Tham khảo tài liệu 'machine vision - david vernon part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Image acquisition and representation distance. Again a pattern of 256 alternately light and dark bars with abrupt edges would require a much higher sampling frequency to represent the image faithfully. 3 2 Inter-pixel distances As mentioned in the preceding chapter video signals assume an aspect ratio of 4 3 horizontal-to-vertical and we noted in Section that although framestores with 4 3 aspect ratios are becoming available they normally use a square aspect ratio of 1 1. The aspect ratio mis-match has serious repercussions for gauging or measuring functions of an inspection system the rectangular picture is being squeezed into a 4 3 aspect ratio video camera signals Image acquisition 75 Image display 1 1 aspect ratio framestores 4 3 aspect ratio video monitor signals Figure Distortion arising when acquiring a video signal with a conventional square framestore. 34 Adjacency conventions square image and hence the effective distance between horizontal pixels is 5 times greater than that between vertical neighbours see Figure . The situation becomes even more unsatisfactory when one considers the distance between a pixel and its diagonal neighbour. While most discussions of inter-pixel distances usually assume the diagonal inter-pixel interval to be J2 . the length of the hypotenuse completing the right-angle triangle formed by the horizontal inter-pixel interval of length 1 and the vertical inter-pixel interval of length 1 . However if we are working with a framestore which has a square aspect ratio and has a video signal which as a 4 3 aspect ratio then the diagonal inter-pixel interval is in fact JI . the length of the hypotenuse completing the right-angle triangle formed by the horizontal inter-pixel interval of length I and the vertical inter-pixel interval of length 1 see Figure . Unfortunately this is not the complete story. The CCIR standard stipulates that a picture comprises 625 lines. However only 576 of these carry visual information
