Sphere mapping is a technique used in computer graphics to apply an image or texture to a 3D sphere. It can be used for environmental mapping or to create textured spheres. The method distorts the image in different ways depending on its use, but it is fast and effective.
In computer graphics, sphere mapping can refer to one of two methods used to apply an image or procedural texture to a three-dimensional (3D) sphere. Typically, sphere mapping is used to apply a two-dimensional (2D) rasterized image to the surface of a sphere, distorting the flat image to match the polar coordinates of the shape. Sphere mapping is also used to describe a form of environment mapping that uses a pre-rendered image of the scene that is mapped onto the surface of a sphere in a way that makes the sphere appear to reflect its surroundings, like a ball made of perfectly reflective metal. Both types of sphere mapping result in a three-dimensional sphere that has texture applied to its surface, although ambient mapping distorts the image in a different way than spherical projection mapping.
When used to apply a texture to a spherical shape, sphere mapping takes a 2D image and projects it onto the textured surface of the sphere. The image follows the coordinate system of the sphere, where each vertex on the surface lines up much like the intersections of lines of longitude and latitude on a globe. This means that, as the image reaches the top and bottom poles of the sphere, the image will begin to compress towards a single vertex, effectively distorting the image. In many cases with a properly created texture image this gives a more natural look to the sphere than using cube mapping or cylinder mapping. This technique is used to create graphics and animations of textured spheres like the planet Earth with satellite images of the oceans and continents.
When used in environmental mapping, spherical mapping is a quick way to create an object that appears to have a mirrored surface. The process calculates vectors of light from the surface of the sphere, and that vector is then translated into coordinates that are used to find color within a 2D image. Unlike the projection sphere mapping, the reflective version distorts the image in a slightly different way.
One advantage of using ambient sphere mapping is that it is very fast compared to other methods, such as ray tracing, which dynamically calculate reflection based on objects in the scene. The method is fast because the reflection is actually a pre-rendered image. This leads to some complications with the technique, because dynamic changes in the environment, or objects moving in the scene, will not show up in the reflection. Also, because the perceived reflection is static, concave shapes that receive the mapping will not reflect, resulting in a sometimes confusing visual effect.
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