Figure 4. Diagram illustrates volume rendering technique. The upper portion of the diagram represents a histogram of the voxel values in the data "ray" depicted beneath it. An opacity of 0% and 100% are assigned for values of 5 or lower and 9 or higher, respectively. The resulting intermediate opacities for values 6, 7, and 8 are 25%, 50%, and 75%, respectively. The lower portion of the diagram shows the equation and progressive computational results used to determine weighted summation along the "ray" through the volume. The resulting displayed value (6) is affected by both opacity (as determined in the graph at the top) and the value of underlying voxels.