Automatic rock depiction via relief shading
ISBN 978-85-88783-11-9
Authors
1Geisthövel, R.
1ETH ZURICH INSTITUTE OF CARTOGRAPHY AND GEOINFORMATION Email: rgeisth@gmx.net
Abstract
Automated cartographic rock depiction is desirable due to the lack of experts in manually produced designs and for the saving of production costs. We present a method for a fully automatic derivation of renderings inspired by Swiss style rock hachuring. The input consists of gridded elevation data and a mask indicating rock areas, as well as global parameters like major lighting direction and output map scale. The output is a vector image consisting of monochrome hachures. Additionally, a shaded relief together with an elevation model that is both smoothed and skeleton line-enhanced are available are created. In order to solve the problem of generating strokes having the properties of rock hachures, we start out by creating a shaded relief of a generalized terrain model. We argue that shaded relief serves well as a basis for rock hachures since both are instances of a terrain visualization scheme induced by the same lighting model, but with shaded relief having a more elementary structure, i.e. one grayvalue per pixel in contrast to meandering lines of variable width and distance. Once the shaded relief is created, the rock mask together with skeleton lines are used to fill the rock areas with Swiss style rock hachures, in accordance to the grayvalues of the shading. The creation of the shaded relief is the most involved subtask of the method. Firstly we smooth the terrain using a directional filter. The filter emphasizes terrain morphology by both aggregating adjacent regions of constant gradient and exaggerating ridges and channels. The smoothed grid is analytically shaded using a Lambertian lighting model blended with an additional perpendicular light source to enhance contrast along ridges and ravines parallel to the main lighting direction. We draw evenly-spaced hachure lines along directions perpendicular to the surface gradient, and perturb the vertices to produce the irregular, wiggly appearance of rock hachures.