Hardware accelerated
volume rendering

Written by Paul Bourke
November 2003


Introduction

The following outlines a programming project aimed at making full use of OpenGL based accelerated hardware to render volume datasets at interactive frame rates (for example, greater than 10 frames per second). It was assumed that only datasets that fit into main memory would be considered, that is, disk spooling would not be considered. In addition to "normal" use on a workstation, another goal was for the software to support stereoscopic projection using either frame sequential or side-by-side stereo pairs commonly used in passive stereoscopic projection environments.

Performance

No matter how fast the graphics hardware there will always be volumes of sufficient resolution to make interactive rates impossible, at the time of writing and with the graphics cards (nVideo fx2000 and Wildcat 6110) available this occurred around the 500x500x500 size. A number of approaches were taken to improve interactivity, they were:

  • The operator may choose the level of subsampling of the native volume resolution. This is typically by a factor of 2, 4, or 8. This enables quick exploration, testing of voxel to colour mappings, and view point considerations. A single keystroke or menu selection changes the subsampling level.

  • When the operator is interacting with the model it is subsampled by a factor of two in all dimensions, typically resulting in a factor of 8 performance improvement. Interestingly the subsampled data often reveals additional structure due to the increased transparency because there are half the number of contributing texture planes.

  • Instead of always looking at the whole volume, a smaller subvolume (shown in yellow in the images on the right) can be moved around the whole volumetric dataset (shown in grey on the right). This was initially chosen for performance reasons but turned out to be a powerful way of exploring large datasets when one was inside the data volume.

Algorithm

The algorithm uses texture mapped planes implemented in OpenGL. The textures are extracted from the volumetric data after applying a colour/opacity mapping function. This function is shown in the lower left corner of the images on the right, the pink represents the volumetric data histogram, the colour map is along the bottom of the histogram and the white curve indicates the opacity. At the time of writing 8, 16, and 32 bit integer voxel value datasets are supported as per the vol file format.

The orientation of the planes depend on the relative position of the virtual camera. One of three sets are used, each set is perpendicular to one of the axes of the volumetric cube. The orientation of the planes used is the one that is closest to being perpendicular to the camera view direction, see figure 1 and 2 for examples of two of the three possible plane orientations (grey lines within yellow subcube). As the camera passes between the 45 degree edges of the inspection sub volume the planes are swapped and the new texture set computed. As long as the camera stays within the current quadrant no reference need be made to the volumetric data. The textured planes are additionally drawn from the back to the front so that the OpenGL blend operators will work correctly (depth test is disabled).


Figure 1: Orientation of planes


Figure 2: Orientation of planes

Details

Usage: glvol [options] volfile
Options
          -h     this text
          -f     full screen
          -s     active stereo
         -ss     dual screen stereo
          -m s   load map file
          -M s   load marker file
          -r n   initial resolution (1,2,3,4)
         -vx n   initial view volume x size
         -vy n   initial view volume y size
         -vz n   initial view volume z size
          -c s   load camera file
          -a     start in auto rotate mode
          -A n   set tween frames for animation
Key Strokes
  arrow keys     move inspection box
       mouse     rotate/roll camera
           h     move camera to home position
           f     focus camera to current inspection cube
           c     center inspection volume
           w     write current window to a TGA file
           W     write frames to TGA file continuously
         i,k     translate, up/down
         j,l     translate left/right
 x,X,y,Y,z,Z     automatic scanning along axes
         <,>     move camera forward/backward
         +,-     change inspection box size
          f1     toggle decoration display
          f2     toggle texture display
          f3     toggle histogram display
          f4     toggle text information
          f5     toggle camera tracking
          f6     toggle autorotate
          f7     toggle 3d cursor
     1,2,3,4     set supsampling level to 1,2,4,8
           q     quit
 


MRI data




Volumetric data courtesy of Brent Tully




Helix waves




2dF subvolume




Screen dump




Ducts






Rendering and visualisation of furnace data

Written by Paul Bourke
May 2004



Temperature distribution





Oxidant distribution





Nitrous Oxide distribution





Flow pattern