1. A method of combining two images to form a fused image, comprising: (a) building, by a processor, a hierarchical structure for each of a plurality of volumes; (b) finding, by a processor, all blocks in each of the hierarchical structures that intersect a slicing plane; (c) dividing, by a processor, each of the plurality of volumes into stacks of parallel slices and sorting the parallel slices by visibility order; (d) choosing, by a processor, a next slice in the sorted parallel slices, the next slice belonging to a current volume; (e) changing, by a processor, rendering parameters if the current volume is different from a previous volume in a previous iteration of step (d); (f) rendering, by a processor, based on the rendering parameters, the next slice by intersecting the slicing plane with the blocks corresponding to the current volume; and (g) repeating steps (d)-(f) until all of the sorted parallel slices of the current volume and the previous volume are rendered together in a fused image.