Research Of Digital Moiré 3D Shape Measurement Technique

Three-dimensional shape measurement technique can realize model reconstruction of actual three-dimensional object in the computer, establishing the contact between real three-dimensional world and virtual digital world. Three-dimensional shape measurement is the basis of digital three-dimensional display, the object profiling and 3D printing, and is widely used in the fields of industrial inspection, quality control, robot vision, reverse engineering, biomedicine, entertainment industry, commodity display, digital museum (heritage digitization) and so on. Optical three-dimensional measurement combines optoelectronics, digital image processing, computer control, machinery and other technology, use the optical image as carriers or means of detection and transmission of information, extracting useful signals from the images. With advantages of large range, non-contact, high-speed, high precision, and system flexibility, Optical 3D shape measurement has good prospects.The proposed method of this thesis belongs to fringes projection profilometry, FPP can be divided into two categories:one is based on the phase to height mapping method, which is developed from the information optics area; the other is based on camera model method, which is developed from computer vision area, and phase is used for stereo matching in such methods. Digital moire 3D shape measurement method belongs to the first category, based on the phase to height mapping method, this thesis gives comprehensive analysis and preliminary experimental verification about the whole process of digital moire shape measurement technology, including digital moire synthesis, phase shifting moire, phase extraction, phase unwrapping and system calibration. The main research contents are as follows:(1) A new method for digital moire synthesis is proposed, which is based on addition/subtraction and band-pass filtering. Firstly, the concept and principle of moire are introduced. Secondly, spectral structures of multiplied and addition/subtraction moire are analyzed by mathematical deduction and spectrum analysis methods. Thirdly, the new method based on addition/subtraction and band-pass filter for moire extraction is proposed and verified by experiments. This method can get better moire with a high signal to noise ratio and standard cosine intensity distribution, which provides a good foundation for 3D shape measurement.(2) The method for generating digital phase shifting moire using single deformed fringe pattern is proposed. Using digital image processing techniques to choose non-deformed fringe region projected on reference plane from the single deformed fringes, implementing Fourier Transform and spectral filtering to generate phase shifting reference fringes, or capturing one reference fringe pattern and using digital picture processing techniques to obtain phase shifting reference fringes; digital phase shifting moire can be produced according to the method proposed in section (1). After extracting phase with phase shifting algorithm and phase unwrapping, we can get the 3D shape information of the surface. The advantages of this approach are:only need one deformed fringe pattern, no longer need the phase shifting device and the amount of phase shifting is precise with image post-processing techniques to achieve the phase shifting, high accuracy of phase extraction. The proposed method can be used to measure the dynamic objects.(3) An improved Fourier Transform Method for 3D shape measurement is proposed, after extracting fundamental frequency in the frequency domain, we needn't to move it to the center of the spectrum. Applying the logarithm or arc tangent function, we get the object surface phase distribution ?(x,y),which is object phase distribution ?(x, y)superimposed with reference fringes phase2?f0x on the reference, subtracting the unwrapped phase of 2?f0x on reference plane from the unwrapped phase distribution ?(x,y)to obtain the absolute phase distribution of objec ?(x,y), we can get better phase unwrapping results.Phase unwrapping and system calibration are two key techniques of optical 3D shape measurement. The precision of phase unwrapping is affected by many factors,for example, methods of generating deformed fringes, and phase extracting, noise, object shape, lighting and so on. System calibration depends on measurement principles, system structure models, structure parameters and other factors. This thesis gives a simple analysis and discussion of phase unwrapping method and system calibration method based on the existing methods for fringes projection profilometry, a lot of work still needs to be done in the practical applications.