit them against zombies that require them to be alert.  Herein lies the key difference between Left 4 Dead's dynamic difficulty the the dynamic difficulty of rubberband AI in racing games: in Left 4 Dead, the increased difficulty can still be overcome or (partially) negated by player skill. An excellent FPS player will still be able to dispatch the extra zombie rush that the AI director throws at it, allowing them to maintain their advantage of having high health. However, not even the best driver can negate the increased difficulty posed by rubberband AI that makes the opposition cars drive at a speed beyond what is physically possible.

Dynamic difficulty is by no means easy to implement well, but Left 4 Dead demonstrates how it can be done. However, its gameplay mechanics, setting, enemies and level design all strongly support the means used to implement that dynamic difficulty. Without this kind of consideration, implementing dynamic difficulty can be a difficult ask indeed.