The most important tool in our volume rendering system is the transfer function widget. It is embedded in the scene to give it a closer relationship with the volume rendering it generates. The horizontal axis maps data value and the vertical axis maps gradient magnitude. The layout of these axes is the same as those shown in Gordon's section.
Since we are only concerned with the zero crossings of the second derivative, we do not show it explicitly. Rather, we restrict opacity to increasing and decreasing second derivative values by manipulating the boundary emphasis slider. At its right-most position, opacity is given to values regardless of their second derivatives; at its left-most, opacity is only given to second derivative values near zero. This is the same pattern of opacity assignments that Gordon showed before.