Two-step Gradient-assisted Phase-shifting Demodulation Algorithm for Fast 3D Reconstruction

The conventional multi-frequency heterodyne method is one of the most widely used methods in non-contact 3D measurement. However, it needs to project different phase-shifting patterns with different frequencies, so a large number of patterns are required. For most conventional methods, the fringe period number of the projected patterns is usually small due to its limited noise tolerance, though the larger fringe period number always means higher accuracy. We propose a two-step phase-shifting demodulation algorithm based on intensity-gradient. In this method, only two patterns for each frequency are required. With the intensity-gradient of the two patterns, we obtain the wrapped phase of each frequency. Next, the absolute phase is retrieved from the three wrapped phases with the heterodyne algorithm. Because only two patterns are required for each frequency, the proposed method is more robust and has higher measuring speed compared with the traditional 3-frequency 4-step heterodyne method. Simulations and experiments prove the feasibility and effectiveness of the method, and demonstrate that the proposed method extends the noise tolerance and achieves high-precision with only a half of the patterns required by the traditional 3-frequency 4-step method.

Enhanced geometric constraint-based phase unwrapping algorithm in binocular stereo vision fringe projection system

Phase unwrapping plays an important and central role in phase-based digital fringe projection profilometry. The unwrapping quality directly influences the three-dimensional measurement accuracy. Recently, an effective geometric constraint-based phase unwrapping algorithm has been proposed to obtain the continuous absolute phase map and the unwrapped phase accuracy was found to be high. However, in this technique the virtual depth plane at z = zmin is often created empirically, which increases the manual measurement error. For this reason, this paper proposes a method for accurately constructing the virtual plane and further applies it to phase unwrapping of objects with a larger depth range. In this method, a binocular stereo vision system is used as the measurement set-up for the virtual depth plane construction and a series of virtual depth planes at z = zimin (i ≥ 2) is automatically built using a computational framework. Then, the phase is unwrapped for each region according to the continuity of the unwrapped phase and a complete absolute phase map is obtained by merging the unwrapped phases in all regions for 3D reconstruction. In this process, the virtual depth planes are created automatically and quantitatively by the measurement system. No human intervention is required and it greatly reduces the manual measurement error. Experiments show that the artificial virtual planes can be built accurately and the phase is unwrapped correctly and readily.

Accuracy estimation of the absolute phase recovery in interferometric synthesized aperture radars data processing.

The problem of accuracy estimation of the absolute phase recovery in interferometric synthesized aperture radars (InSAR) data processing is considered. A method for accuracy estimation based on the calculation of the standard deviation of the reference phases and the absolute phases measured by the interferometric system in the radar image coordinate system "azimuth – slant range" is developed. The method includes converting the coordinates of the reference topographic height marks from the geographical coordinate system to the radar image coordinate system and then calculating of the transformation parameters between the reference heights and the measured absolute phases using the least squares method. The latter allows one to evaluate the performing effectiveness of individual stages of interferometric processing (multilooking, phase noise suppression, phase unwrapping) and experimentally determine the most effective processing algorithms that provide the best accuracy of the phase recovery and their optimal parameters. The method is tested using the ALOS PALSAR radar images obtained under various imaging conditions.

A Binocular Vision-Based 3D Sampling Moiré Method for Complex Shape Measurement

As a promising method for moiré processing, sampling moiré has attracted significant interest for binocular vision-based 3D measurement, which is widely used in many fields of science and engineering. However, one key problem of its 3D shape measurement is that the visual angle difference between the left and right cameras causes inconsistency of the fringe image carrier fields, resulting in the phase mismatch of sampling moiré. In this paper, we developed a phase correction method to solve this problem. After epipolar rectification and carrier phase introduction and correction, the absolute phase of the fringe images was obtained. A more universal 3D sampling moiré measurement can be achieved based on the phase match and binocular vision model. Our numerical simulation and experiment showed the high robustness and anti-noise ability of this new 3D sampling moiré method for high-precision 3D shape measurement. As an application, cantilever beams are fabricated by directed energy deposition (DED) using different process parameters, and their 3D deformation caused by residual stresses is measured, showing great potential for residual stress analyses during additive manufacturing.