Abstract

Iso-energy directional signatures are measured in 2-D Fourier spectra of aerial photographs. They combine the respective advantages of both the ring-shaped and wedge-shaped features usually measured such spectra, namely, the sensitivity to the coarseness and directionality of textures. Different terrain textures characterize the spatial structures displayed in aerial photographs recorded in the visible domain. The study is restricted to three classes of textures associated with fields, woods, and urban regions. Invariant signatures are defined by an automated application of the technique of Fourier descriptors for plane closed curves to contour lines selected in the spectra of the photographs. In a second step the dimensionality of these descriptors is reduced by a partial expansion on an orthonormal polynomial basis. Their statistical significance is determined by using nonsupervised classification methods. As a result, an oblique basis of signatures matched to the three classes of textures is proposed, which is invariant under translation, rotation, and change of scale. The identification rate is optimized by a further orthogonalization of this basis, using the Gram-Schmidt procedure. A comparison between the results of terrain classifications performed using either the direct signatures or the two proposed bases is presented.

© 1984 Optical Society of America

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