TAILIEUCHUNG - The Essential Guide to Image Processing- P18

The Essential Guide to Image Processing- P18: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. | Approaches for Color and Multispectral Images 519 a b c FIGURE Canny edge detector of Eq. applied after Gaussian smoothing over a range of a a b a 1 c a 2 and d a 4. The thresholds are fixed in each case at Tu 10 and TL 4. d only computational cost beyond that for grayscale images is incurred in obtaining the luminance component image if necessary. In many color spaces such as YIQ HSL CIELUV and CIELAB the luminance image is simply one of the components in that representation. For others such as RGB computing the luminance image is usually easy and efficient. The main drawback to luminance-only processing is that important edges are often not confined to the luminance component. Therefore a gray level difference in the luminance component is often not the most appropriate criterion for edge detection in color images. 520 CHAPTER 19 Gradient and Laplacian Edge Detection a b c d FIGURE Canny edge detector of Eq. applied after Gaussian smoothing with a 2 a Tu 10 Tl 1 b Tu Tl 10 c Tu 20 Tl 1 d Tu Tl 20. As Tl is changed notice the effect on the results of hysteresis thresholding. Another rather obvious approach is to apply a desired edge detection method separately to each color component and construct a cumulative edge map. One possibility for overall gradient magnitude shown here for the RGB color space combines the component gradient magnitudes 24 fc x y VfR x y Vfc x y f x y . The results however are biased according to the properties of the particular color space used. It is often important to employ a color space that is appropriate for the target Approaches for Color and Multispectral Images 521 application. For example edge detection that is intended to approximate the human visual system s behavior should utilize a color space having a perceptual basis such as CIELUV or perhaps HSL. Another complication is the fact that the components gradient vectors may not always be similarly oriented making the search for local maxima

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