1. Computer comprising: processor; computer readable storage medium accessible to the processor and bearing instructions executable by the processor to cause the processor to: generate plural one-dimensional histograms using respective rows of pixel values in an image; generate a two-dimensional histogram using the one-dimensional histograms; determine at least one characteristic value of the two-dimensional histogram; and apply the characteristic value to the image to change at least one pixel value of the image, wherein the instructions when executed by the processor configure the processor to determine the characteristic value using at least first and second evaluations of each bin element of a one dimensional histogram.