Mobj

mobj, short for moving object, is the core object type used by the Doom engine and its derivatives to represent any entity that can move or interact in the game world. It covers players, monsters, projectiles, items, and other dynamic objects that populate a map.

In the engine, a mobj contains spatial data, collision information, and state data. Typical fields include the world coordinates (x, y, z), a bounding radius and height that define its physical collision box, and orientation (angle). A mobj also carries movement and physics data such as velocity components, momentum, and gravity-related values. It references a current state for animation and behavior, a health value, and various pointers and flags that influence how it interacts with the world and other objects. Additional links include pointers to targets, attackers, and other objects, along with timing fields that govern actions and state transitions.

Behaviorally, mobj instances are updated by a thinker mechanism each game tick. Each mobj has a state machine defined by a sequence of states that control sprite frames, duration (tics), and transitions. The engine handles movement, collision with walls and floors, gravity, friction, and interactions with other objects (damage, item pickups, triggers). Flags determine solidity, damageability, visibility, and other attributes that affect gameplay and AI.

Lifecycle-wise, mobj objects are spawned by maps or scripts, persisted in the game world while active, and removed when destroyed, collected, or no longer needed. The mobj concept remains foundational in Doom-era engines and is extended in modern derivatives to support advanced features while preserving backward compatibility.