techniques for determining the size distribution of particles in an image are an important part of the field of granulometry. Morphological techniques can be used to measure particle size distribution indirectly; that is, without identifying explicitly and measuring every particle. For particles with regular shapes that are lighter than the background, the basic approach is to apply morphological openings of increasing size. For each opening, the sum of all the pixel values in the opening is computed; this sum sometimes is called the surface area of the image. The following commands apply disk-shaped openings with radii 0 to 35 to the image in Fig. 9.25(a):