Abstract
In this work, a new method of measuring surface shape based on Brox optical flow estimation is presented. The measuring system consists of a projector, a measured object and a charge coupled device (CCD) camera. The grating fringes are projected onto the reference plane at a small angle. Two fringe images—before and after placing the measured object on the reference plane—are captured, respectively. Then, the optical flow field between two images is evaluated by using Brox optical flow algorithm. The theoretical relationship between the optical flow field and the height of the measured surface is established. According to the relationship, the height distribution of the measured object can be retrieved quickly without phase-to-height transformation. However, the calculated height distribution has been found to be deviated from its true value. To solve the problem, a correction scheme suitable for the optical flow method is proposed. By using the correction scheme, the accuracy of the calculated result is greatly improved. Simulations and experiments are completed to verify the feasibility of the proposed method and the accuracy of the correction method. The results show that the proposed method is more accurate than that of the Fourier transform method. Compared with traditional surface shape measurement, the optical flow method has some obvious advantages: (1) Only two frame images are required to recover the height distribution. (2) Relatively simple in measurement process and calculation so less time consuming. (3) Because the optical flow method contains time factor itself, it is more suitable for dynamic measurement. (4) No restrictions on projection pattern.
Highlights
Three-dimensional (3-D) surface shape profilometry is an important method to obtain object shape features, and it has many applications in cultural relic protection, computer vision, surface detection, quality inspection and so on [1,2,3,4]
A grating fringe image with a gray level of cosine distribution is projected onto the measured object surface by using a projector
From the introduced due to the measured surface when the point is observed from the sensor, which analyses above, we know that there are no deviation errors when the projector is located at the point P0 is represented byCCD
Summary
Three-dimensional (3-D) surface shape profilometry is an important method to obtain object shape features, and it has many applications in cultural relic protection, computer vision, surface detection, quality inspection and so on [1,2,3,4]. The projection fringe correction method, more suitable for measuring large objects, requires that the optical center of the projector and that of the CCD camera are at the same distance from the reference surface. Different from the projection fringe correction method, the generalized fringe projection method has high accuracy, but needs to capture a series of images in the experiment, and requires other operations such as phase extracting and phase unwrapping, fringe order offsetting and parameter detecting. This method is time-consuming and complex in the experiment process. Any image with varying gray values can be used for projection
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