Heads-up display

In technology a heads-up display is a set of data that appears before a viewer without obstructing view. In the Doom games it consists of the status bar on the bottom of the screen and text that appears in the upper left corner of the screen, normally consisting of one-line text message in red lettering.

A message is normally displayed in the following circumstances:
 * An item has been picked up.
 * A key is needed to open a door or flip switch.
 * In multiplayer mode, a player has sent a message, or left the game.
 * A game mode (messages, detail, gamma) has been changed.
 * A cheat code has been activated.

Messages are displayed for approximately four seconds and then disappear. The "Enter" key will re-display the previous message. The "F8" key will disable game-generated messages (and subsequently reenable them). ("F8" does not affect messages from other players.)

In multiplayer mode, there is a second line in the heads-up display that is used to compose messages to be sent to the other players. The "T" key begins message composition, and a horizontal caret appears. A message may then be typed, and sent with the "Enter" key, or one of several predefined messages (known as "chat macros" or "chat strings") may be sent by holding "Alt" and pressing the assigned number key. If there are more than two players, a message may be sent to a single specific player by using the first letter of that player's color instead of "T" ("G" for green, "I" for indigo, "B" for brown and "R" for red).

Many source ports have made enhancements to the heads-up display, such as scrolling message lines, additional game-generated messages, varying color and position of text, and modes that replace the status bar at the bottom with heads-up display data (commonly known as HUD mode).

Technical notes

 * The heads-up display code is also used for text messages in menu mode and in automap mode (including the level name).
 * Different messages are displayed when a key is needed for a local door versus a remote door:
 * "You need a color key to open this door"
 * "You need a color key to activate this object"
 * this is because this may be not a door at all but a switch which make the floor sink for example