The Essential Guide to Image Processing- P5:We are in the middle of an exciting period of time in the field of image processing. Indeed, scarcely a week passes where we do not hear an announcement of some new technological breakthrough in the areas of digital computation and telecommunication. | 118 CHAPTER 5 Basic Tools for Image Fourier Analysis FIGURE Image peppers processed with the left mid-frequency DFT mask and the right high-frequency DFT mask. FIGURE Image fingerprint processed with the left low-frequency DFT mask and the right midfrequency DFT mask. significantly enhanced. A similar effect was produced with the higher frequency mask but with greater emphasis on sharp details. Frequency Orientation The orientation of an image frequency refers to its angle. The term orientation applied to an image or image component describes those aspects of the image that contribute to an appearance that is strongly characterized by the frequency orientation portrait of the DFT. If the DFT is brighter along a specific orientation then the image contains highly oriented components along that direction. The image fingerprint with DFT magnitude in Fig. c is also an excellent example of image orientation. The DFT contains significant mid-frequency energy between Understanding Image Frequencies and the DFT 119 the approximate orientations 45 135 from the horizontal axis. This corresponds perfectly to the orientations of the ridge patterns in the fingerprint image. Figure shows the image planks which contains a strong directional component. This manifests as a very strong extended peak extending from lower left to upper right in the DFT magnitude. Figure escher exhibits several such extended peaks corresponding to strongly oriented structures in the horizontal and slightly off-diagonal directions. Again an instructive experiment can be developed by defining zero-one image frequency masks this time tuned to different orientation frequency bands instead of radial frequency bands. Several such oriented frequency masks are depicted in Fig. . As a first example the DFT of the image planks was modified by two orientation masks. In Fig. left an orientation mask that allows the frequencies in the range 40 to 50 only as well as the .
