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In some video games, noclip is a video game cheat command that prevents the first-person player character camera from being obstructed by other objects and permits the camera to move in any direction, allowing it to pass through such things as walls, props, and other players.
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 the "No-Clip" code probably came from id Software's popular game series, Commander Keen.
Noclip modes (and other 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 given 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 you use "no collision" mode to walk through it. It is unclear why id Software chose to call a "no-collide" code a "no-clip" code, unless the code also turned off clipping, and that was the (developer-intended) point to the cheat: to test the system when everything in the level was being drawn at once, and also remove collision so that one could quickly inspect the level for problems.
Some developers have continued in the fashion of id Software, and refer to this cheat as "clipping" even when it only turns off collision detection, perhaps due to user familiarity with id's code. Other developers call the toggle by its proper name.
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.
No-clipping can conflict with other elements of the game. For instance, in Duke Nukem 3D, and the aforementioned Commander Keen Series, having no-clip 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 most source ports for Duke Nukem 3D, this problem is corrected and it instead behaves more like Doom.
Uses of noclipping 
Noclipping can be used to cheat, avoid bugs (and help developers debug), find easter eggs, and view areas beyond a map's physical boundary.
Visual glitches 
In the classic first-person shooter Doom (but less common in newer titles, and completely removed when a skybox is present), an effect known as the 'Hall of Mirrors' can occur as a result of design error or player experimentation.
Games of the day had been programmed under the assumption that the player's viewpoint would always be completely contained within sealed areas.
This made it unnecessary to erase the contents of the screen before drawing the next frame; the new frame would completely cover its predecessor anyway. However, when the player used noclip mode to maneuver the virtual camera so as to violate this assumption, the game would generate frames with "holes" in them where there was no content to display. These frames would fail to completely cover the older ones, leaving whatever had been drawn a moment before still visible in certain regions of the screen. As the player further moved the virtual camera, numerous un-erased regions would blend together to produce visual patterns resembling those encountered in a physical hall of mirrors.
Typing "gl_clear 1" in Quake engine games is a way to empty the buffer of the image of what was last rendered. With this setting, a magenta color is drawn outside of the world, clearing the screen and aiding navigation when outside of the level.
There are bugs in games such as Sonic Adventure 2, Dead Rising, Lost Planet and We Love Katamari at which the character's shadow appears on all floors below the character instead of only the floor immediately beneath the character.
This can also happen in Source engine games including Half-Life 2, Counter-Strike: Source, Left 4 Dead, and Team Fortress 2, where if a mapmaker has not set the correct properties on a floor (disable shadowing, etc.), the shadows of the players above will show through. This is not helpful in the least for players in upper rooms trying to hide or avoid detection from players below them. In Team Fortress 2, no clipping can be used to build buildings in spawns; walk through walls; float everywhere; receive health, ammo and metal; and move players into enemy spawns, if enabled by the server. In Counter-Strike and the Left 4 Dead series, after the player has died, they have the option to select "free mode" as they wait to respawn or are a spectator. This allows the player to move anywhere on the map. This ability also helps to find secret areas of the map that are unreachable or unable to be seen in normal play. Such as seeing the credits for the map de_Dust2 (and many other maps) outside the Counter Terrorist's spawn.
In the original Metroid for the NES, there is a secret world that can be found by taking advantage of an in-game glitch involving bomb jumps and an open door. The area where the player ends up is similar to spaces you can find using a Noclip code, in that it stores sprites that were not used in the final game.
In Rygar for the NES, there is a place on the overworld "Garloz Plains" there is a spot where the player can shimmy through the wall and onto the green grass. Once there, the player can go left and will see a 2-D "hall of mirrors" like effect, with the room full of the same sprite.
In Super Mario Bros. for the NES, there are three ways to walk through walls, an example being the trick to access the minus world (World -1) and the shortcut on World 4-2 or by simply using a Game Genie code.
In many Unreal Engine games there are several cheats that can be typed to enable noclip mode. The most common one is a single typing of the ghost cheat. The player typing the cheat will be able to fly around the level and through geometry.
In the MMORPG RuneScape a glitch was discovered that gave players certain properties of noclip, such as being able to go past almost any barrier and allowed players to cheat in in-game activities. This glitch was patched some months after its discovery in 2008.
In the racing game "LA Rush" for the PSP, players can leave the map by driving through a store window in the Hollywood section. They can then reach areas blocked off before.
In the games Call of Duty 4: Modern Warfare, Call of Duty: World at War, and Call of Duty: Black Ops for the Xbox 360 and more recently the PlayStation 3, players could copy their game save data to their computer and modify it so they could use noclip in online multiplayer matches. Players primarily used the noclip mode to cheat, as it allowed them to "fly" across the map with great speed and access normally inaccessible parts of the map in addition to turning off the hit detection (which effectively made them invincible). However, while the noclip mode was active, players could in no way interact with the game environment (i.e. shoot other players). This exploit has since been patched for both versions of the game on the Xbox 360 and PlayStation 3(Although there are some rumors[who?] that a software called "Modio" can still be used to cause noclip to occur on World at War and Modern Warfare for Xbox 360). However, some games, such as Team Fortress 2 have not patched this on the console version. On PC versions the noclip mode is accessible in single player maps as well by entering "noclip" into the console.