Abstraction
The application of volume rendering techniques to the display of surfaces from sampled scalar
functions of three spatial dimensions is explored. Fitting of geometric primitives to the sampled
data is not required. Images are formed by directly shading each sample and projecting it onto
the picture plane. Surface shading calculations are performed at every voxel with local gradient
vectors serving as surface normals. In a separate step, surface classification operators are
applied to obtain a partial opacity for every voxel. Operators that detect isovalue contour surfaces
and region boundary surfaces are presented. Independence of shading and classification calculations
insures an undistorted visualization of 3-D shape. Non-binary classification operators insure
that small or poorly defined features are not lost. The resulting colors and opacities are composited
from back to front along viewing rays to form an image. The technique is simple and fast, yet
displays surfaces exhibiting smooth silhouettes and few other aliasing artifacts. The use of
selective blurring and super-sampling to further improve image quality is also described. Examples
from two applications are given: molecular graphics and medical imaging.
