Sophisticated techniques exist for converting real-world elevation data into a terrain skin appropriate for graphical rendering [6,9,10,16]. In most such systems, however, visual realism suffers because of the stylized texture maps or simplistic coloring used to pattern the terrain skin. To improve image quality, some high-end real-time visual simulators now support geospecific texturing in which texture maps are derived from aerial images registered to the terrain skin. Software techniques are also available for draping large aerial images over terrain data [4].
 |
While draping aerial imagery onto a terrain skin adds a great deal of visual richness (Figure 1), visual realism still suffers in a number of important ways. Available high-resolution imagery is almost always panchromatic. Shadowing in the source imagery makes rendering views for times of day other than when the imagery was acquired problematic. Similar problems occur when simulating views at different times of year, particularly in alpine terrain when snow cover is an issue. Finally, texture mapping with aerial imagery in and of itself does not provide the fine scale three-dimensionality that is critical to perceiving a sense of scale over long distances. In the case of alpine environments, in particular, the three-dimensionality of trees is apparent even over distances of several kilometers.
The rendering shown in Figure 2 was produced by starting with the same source data as used to generate Figure 1. Using no additional data, terrain types and vegetation cover were mapped and this information was used to color the image. Existing shadows were removed and new shadows, appropriate to a different time of day, were added. Three-dimensional trees and brush were added at locations consistent with their appearance in the original aerial image. Finally, snow cover was added in locations appropriate for a particular time of year.
