Illustrating transparent surfaces with curvature-directed strokes

Transparency can be a useful device for simultaneously depicting multiple superimposed layers of information in a single image. However, in computer-generated pictures-as in photographs and in directly viewed actual objects-it can often be difficult to adequately perceive the three-dimensional shape of a layered transparent surface or its relative depth distance from underlying structures. Inspired by artists\´ use of line to show shape, we have explored methods for automatically defining a distributed set of opaque surface markings that intend to portray the three-dimensional shape and relative depth of a smoothly curving layered transparent surface in an intuitively meaningful (and minimally occluding) way. This paper describes the perceptual motivation, artistic inspiration and practical implementation of an algorithm for "texturing" a transparent surface with uniformly distributed opaque short strokes, locally oriented in the direction of greatest normal curvature, and of length proportional to the magnitude of the surface curvature in the stroke direction. The driving application for this work is the visualization of layered surfaces in radiation therapy treatment planning data, and the technique is illustrated on transparent isointensity surfaces of radiation dose.