A method for combining at image data sets to generate a composite image. The method divides each of the data sets into a plurality of bricks along three mutually orthogonal axes. The method includes (a) building a hierarchical structure for each one of the at least two data sets, each structure comprising higher level blocks of voxels and lower level blocks of voxels, the higher level blocks having a larger number of voxels than the lower level blocks; (b) expanding boundaries of each one of the structures into corresponding expanded hierarchical structures, such boundary expanding comprising adding additional virtual processing bricks to the initial processing bricks, such virtual processing bricks comprising semi-unbounded blocks to provide the expanded boundaries of the expanded hierarchical structures; and (c) rendering each one of the bricks in each one of the expanded hierarchical structures into a two dimension image.