Volume Rendering

Volume rendering is a technique for displaying a 3-dimensional field as a semi-transparent colored fog. Though volume renderings of some physical variables don't look, others can be displayed very effectively with the right color mapping.

The volume rendering feature is available in Vis5D on almost all systems. Be warned that systems without 3-D graphics hardware (i.e. those using Mesa) will render volumes very slowly.

The sixth column of buttons on the control panel are the volume buttons. Only one may be displayed at a time. When a volume rendering is activated a pop- up window with a color table appears. This color table is used in exactly the same way as described for colored slices above. That is, using the mouse or keyboard you can change the function which maps data values to color and transparency. Again, the transparency can be changed while holding down the SHIFT key and drawing a curve with the mouse or pressing the up/down keys.

For those who are curious about the implementation of this feature, the volume rendering is made as follows:

  1. Examine the current viewing transformation to determine which axis of the 3-D box is most nearly parallel to the view direction.

  2. Create a number of colored slices perpindicular to that axis which map data values to colors and opacity.

  3. Render the colored slices in back to front order. The alpha values at vertices are interpolated and blended to make smooth transitions between and within slices.

Despite the simplicity of the algorithm, most fields are rendered acceptably. Those that aren't can be improved by adjusting the color and opacity mappings. While more attractive volume rendering techniques are known, this technique can be implemented quickly on many systems.