where the latter is the central moment and p is the moment order. Normalized moments invariant to translation, rotation, and scaling [1,9] are obtained with

This orientation and the one orthogonal to it are the principal axes of the object. The eccentricity of an object is given by m mp

and that ranges from 0 for a perfect circle to 1 for a straight line.

Spatial moments have been used to quantify breast tumors [2,4], mammographic calcifications [1], and blood cells [6]. The effectiveness of spatial moments has been shown to deteriorate when the object is less than about fifteen pixels wide or when parts of the objects are relatively small [7]. A modified set of invariant spatial moments normalized with respect to standard deviation [8] has been reported to improve classification and sensitivity to noise.

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