This paper describes a local thresholding algorithm for binarizing gray scale images of aggregates from a
gravitational ¯ow±falling particles. The algorithm repeatedly thresholds an aggregate image until no object can be
thesholded any further. Before thresholding an object, in each iteration of the thresholding process, the algorithm
calculates the size, shape and range of gray levels of the object. The algorithm has been implemented in an on-line
system and the ®eld test result show that it works under certain conditions. # 1999 Elsevier Science Ltd. All rights
reserved.