Browse Theses

Presented herein is the first method for interpreting the boundaries of shadows cast by arbitrarily shaped objects on arbitrarily shaped surfaces. The entry-exit method separates shadow boundaries into segments across which projected light rays either enter or exit the shadow. The relative orientation of each entry-exit vertex then identifies it as a juncture of specific types of segments, e.g., a shadow-making line connected to either its shadow line or its occluding line. The entry-exit approach is distinguished by being purely geometric and not requiring a priori knowledge of the scene.

Of even greater significance, this new method provides a general structure for understanding shadow boundaries. With simple elegance the entry-exit method extracts the topographic information contained in shadow boundaries. It does this by identifying the basic segments that form shadow boundaries and establishing the basic relationships that exist between them. While shadows have been widely used to help identify specific objects in aerial photographs of ground scenes, the shadows themselves are not well understood. Their complex intermingling and overlapping has heretofore obscured their purely geometric nature. As a result shadow analysis has been limited to scene-specific characteristics. The entry-exit method now provides a much broader perspective for understanding shadows.

Furthermore, the entry-exit method establishes a foundation on which elaborate scene-specific techniques can be built. The method recognizes ambiguities in shadow boundaries and identifies the information needed to resolve them.

The new method is demonstrated on scenes of irregularly shaped objects with overlapping shadows. It is shown to work in extremely complex situations.

Shadows are of great value when they are cast by inaccessible surfaces that contain abrupt protrusions of interest. In general shadows can be analyzed to determine texture, detailed relative relief curves, and under certain suppositions, 3-dimensional profiles. Valuable shadows appear in satellite images of sunlit cloud formations. Storm towers, whose existence and size are of great interest in meteorology, over-shoot cloud decks and their shadows become distinctly visible to the satellite. Cloud shadows, in fact, spurred the interest to develop a method of segmentation capable of dealing with rather arbitrary forms of shadows. The entry-exit method can now serve as the key to cloud shadow interpretation.