In this method, each pixel is defined by projecting an imaginary light ray through the space between voxels in a straight line. One of the most common methods is known as alpha-blending. There are several ways to render volume data, be it as an octree or the entire voxel set. Marc Levoy, an assistant professor at Stanford University who is well-known for his work in volume rendering, predicts that within five years the average desktop machine will be powerful enough to skip the octree optimization as well. These specialized machines can blindly process every single voxel and still achieve real-time performance. On the other hand, if your pockets are much deeper and you can get a machine optimized for image rendering, such as SGI's Onyx/Reality Engine at US$100,000, the octree step isn't necessary. This is a slow process on desktop machines and precludes real-time display. It's a clever scheme but comes with a significant caveat: You can quickly rotate the image or change the lighting, but if you alter the opacity of any tissue within the scan, the entire octree must be recomputed.
