TOWARDS OPTIMAL RELIEF REPRESENTATION FROM MARS IMAGERY BY COMBINATION OF DEM AND SHAPE-FROM-SHADING
Egon Dorrer, Xiuguang Zhou · 1998
Described are experimental results of a first approach towards optimal shaded relief representation of the surface of Mars derived from Viking Orbiter digital imagery. The complex problem addresses homogenization and optimization of reliefshading caused by both heterogenious and cartographically improper directions of terrain illumination. The paper deals with the task of removing actual (natural) shading in the original images by a method denoted “de-shading “ and subsequent artificial “re-shading “ simulating physiologically better illumination direction. Due to space-variant reflectance properties of the physical surface, occurrences of shadows, and mostly non-existence of a high quality digital terrain model, the mathematical problem is, in principle, ill-posed and requires certain regularity and smoothness conditions for the derivation of reasonable solutions. Shape-from-shading (SFS) methods alone can provide relative shape information only for surfaces with constant albedo and, at least, two different illumination directions. Since this is unrealistic for planetary surfaces, an integral estimation approach utilizing even low quality DEM’s generated by stereo photogrammetry may lead to better solutions. In the paper a modified iterative SFS-algorithm originally published by Zheng and Chellappa is described by using the simple Lambert photometric model. For the first time, space-variant albedo could be estimated by means of a special region growing algorithm. Experimental results for a Viking orbiter image of Mars, for which no height information was available, and for a Clementine orbiter Lunar image with a fairly smooth digital elevation model given, are discussed. They indicate the principal possibility of recovering albedo as well as improving the DEM; however, also the deficiencies of the method become apparent. 1