Issues on Displaying 3D Data for Scientific Visualization Thomas Theußl theussl@cg.tuwien.ac.at Institute of Computer Graphics Vienna University of Technology Vienna, Austria

Curvature-directed Strokes

  Not all surfaces can be sufficiently characterized by feature lines. There are situations in which a more continuous representation of both areas where the surface shape is changing and areas across which it remains relatively uniform would be desirable. The use of curvature directed strokes [6] was again inspired by artist's use of lines to show shape. Our perception of a surface's form is strongly affected by the choice of line direction used to represent it. Several different basic techniques are commonly used. Strokes uniformly directed across an entire image tend the objects appear to be flattened, vertically-oriented strokes emphasize height and horizontally-oriented strokes emphasize width. But the effect that has been chosen to pursue in this work is to align the stroke direction with the direction of strongest curvature of the surface.
The first step is to iteratively select points as evenly distributed as possible over the surface around each texture element will be centered. Next, principal direction, for the direction of the stroke, and principal curvature, for the length of the stroke, are computed at each centerpoint. Each stroke is now modelled as a ``slab'' with a length dependent of the principal curvature, a height large enough to allow each slab to contain the surface across its fullest possible extend without being so large that it opacifies the surface in unintended areas, and a properly chosen width. The geometrical definition of each individual stroke is now simply passed to the volume renderer (see Figure 6).

  
Figure 6: Transparent outer shell with principal direction texture on the left compared to a transparent outer shell textured with randomly-oriented constant length strokes on the right.