An exemplary method of combining two images to form a fused image is provided. A hierarchical structure for each of a plurality of volumes is built. All blocks in each of the hierarchical structures that intersect a slicing plane are found. Each of the plurality of volumes is divided into stacks of parallel slices, and the parallel slices are sorted by visibility order. A next slice in the sorted parallel slices is chosen. The next slice belongs to a current volume. Rendering parameters are changed if the current volume is different from a previous volume associated with the chosen slice in a previous iteration of the step of choosing the slice. Based on the rendering parameters, the next slice is rendered by intersecting the slicing plane with the blocks corresponding to the current volume. The steps of choosing the slice, changing the rendering parameters, and rendering the slice are repeated until all of the sorted parallel slices are rendered.