Skybox

A “Skybox” is a catch-all term used to describe ways of representing a sky or background that that 'wraps around' a computer game-world – the name coming from the traditional method of using of a big literal cube to enclose the level, with a seamless texture applied across all 6 faces. Other game engines may use a large dome or sphere instead.

Traditionally, a skybox is simply a static image that represents the entire background of a level – including the sun, clouds, distant mountains, buildings, etc. More modern games will typically break up these elements either as separate 2D image planes or low poly objects, allowing the developers to apply animation to the textures to give the appearance of moving clouds or distant traffic, as well as allowing the player to come closer to the background elements as they progress though the level. A notable example of this is the Citadel in Half-Life 2, which appears increasingly larger as the player progresses further towards it.

Some game engines, such as the Source Engine, treat the skybox a distinct element of the level, where the level designer creates a miniature enclosed area somewhere on the map, which is then projected many times larger within the area outside the playable level. Other game engines, such as Unreal Engine, make no distinction – the “Skybox” merely describes the background elements placed outside the level’s intended playable area, and although the engine comes with a built in “sky sphere”, this is simply just a large mesh that can be optionally placed in within the level.

The Skybox equivalent in CG films and VFX is typically called a “Backplate”, and is usually either a large dome or 2D image projected behind the CG elements, which can either be part of a live action shot, a matte painting, or a mixture of both. Typically however the CG elements and the background will be rendered separately and later composited together in post-production.


Examples

  • Used in Sonic Adventure 2. The levels look like they occur in the real world, but there are glitches that allow the user to jump out of a level. Doing so reveals that the levels are hallways built in a large skybox.
  • In Spyro the Dragon, flying outside the bounds of the flight levels reveal that you are inside a skybox.
  • One of the maps in Team Fortress 2, a train-yard called "Well", uses the skybox to conceal the interiors of the teams' bases, which are Bigger on the Inside. The top half of the buildings' façades are 3D miniatures projected onto the skybox. Generally this is not feasible in the game's engine, since the technique requires that there not be a direct line of sight to any part of the area being hidden.
  • In Mabinogi: Fantasy Life, the skybox moves as time flows; since time is sped up in the game compared to real life, you can actually watch the clouds/moon/sun move across the sky.
  • A toggelable option in Runescape. Used most notably in the Clan Citadels.
  • Roblox has tons and tons.
  • Modern Game Engines come with built-in skyboxes; an example is Unreal Engine from Epic Games. It's most likely the first thing you see when you load up the development environment.
  • Some especially enterprising Kerbal Space Program modder came up with a utility for replacing the default skybox, among other features.
  • The Source Game Engine used by Valve Software supports both standard 2d skyboxes, and specialized 3d skyboxes; level designers can designate an enclosed area as the 3d skybox, which cause anything placed there to appear sixteen times larger for the rest of the map. The 3d skybox is used heavily to make the otherwise very small levels in Half-Life 2 appear much larger. Some Team Fortress 2 mods can abuse the 3d skybox (which does the same thing for Half-Life 2) for shits and giggles with ricocheting rockets from the Soldier, all of which can bounce off the corners of the skybox, which have a slim chance of actually hitting something.
    • In the final scene of Half-Life: Opposing Force, the skybox was cleverly used to present a teleportation event without breaking the flow with a loading screen. Since the player has a very limited view of "outside" through the open door of an Osprey, giving each side of the skybox a different texture was sufficient to create the illusion of seeing Earth one moment and Xen the next. An unseen side of the skybox contains a hidden message which alludes to this "hack".
  • SCP Foundation has SCP-1165, titled "Minus Level", a deconstruction of Minus Worlds. There's a recreation of the Eiffel Tower in the distance, but it turns out to be part of the world's equivalent of a skybox; if one tries to reach it, they'll just notice the tower remaining at the same distance from them no matter how far they go. One survey team sent to investigate it had to call off their journey because lethal things would happen to them if they went beyond 100 km of their starting point within the strange world.

http://tvtropes.org/pmwiki/pmwiki.php/Main/SkyBox