Specialization

About the project

This specialization project is an AI-Director feature which controlls and manages a simple zombie survival game. The player is stuck in the middle of the screen while having to shoot hordes of zombies running at and attacking them from outside the screen. Exactly how these zombies are spawned is controlled by an AI-Director that continuously analyses and reacts to the players actions and behavior. These reactions come in the form of dynamically changing the difficulty of the game in several ways depending on how skilled the player is at certain things.

State structure

The first thought that came to mind when i was researching in pre-production was to look up Left4Dead’s infamous AI-Director, especially since Left4Dead is conceptually similar to this project with hordes of enemies swarming the player as the core game-loop. The biggest structure I found in Left4Dead’s AI-Director is the different states: Buildup, Peak and Relax. They are used to regulate the intensity of the player experience and make it dynamically increase and decrease over time giving the player waves of different intensity levels.

Buildup

The first milestone I implemented using this intensity state structure where the buildup state as the name suggests builds up the intensity and spawns more and more enemies. The starting point of the build up is dependent on the player skill level. At the very start of the game a first wave is spawned to determine the player's very preliminary skill level by measuring how many shots per second the player is hitting on enemies. After the first wave of intensity this skill level measurement is replaced by a continuous analysis.

Peak

After the build up when the intensity level is at its highest the AI-Director determines a switch to the peak state. This switch is based on how many enemies are on the screen in a ratio to the players skill level. The main use of the peak state is to spawn one final wave of enemies to cause one final increase of intensity for the player before the relax state.

Relax

The relax state is active when the player has cleared most of the peak’s final wave of enemies. The only thing the relax state does is wait for a few moments to let the player wind down and mentally prepare for the next wave. This is done by only spawning a couple of enemies every second for about 5-10 seconds. After this the process is repeated with the build up state.

Enemy spawn angles

The second big milestone of the project was implementing a way to spawn enemies in different angles to the player depending on how good the player is at shooting enemies far apart in rapid succession. This further helps the AI-Director dynamically ajust the gameplay to the player’s skill level not only with how many enemies there are but also by how hard they are to take down as a group and not only as individual. This is done in two different steps; Measure where the skill of the player is measured and Modify where the values controlling the gameplay experience are modified depending on the previously measured values.

Measure

The first step in this process is to measuring the time between every shot the player hits on a different enemy as a ratio to how far apart the two enemies are; angle per time. This ratio is then normalized to a value between zero and one where one is the maximum value the player could possibly get with two perfect shots. These normalized ratios are then stored in a sorted list where a certain top percentage of them are used as modifiers to how far apart enemies are allowed to spawn from each other in a single wave of enemies.

Modify

The second step in the process is to use the value calculated in the previous step to change how the enemy waves are spawned. This is done by multiplying and offsetting the value with constant modifiers and then multiplying with our spawn angle which has been randomized withing a certain range.