Detecting Imagery Patterns of Circular Disks in Regular Formation by Hough Transform

• Main Authors: Liu, Hsin-Hung, 劉信宏
• Other Authors: 鍾崇斌
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/67521801575882056218

碩士 === 國立交通大學 === 資訊科學與工程研究所 === 103 === Consider the case where a circular boundary consists of 4 equal-sized arcs, the upper/lower and left/right ones, derived from an array of circular disks lying horizontally in equal distance. It’s readily observed that all upper-arcs “lie” in the same line, as “lie” all lower-arcs in another. The same phenomenon exists for left/right arcs on vertically distributed disks. The thesis addresses the subject of detecting imagery patterns of somewhat regularly distributed circular disks via Hough transform. The conception arises from the observation that the oriented-fragments in an edge-map derived from an image of circular disks in quasi-regular formation appear to lie on a same line in the very same orientation. A prototype system was setup for a series of detecting task in which quite some images of repetitive patterns, besides circles, are examined. The detecting process takes five steps as follows. (1) Extraction of horizontal and vertical arcs from disks in the input image, resulting in 2 arc maps; (2) Derivation of counterparts in Hough-space of the 2 arc maps, followed by proper threshold; (3) Inverse-transform of the 2 Hough-space maps for lines of arc-in-line in the 2 arc maps, followed by the discard of “flawed-arcs” which are unlikely to be parts of circles; (4) Hough-transform of the 2 refined arc maps in (3), followed by Inverse Hough-transform and subsequent flaw-arc removal as done in (3); (5) Congregation of the 2 refined arc maps for identifying the image regions where quasi-circular patterns exist. The initial results shown by the pilot system are deemed as satisfactory for images with spotted or circular patterns on people’s clothing where high contrast and tight spacing is generally found. For repetitive patterns accompanied with medium or relatively low contrast and in relatively large distance, as already being encountered in images of buildings in urban scene, a similar line of process as deployed in the prototype could be applied, though not yet implemented.