AUTOMATIC DETECTION OF SHADOW POINTS IN DIGITAL IMAGES FOR AUTOMATIC TRIANGULATION

摘要

This paper presents a novel method for automatic detection of shadow points in digital images in order to improve the reliability of automatic triangulation. The new method is based on the model of shadow points, which contains the radiometric and geometric properties of shadow in the image. The first is that there is a distinct contrast along the edge of a shadow in the image. The second is that the surface around a shadow is not a smooth surface and the points in the shadow area have smaller elevation than the objects casting the shadow, such as trees or buildings. The proposed method works in two steps: it first detects points with high contrast by image processing, and then finds points where the terrain slope changes sharply in its surrounding area by using a local digital surface model (DSM) which is generated by image matching. The proposed approach has been implemented within Z/I Imaging's automatic triangulation product - ISAT. The tests on different projects show that the proposed method can detect shadow points effectively and thus improve the reliability of the triangulation results.