1. A method, comprising: representing a first image as a first graph and a second image as a second graph, wherein each vertex in the first and second graphs corresponds to a pixel in the respective images; assigning a weight to each edge between vertices in each undirected graph, wherein the weight corresponds to a difference between adjacent pixel values; deriving a first family of trees from the first graph and a second family of trees from the second graph, wherein graph metrics are embedded in the families of trees; and determining a difference measure for the first and second images based on a difference measure between the first and second families of trees.