Synthetic image of human body is an important task in computer graphics, in which hair generation technique is especially attractive. The key point in hair generation is its fine shapes and large quantity. A method for describing 3D texture space with two concepts, curved texel and texid, i.e. 3D texture solid, is presented for hairs. The authors also give mathematical expressions of curved texels and texids for the different hair styles. Texids and curved texels are structural organizations of 3D textures. Scenes with 3D textures are made up of texids and curved texels, and a texid can be divided into curved texels, which indicates the structure of 3D texture space, and solves the modeling problem of 3D texture surfaces. The application value of structural modeling for hair description is proved through several examples.