Direct volume rendering techniques use semi-transparency to visualize much more important features in the three-dimensional scalar sample volumes than isosurface rendering methods. However, they are computationally expensive and far from interactive manipulation. In fact, on the base of the Boundary Representation Scheme for the volumetric data, the 3D spatial coherence can be fully exploited to drastically reduce the rendering time and the memory space for arbitrary viewing orientation. A PC-based rendering algorithm in use of the Boundary scheme is proposed in this paper, which produces results visually similar to the volume rendering techniques, but speedups at least an order of magnitude. Examples from medical CT studies are given in contrast to the results from ray casting method both in image quality and in processing time.