1. A method for analyzing digital image data, comprising: (a) loading into a computer memory a plurality of datastacks wherein each datastack comprises pixel intensity data for a plurality of images, the pixel intensity data expressed as a function of: (i) pixel position, (ii) a first non-positional variable, and (iii) a second non-positional variable, wherein within a datastack, the value of the first non-positional variable is not constant, and the value of the second non-positional variable is constant, and wherein between datastacks, the value of the second non-positional variable differs; (b) generating for a plurality of pixels within a first datastack, a plurality of first functions that relate pixel intensity to the first non-positional variable; (c) sorting the pixels within the first stack according to a first value obtained by applying a mathematical operation to the first functions generated for the plurality of pixels; (d) selecting a first set of sorted pixels; (e) generating for a plurality of pixels within the first set, a plurality of second functions that relate pixel intensity to the second non-positional variable; and (f) sorting the pixels within the first set according to a second value obtained by applying a second mathematical operation to the second functions generated for the plurality of pixels within the first set.