Second-order statistical methods include graylevel co-occurrence matrices (GLCM) (Haralick
et al., 1973) and gray-level run-length matrices (Dasarathy and Holter, 1991). Haralick et al.
(1973) proposed a set of 14 features calculated from a co-occurrence matrix, whose elements represent estimates of the probability of transitions from one gray level to another in a given direction at a given inter-pixel distance. The features derived from GLCM include contrast, entropy, angular
second moment, sum average, sum variance and measures of correlation. Parkkinen and Oja
(1990) showed thatGLCM can be applied on different inter-pixel distances to reveal periodicity in the
texture. However, there is an inherent problem to choose the optimal inter-pixel distance in a given
situation. Also, the GLCM method, in general, is not efficient since a new co-occurrence matrix needs
to be calculated for every selected angle and interpixel distance. However, all these feature extraction
methods increase the computational time of the classification process (Haering and Lobo, 1999).