Noclip mode

In video games, noclip mode (also known as "noclipping") is a cheat that prevents the first-person player character camera from being obstructed by other objects and permits the camera to move in any direction, simply removing all physics so the player is granted the ability to pass through such things as walls, props, and other players. Noclipping can be used to cheat, avoid bugs (and help developers debug), find easter eggs, and view areas beyond a map's physical boundaries.
Description
The term was popularized by the games of id Software in the 1990s. The name is derived from the command traditionally used to activate it: typing "idclip" in the game's command PC game console for Doom II (the previous one was "idspispopd"). The cheat is commonplace, particularly in action-oriented first-person shooters such as Quake or Half-Life. John Carmack of id Software has told fans that he derived the term from the concept of "clipping a movement vector". The first instance of noclip code came from id Software's popular game series Commander Keen.
Noclip modes (and similar modes) often originate as a means by which developers test games. If a new feature is implemented in a game but requires play to determine whether it works, it saves time if a developer can quickly reach the relevant portion of the game by avoiding death or by "flying" over time-consuming regions of the game environment. This source of god modes often manifests itself in the route by which players activate these modes - for example, running a game with a development mode flag.
The equivalent code for a game may also turn off clipping, but this is not the reason why the player can walk through walls. The code turns off collision detection, an entirely separate toggle. The code generally does not turn off back-face culling, which is why the other side of a one-sided wall is not drawn when the player uses "no collision" mode to walk through it.
Some developers have continued in the fashion of id Software.
Generally speaking, walls and objects have no "substance" unless advanced in-game physics is being used. Collision detection refers to the intersection of a wall or object with the player's avatar. If there is an intersection (collision is on), the game stops the player's motion, as if they had bumped into the intersecting object. Otherwise, the avatar will not interact with the object and will pass through it. This is a relatively simple method of implementing in-game physics with walls.
Noclipping can conflict with other elements of the game. For instance, in the MS-DOS registered 1.3D version of Duke Nukem 3D, and the Commander Keen series, having noclip mode on and walking outside the level area causes death, and if the player has god mode activated the game will be left in an infinite loop or crash due to the way god mode was implemented. In the MS-DOS Plutonium Pak 1.4 / Atomic Edition 1.5 and in source ports for Duke Nukem 3D, this problem is corrected and it instead behaves more like Doom. Disabling collision detection may also interfere with the game's programming in other ways; the player may trigger pre-programmed events at the wrong time, prevent a critical event from activating at all, or inadvertently disable access to other sections of the game world.
 
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