There is a good chance that your 3D volume visualization application not only needs to display high-quality images for your customers, but also needs to allow them to interact intuitively with the data. Part of this depends on your ability to identify and select parts of the volume that correspond to a given point on the screen (e.g. under the mouse cursor).

This video illustrates this technique. We create a grid of boxes at regular positions on the screen, and then use the depth buffer to place the boxes on the surface of the object which is being rendered. This technique copes naturally with varying opacity curves.