1. A contrast enhancement method comprising: computing a global contrast measure based on a count of occurrences of a selected spatial arrangement of two or more pixels or voxels having selected pixel or voxel intensities in an analysis image or image portion; adjusting an adjustable tone transformation function to generate a contrast-enhancing tone transformation that optimizes the global contrast measure; and enhancing contrast of a target image or image portion using the contrast-enhancing tone transformation; wherein the computing, adjusting, and enhancing are performed by a digital processor wherein the computing of the global contrast measure comprises: computing at least some terms or a function of term of a co-occurrence matrix for the analysis image or image portion; and computing the global contrast measure based on the computed terms or function, wherein the global contrast measure comprises a double integral or double summation of: 
 C ij ·D(T(i), T(j)) where indices i and j denote intensity values over which the double integral or double summation is computed, C ij denote computed terms or a function of terms of a co-occurance matrix, D(T(i), T(j)) denotes an intensity distance metric between T(i) and T(j), and T( ) denotes the adjustabletone transformation function.