Circle, line and circular arc are the common basic elements in industrial computed tomography (ICT) image. The algorithm of recognizing such elements is the key to industrial CT image precise vectorization. An industrial CT image vectorization system has been studied, including different recognition methods for these elements. Firstly, based on facet model, the sub-pixel edge of an industrial CT image is extracted. Then, the circles are recognized by an improved algorithm based on probability of existence map, while the lines are recognized with the set intersection algorithm of fitting a straight line, and the circular arcs are recognized by the combination of the perpendicular bisector tracing algorithm and least squares function. Finally, the element parameters are measured according to recognition results, and the drawing exchange file (DXF) is produced and transmitted into the computer aided design (CAD) system to be edited and consummated. The experimental results show that these methods are capable of recognizing graphic elements in industrial CT image with an excellent accuracy, besides, the absolute errors of circles are less than 0.1 mm, the relative errors are less than 0.5%. It can satisfy the industrial CT vectorization requirements of higher precision, rapid speed and non-contact.
For ICT wide fan-beam scanning, there is a geometrical supposition that the object rotation center and the radiation source center intersect the image reconstruction center. In practice, the existing intersection deviation has influence on the image reconstruction precision. The image reconstruction mathematical model for shifted rotation center was established, and the relationship between the deviation error and reconstructed image precision was studied by simulation. As a result, for 512×512 CT reconstructed image, there is no distinctive difference between the reference image and the reconstructed image with eccentricity 0.1 pixels; however, with 0.2 pixels or more, the difference is obvious. So, for 512×512 CT image, the maximum permissible deviation of the rotation center is within 0.1 pixel dimension.
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