1. A method of segmenting a digitized image comprising the steps of: providing a digitized image comprising a plurality of intensities corresponding to a domain of pixels on a grid; marking a subset of pixels in the image; defining edge conductances between each pair of adjacent pixels in the image based on the intensity difference of each said pixel pair; associating a probability potential with each unmarked pixel; and using a multigrid method and a computer to solve for the probability potentials for each unmarked pixel, wherein a restriction operator from the image grid to a coarse grid is calculated from a conductance-weighted average of the edge conductances on the image grid, coarse grid conductances are calculated from the edge conductances using a Δ-Y conversion, and a prolongation operator is calculated using a conductance-weighted interpolation of the coarse grid conductances.