Contents
- Background and Introduction
- Occlusions are insufficient
- Formulated Silhouettes
- Applications
- Future Work
- Conclusion
Paper
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Abstract
The mathematical definition of silhouettes within NPAR is based on the
assumption that silhouettes are the eye-centred projections of occluding contours on an
imaging plane. However, occluding contours are insufficient for sketching the silhouettes
of hills as drawn by cartographers. The research reported in this paper is based on the
proposition that silhouettes are mental visualisations of the outlines of objects which
arise from knowledge and experience of the visual world. This paper does not seek to
provide an alternative definition of silhouettes. Instead, it tests the proposition that
missing silhouette elements should be drawn on those visible surfaces which will become
occluded if the camera position is lowered. In the upright untransformed
model, these candidate locations will have negative gradients in the view direction.
We term these candidate locations visible slopes with negative gradients. In this study, which uses a heightfield
representation of terrain, parts of surface profiles parallel to the view direction are
used as surrogates for these candidate locations. A
slope-based filter selects points from these candidate parts of these visible negative
slopes for thinning and chaining into 3D polylines, called Formulated Silhouettes. These
silhouettes do resemble the outlines drawn by people, encouraging their immediate use in
the visualisation of other layers of thematic data within Digital Cartography and
GIS. Some limitations of the current prototype approach are presented to spur future
hypotheses-based research on silhouettes.
KEYWORDS: Silhouettes, Occluding Contours,
Algorithmic Sketching, Terrain Visualisation, P-strokes
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