In the visual effects industry it is common to shoot a picture of a mirror ball (also known as a light probe) on set, as well as a gray ball for lighting reference.
Ideally, you shoot these at multiple exposures and use a tool to combine them into a single high dynamic range image and/or unwrap the mirrored/gray ball into a spherical environment map.
However, it is often difficult to regain the proper orientation of the spherical map such that it matches the camera used to render the CG scene. Furthermore, a single photo of a mirror/gray ball contains poor data for certain angles that you want to avoid seeing in the final render.
The Gray Ball and Spherical Mapping shaders are intended to simplify a particular special case where the mirror/gray ball is already shot from the exact camera angle from which the final image will be rendered. These shaders use the mental ray camera coordinate space and apply the mirror/gray ball in this space, hence the orientation of the reflections will always "stick" to the rendering camera.
The Gray Ball shader does a texture look-up based on the direction of the surface normal. It will map the normal vector direction to a point on the gray ball and retrieve its color. The parameters are identical to those used by the Spherical Mapping shader.
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