Warning feedback alerts players to impending danger. The kind of danger the game warns players about is determined by the game designers, and can vary game to game. Some games focus their warnings on circumstances that will cause the player to die or take damage. Some games warn players that other game entities are searching for them. Other warnings include notification that time to complete a required task is almost expired. Whatever the warning alerts the player to, it can be represented through the game's visual, aural or tactile output channels, and many warnings come over multiple channels at once.
Falcon 4.0 [Blankenship and Gilman, 1998] provides audible and visual alerts when another fighter plane has locked missiles on the player's fighter plane. An alarm siren sounds and red lights flash within the representation of the fighter cockpit to warn the player that he will have to dodge incoming missiles.
Final Fantasy X [Nakazato et al., 2001] warns players when their characters are close to death by causing characters' on screen representations to slump as if at the point of collapse. This visual cue is supplemented by changing the color of a critically damaged character's hit point display yellow, rather than the normal white color. These visual cues inform players when their characters need to be healed or they will likely die in combat.
In Doom, the player is warned of nearby enemies audibly by hearing the idle noises they make.
In Doom, the player is warned of dangerous condition by the appearance of the avatar's face in the HUD; while the player has an amount of health clearly displayed, the face of the main character visibly because more bruised and bloodied as the health becomes closer to zero, giving a non-numerical indication of health. This is particularly important when the player is very close to death. This is a weak example because while the use of the face in the interface serves to warn to player of his health status, the warning is most important and effective when the player is heavily wounded. In this sense it is a sort of indirect warning.
Blankenship, S. and Gilman, L. (1998). Falcon 4.0. Hasbro Interactive, windows edition.
Nakazato, T., Yoriyama, M., and Tsuchida, T. (2001). Final Fantasy X. Square Electronic Arts L.L.C., playstation 2 edition.