Note from the profile that an edge transition from

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Unformatted text preview: t an edge (transition from dark to light) is modeled as a smooth, rather than as an abrupt, change of gray level. This model reflects the fact that edges in digital images are generally slightly blurred as a result of sampling. Fig 4 (a) shows that the first derivative of the gray level profile is positive at the leading edge of a transition, negative at the trailing edge, and, as expected, zero in areas of constant gray level. The second derivative is positive for that part of the transition associated with the dark side of the edge, negative for that part of the transition associated with the light side of the edge, and zero in areas of constant gray level. Hence t he magnitude of the first derivative can be used to detect the presence of an edge in an image, and the sign of the second derivative can be used to determine whether an edge pixel lies on the dark or light side of an edge. Note that the second derivative has a zero crossing at the midpoint of a transition in gray level. Zero crossings provide a powerful approach for locating edges in an image. Although discussion so far limited to a 1-D horizontal profile, a similar argument applies to an edge of any orientation in an image. We simply define a profile perpendicular to the edge direction at any desired point and interpret the results as in the preceding discussion. The first derivative at any point in an image is obtained by using the magnitude of the gradient at that point. The second derivative is similarly obtained by using the Laplacian. Complete segmentation • • set of disjoint regions uniquely corresponding with objects in the input image cooperation with higher processing levels which use specific knowledge of the problem domain is necessary Partial segmentation • • • regions do not correspond directly with image objects image is divided into separate regions that are homogeneous with respect to a chosen property such as brightness, color, reflectivity, texture, etc. in a complex scene, a set of possibly overlapping homogeneous regions may result. The partially...
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This document was uploaded on 03/12/2014 for the course MECHANICAL 214 at University of Manchester.

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