During the shading process, the material shader components calculate shading values by accessing a set of global input variables that completely characterize the surface point to be shaded.
This information includes the location of the point being shaded in both the object's local coordinate system, and the world (global) coordinate system. Also provided are a geometric surface normal vector, a shading normal vector, and an orientation vector with respect to the view direction. Any or all of these geometric values may be employed by a material shader component in order to perform its calculations. Material shader components that calculate their results based upon the shading point's position in space are able to generate arbitrarily complex and coherent patterns. Examples of these are a wood or a marble color source shader.
Shader elements with this property can be classified according to the nature of the positional information upon which the calculations are based.
The first type of material shader element is generally known as a solid texture. Solid textures use the three-dimensional coordinate system of the shading point from which to derive their results. Consequently, they create the appearance of a primitive having been carved out of a solid block of the particular material, and the visual characteristics at any point on its surface are those taken on by the solid material at the places that coincide with the object's exterior.
The second type is known as a wrapped texture. Unlike solid textures, which operate within a three-dimensional coordinate system, wrapped textures operate within a two-dimensional coordinate system known as texture space.
A simple example should help you understand the difference between solid and wrapped textures. In the following pictures, the same vase model has been shaded using similar color source components of their materials, but using a solid texture in the first case, and a wrapped texture in the second.
The vase in the first picture seems to be carved out of a solid piece of material with these properties. Where the surface of the vase passes through the center of one of the spheres, the dots appear larger, whereas in the places where the surface of the vase passes through the edge of one of the spheres, the dots appear smaller. Furthermore, in some places distortions in the pattern are apparent, while in others large areas of the surface may be void of any circular patterns where these areas do not coincide with any of the spheres. Compare this with the second picture that uses a wrapped texture. In this case, the pattern corresponds to a grid of circles against a blue background. This pattern may be thought of as having been printed on a sheet of paper which has first been wrapped around the vase and then shrunk so that it fits tightly and without distortion. In this image, the circles appear uniformly and distributed around the surface.