Until recently, little volume rendering has been done in virtual environments because the time required to render a single animation frame has defeated the need in VR for real-time animation. However, on the SGI ONYX, 3D texture mapping, done in hardware, can be used for volume rendering in real time. In brief, the data is defined as a 3d texture which is then sampled by a stack of equally-spaced slicing planes. There are many ways to use 3D texture mapping in direct volume rendering. We currently let the input values act as indices into a texture lookup table, which in turn, contains the brightness values and opacities that are actually displayed.
Widgets are available for separate control of brightness and opacity.
This arrangement allows the user to segment the data by making certain ranges of values visible or invisible. The opacity can be adjusted to vary the display from translucent to opaque, the latter being equivalent to an isosurface display. Brightness can be adjusted for a pseudo-lighting effect. The data may be rotated in texture space to be viewed from any orientation. Results are best if the slicing planes are kept perpendicular to the user's line of sight.