An image comparison method, the method comprising: receiving a first image and a second image; point matching said first image with said second image to identify corresponding points in said first and second images; applying a Delaunay triangulation to said identified points in said first image, thereby defining a plurality of first image tiles within said received first image; defining a plurality of second image tiles within said received second image, each of said plurality of second image tiles corresponding to a respective one of said plurality of first image tiles such that each of said plurality of second image tiles is defined as a triangle whose vertices are said identified points in said second image which correspond to said identified points in said first image which are the vertices of said corresponding first image tile; for each of said first image tiles and second image tiles, adjusting the intensity of each of a set of pixels by replacing the respective pixel intensity with the cumulative relative frequency of the intensity level of said respective pixel within said respective tile; for each of said intensity adjusted image tiles, applying a plurality of non-linear functions to said respective image tile, said non-linear function being linearly independent; for each of said applied non-linear functions, separately determining a moment of the outcome of said respective non-linear functions for each of a plurality of pixel coordinate axes of said respective image tiles thereby defining a plurality of vectors, each of said plurality of vectors defined by said determined moments for a respective one of said plurality of pixel coordinate axes, each of said defined vectors being a respective column of a matrix, said non-linear functions selected such that the matrix is full rank; for each of said first image tiles, determining a distance between a subspace which is spanned by said defined vectors of said respective first image tile and a subspace which is spanned by said defined vectors of said corresponding second image tile; for each of said first image tiles, responsive to said determined distances and a distance rule, determining whether said respective first image tile and its corresponding second image tile comprise observations of the same portion; and outputting an indication of said portions determination. receiving a first image and a second image; point matching said first image with said second image to identify corresponding points in said first and second images; applying a Delaunay triangulation to said identified points in said first image, thereby defining a plurality of first image tiles within said received first image; defining a plurality of second image tiles within said received second image, each of said plurality of second image tiles corresponding to a respective one of said plurality of first image tiles such that each of said plurality of second image tiles is defined as a triangle whose vertices are said identified points in said second image which correspond to said identified points in said first image which are the vertices of said corresponding first image tile; for each of said first image tiles and second image tiles, adjusting the intensity of each of a set of pixels by replacing the respective pixel intensity with the cumulative relative frequency of the intensity level of said respective pixel within said respective tile; for each of said intensity adjusted image tiles, applying a plurality of non-linear functions to said respective image tile, said non-linear function being linearly independent; for each of said applied non-linear functions, separately determining a moment of the outcome of said respective non-linear functions for each of a plurality of pixel coordinate axes of said respective image tiles thereby defining a plurality of vectors, each of said plurality of vectors defined by said determined moments for a respective one of said plurality of pixel coordinate axes, each of said defined vectors being a respective column of a matrix, said non-linear functions selected such that the matrix is full rank; for each of said first image tiles, determining a distance between a subspace which is spanned by said defined vectors of said respective first image tile and a subspace which is spanned by said defined vectors of said corresponding second image tile; for each of said first image tiles, responsive to said determined distances and a distance rule, determining whether said respective first image tile and its corresponding second image tile comprise observations of the same portion; and outputting an indication of said portions determination.