EDGE DETECTION Changes or discontinuities in an image amplitude attribute such as luminance or tristimulus value are fundamentally important primitive characteristics of an image because they often provide an indication of the physical extent of objects within the image. Local discontinuities in image luminance from one level to another are called luminance edges. Global luminance discontinuities, called luminance boundary segments, are considered in Section . In this chapter the definition of a luminance edge is limited to image amplitude discontinuities between reasonably smooth regions. Discontinuity detection between textured regions is considered in Section . | Digital Image Processing PIKS Inside Third Edition. William K. Pratt Copyright 2001 John Wiley Sons Inc. ISBNs 0-471-37407-5 Hardback 0-471-22132-5 Electronic 15 EDGE DETECTION Changes or discontinuities in an image amplitude attribute such as luminance or tristimulus value are fundamentally important primitive characteristics of an image because they often provide an indication of the physical extent of objects within the image. Local discontinuities in image luminance from one level to another are called luminance edges. Global luminance discontinuities called luminance boundary segments are considered in Section . In this chapter the definition of a luminance edge is limited to image amplitude discontinuities between reasonably smooth regions. Discontinuity detection between textured regions is considered in Section . This chapter also considers edge detection in color images as well as the detection of lines and spots within an image. . EDGE LINE AND SPOT MODELS Figure is a sketch of a continuous domain one-dimensional ramp edge modeled as a ramp increase in image amplitude from a low to a high level or vice versa. The edge is characterized by its height slope angle and horizontal coordinate of the slope midpoint. An edge exists if the edge height is greater than a specified value. An ideal edge detector should produce an edge indication localized to a single pixel located at the midpoint of the slope. If the slope angle of Figure is 90 the resultant edge is called a step edge as shown in Figure . In a digital imaging system step edges usually exist only for artificially generated images such as test patterns and bilevel graphics data. Digital images resulting from digitization of optical images of real scenes generally do not possess step edges because the anti aliasing low-pass filtering prior to digitization reduces the edge slope in the digital image caused by any sudden luminance change in the scene. The one-dimensional .
