Issue 30

W. Tao et alii, Frattura ed Integrità Strutturale, 30 (2014) 537-544; DOI: 10.3221/IGF-ESIS.30.64 540 M EASURING THE PARAMETERS OF UNDERWATER BRIDGE CRACKS n dealing with the image, the target description is mainly boundaries and regions. The object boundary is generally represented by a chain code. The target area usually has 2 representations method of 4-neighborhood and 8- neighborhood. But this article will not expand on this in detail. Feature extraction Common regional features include size, external ellipse, external rectangles, gravity center, circularity, perimeter, etc.. Grayscale features have a highest gray value, a minimum gray value and an average gray value, etc.. Contour features include contour length. The basic steps of the feature extraction process are shown in the following picture. In order to detect the cracks it is necessary to find the cracks in the image, confirm the crack area and obtain the crack region area, length, width, perimeter and other parameters. 1) Measurement of proportion In regular binary image processing, the calculation of proportion, in fact, is about the geometry characteristics quantity for measurement of the size of connected area after binaryzation. The specific definition is the total amount of pixel in connected area. We speculate that the pixel value of cracks in binary image is 1. Therefore, the calculation of the proportion can be simply expressed as:     , , x y s A g x y    S is expressed as the connected domain that needs to be measured;   , g x y is the pixel value of point   , x y . Although the overall proportion measurement of the target is very accurate, the proportion refers to the total proportion of all targets in the image. It may involve the fracture section, various complex cracks or some small cracks that have interference (eg. the unfiltered random noise). Above all, the complex cracks such as massive cracks or crocodile cracks cannot be clearly distinguished into single cracks. Thus, the big error exists. Therefore, we propose a new means to solve the problem. We use the template and target to conduct bitwise AND, in fact, logic and operation on binary image to calculate the area of cracks. We select curve   , f x y and use binary logic operation to obtain more accurate value. Binary logic and operation meet the following formula: 0 & 0 0  ; 0 & 1 0  ; 1& 0 0  ; 1& 1 1  (In binary image, the pixel value of points in the image only have two kinds of values; in the specific calculation process, we adopt bitwise and operation on the binary image array.) By doing this, the false data that may be contained around crack and in crack can be removed. That can make the image area calculation become more accurate. In addition, bit manipulation, the most basic operation in the computer, occupied the absolute advantage in calculation speed because the hardware of the computer only identifies 0 and 1. 2) Measurement of length and width In practical crack image, absolute across and down do not exist. Generally, the cracks have radian and even burr. The middle part may also break. Therefore, we adopt the following assumption for the easy disposal of computer. 1) In the segmented sub-block, axis of cracks in the target area is expressed as curve   , f x y . It is a connected fracture section with unit pixel width; 2) Cracks in the target area is the point with the minimum gray level in the non-growth direction area of that point; 3) Curve   , f x y can fit a straight line in subsection. Read image Binaryzation Noise  removal Mark area Calculate  feature  quantities I