Interactive Music

Interactive music can be related to various terms such as adaptive, reactive and dynamic. Karen Collins describes dynamic music to cover interactive and adaptive music.
The main idea is to support the gameplay / action and communicate the story and emotions of the game so that the music will react with the narrative, game environment, non player characters as well as react or interact to player actions and decisions.
Doing this successful can be very tricky due to the non-linearity of games making the length of gameplay sections undefinable and the exact decisions a player makes upredictable. Ideally an interactive music system will be able to adapt to the undefined events that occur in a game as well as the varying parameters that occur such as health and status. On top of this the music needs to be interesting and maintain musical integrity, as well as serving the interactive needs of the game.

I decided to look at 'No One Lives Forever 2' and try describe / illustrate how the interactive music sytem works.
It seems to be mainly based around horizontal re-sequencing with some elements of varying layers the level shown below has 3 modules for various intesities of gameplay -

• A continual module that contains various layers but is pretty much a loop, this plays when you are in the level but not interacting with anything
• An alert module which raises the tension when you are spotted by an enemy
• A combat module that is triggered when you engage in a fight with an enemy

Each module will probably have defined points when it can transition into another module and when a transition occurs it will wait until it gets to one of these points. This helps to maintain the musical integrity but in turn doesn't always support the action perfectly.
There seems to be some transition sections that only occur when a transition takes place. There is very little cross fading between sections as the musica has been written with phrases that can switch between modules and enter transition phrases nicely.
The continual module plays until you either alarm the enemy of your presence or fully engage in battle. When alarmed of your presence the music will transition from continual to alert at the next transition point then do one of the following;

• If you are not physically in view of the enemy the alert music will play for a length of time then go back to continual.
• If you kill the enemy before a certain period of time the music will transition back to continual or play a short phrase and go back to continual
• If you are in view of the enemy but do not kill them before a certain time has passed then the combat music will commence
  

When you are engaged in combat the combat music will generally play until all enemies in range have been killed then transition at the next transition point back to continual or sometimes alert. The combat music sometimes continues even though there isnt an enemy within close range. I can only guess this is to do with some kind of alert parameter.