IMAGE DETECTION AND REGISTRATION This chapter covers two related image analysis tasks: detection and registration. Image detection is concerned with the determination of the presence or absence of objects suspected of being in an image. Image registration involves the spatial alignment of a pair of views of a scene. | 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 19 IMAGE DETECTION AND REGISTRATION This chapter covers two related image analysis tasks detection and registration. Image detection is concerned with the determination of the presence or absence of objects suspected of being in an image. Image registration involves the spatial alignment of a pair of views of a scene. . TEMPLATE MATCHING One of the most fundamental means of object detection within an image field is by template matching in which a replica of an object of interest is compared to all unknown objects in the image field 1-4 . If the template match between an unknown object and the template is sufficiently close the unknown object is labeled as the template object. As a simple example of the template-matching process consider the set of binary black line figures against a white background as shown in Figure . In this example the objective is to detect the presence and location of right triangles in the image field. Figure contains a simple template for localization of right triangles that possesses unit value in the triangular region and zero elsewhere. The width of the legs of the triangle template is chosen as a compromise between localization accuracy and size invariance of the template. In operation the template is sequentially scanned over the image field and the common region between the template and image field is compared for similarity. A template match is rarely ever exact because of image noise spatial and amplitude quantization effects and a priori uncertainty as to the exact shape and structure of an object to be detected. Consequently a common procedure is to produce a difference measure D m n between the template and the image field at all points of 613 614 IMAGE DETECTION AND REGISTRATION FIGURE . Template-matching example. the image field where -M m M and -N
