A texturing artifact of 3d games where a perspective correct textured1 polygonal sky is used.

This is produced by the fact that the corners of the sky model will invariably be more distanced than the centres of those polygons, provided that the viewer is at the centre of the sky model, and the sky model is a regularly shaped one. Thus, at the corners, the texels2 will be more widely spaced when rasterized, causing lower detail of the sky in these areas.
This effect can be dampened slightly by bilinear filtering3 of the texture and other such texture interpolation techniques, as many other texturing artifacts can be. The effect is amplified by a low field of vision.
The most effective solution to this artifact, when using hardware rendering, is using a higher detail sky model. For example, a sphere approximation4 with twenty polygons would make this artifact greatly less noticable, than using a cube with six. Whether this extra overhead is worthwhile, however, is an entirely different matter. If software rendering is used, ray tracing5 may produce the best results, graphically. This also allows for use of procedural textures and the like, which are seldom implemented when hardware rendering is used.

It appears to be most prominent in games utilising the Quake 2 engine.

1 - This produces more realistic results on large polygons - otherwise, the texture is linearly interpolated over the polygon, with the distance of each point not taken into account.
2 - A texel is an individual element of the texture map, when it is scaled onto a polygon.
3 - When the colour of the pixel to be drawn is computed by a linear interpolation of all four neighbouring texels. See also, bicubic interpolation.
4 - A sphere approximation is a perfect sphere, simplified into basic polygons.
5 - This is, in general, much slower than polygon rasterization, but is able to produce smoother, and more realistic results.

(nodeshell rescue)

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