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Skybox (Video Games)A skybox is a pre-rendered backdrop of "infinite-distance" surroundings used in 3D video games. It is used to create the illusion of distant three-dimensional surroundings. In the real world, distant objects such as clouds, stars, and mountains on the horizon appear to travel along with the viewer's motion, because their distance causes the viewer's translation to be negligible relative to them. A skybox is a cube comprised of six images, whose inside faces are displayed behind all other three-dimensional objects, and its position is fixed so that its center is always aligned to the camera's location. The six images are pre-rendered so that the seams of the cube are invisible, so it does not appear to be a cube. The resulting illusion is that objects pre-rendered in the skybox appear to be distant. Skyboxes are not strictly a box though. An alternative to the cube used in some games is a pre-rendered sphere or dome (in which case it is technically a skydome). Later video games have expanded the use of the skybox to include other objects in addition to the pre-rendered cube, so that the skybox creates a better illusion of depth and can include animated objects. Some games use an actual 3d construct as a sky. A skyline is created in miniature, in a separate part of the game level. A camera is placed fairly low within this construct. The camera rotates with the player movement, and all sky areas within the map display what the camera sees. This allows for the easy simulation of a moving sky, and for the placement of objects to create skies that change with each new level, instead of having to make a new render or artwork for a conventional skybox. A perfect example is the way that the sky changes a little with each map (well, the ones in which there are sky), in the game Unreal. The look of the sky changes as the time of day changes, and the positions of stellar objects such as the sun and moons changes as well. The camera is fixed in one spot, so any features in the skybox still appear infinitely distant, due to their lack of movement. The Source engine used by Half-Life 2 uses a similar idea, but unlike Unreal, the camera is not fixed in one spot. Instead, it can replicate the player movements in miniature. This allows for the skybox objects to appear to move relative to the player movements, called parallax. Thus, mappers can create convincing mid range features, such as hills and cityscapes.
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