The preceding chapters deal primarily with two-dimensional (2D) images. The subjects of some images may themselves be two-dimensional, such as printed text on a page. Or, depending on the scale, they may be treated as two-dimensional, such as particulates scattered on a substrate, or buildings on the surface of the earth viewed from a satellite. The measurement of surfaces that have relief and a partially three-dimensional (3D) character is dealt with in Chapter 15.