Sketch-based evaluation of line filtering algorithms

This is a record of a talk given by Mahes Visvalingam at GIScience 2000 (Oct 28-31, 2000 at Savannah, USA). The Douglas-Peucker algorithm is better than Visvalingam's algorithm for minimal simplification of 2D lines. However, the sketches abstracted with this algorithm lacked Gestalt qualities of coherence and pregnance. Whereas the P-strokes generated with Visvalingam’s algorithm were perceived to be on a single terrain surface, those derived with the Douglas-Peucker algorithm did not. The talk explains how P-stroke sketches were used for cognitive evaluation of these line filtering algorithms

Beyond factual to formulated silhouettes

When sketching terrain, a view-dependent framework of silhouette-related cues is required. This framework is prominent in manual sketches and is especially important in small-scale depictions viewed obliquely from above. Occluding contours, namely the lines delineating depth discontinuities in the projected surface, are insufficient for forming this framework. The role which the occluding contour, or Factual Silhouette, plays in structuring the sketch becomes increasingly minimal as more of the terrain becomes visible, as the viewpoint is raised.The aim of this research is to extend the set of occluding contours to encompass situations that are perceived as causing an occlusion and would therefore be sketched in a similar manner. These locations, termed Formulated Silhouettes supplement the set of occluding contours and provide a successful structuring framework. The proposed method processes visible areas of terrain, which are turning away from view, to extract a classified, vector-based description for a given view of a Digital Elevation Model. Background approaches to silhouette rendering are reviewed and the specific contributions of this thesis are discussed.The method is tested using case studies composed of terrain of varying scale and character and two application studies demonstrate how silhouettes can be used to enhance existing terrain visualization techniques, both abstract and realistic. In addition, consultation with cartographic designers provides external verification of the research. The thesis concludes by noting how silhouette contours relate to perceived entities rather than actual occlusions