Enhanced phase-coding method for three-dimensional shape measurement with half-period codeword

Fringe-based depth segmentation via minimum-fringe-period-based singular points extraction

In the field of machine vision, depth segmentation plays a crucial role in dividing targets into different regions based on abrupt changes in depth. Phase-shifting depth segmentation is a technique that extracts singular points to form segmentation lines by leveraging the phase-shifting invariance of singular points in different wrapped phase maps. This makes it immune to color, texture, and camera exposure. However, current phase-shifting depth segmentation techniques face challenges in the precision of segmentation. To overcome this issue, this paper proposes a singular points extraction technique by constructing a more comprehensive threshold with the help of the minimum period of the phase map. Taking full advantage of the proposed technique, mean-value points and order singular points are accurately filtered out, and the integrity of segmentation lines in high-curvature regions can be guaranteed. During optimization processing, the precision of segmentation is improved by employing a low-cost morphology-based optimization model. Simulation results demonstrate the segmentation accuracy reaches up to 98.58% even in a noisy condition. Experimental results on different objects indicate that the proposed method exhibits good generalization and robustness.

Absolute phase measurement method based on bidirectional coding patterns

The stair-phase-coding patterns have been widely used to determine the fringe order for phase unwrapping of the wrapped phase in three-dimensional shape measurement. Although the special coding sequence algorithm can achieve with a large number of codewords, it needs the current codeword and its adjacent codewords to jointly determine the fringe order. If any codeword of the grouped adjacent codewords is incorrectly recognized, it will result in many false fringe orders. It increases the probability of fringe order error in the decoding process. And it is challenging to significantly increase the number of codewords. To solve this problem, we propose an absolute phase measurement method based on bidirectional coding patterns. The wrapped phase of the object is obtained by four-step phase-shifting patterns, and the fringe order is obtained by bidirectional coding patterns. When generating the bidirectional coding patterns, we code two groups of stair phase with different frequencies along the horizontal direction, which respectively represent local fringe order and partition information. Then, we alternately repeat the two groups of stair phase along the vertical direction in the whole pattern to obtain the bidirectional coding patterns. Each local fringe order information and the corresponding partition information in a small region jointly determine the fringe order of pixels in this small region. Fringe order errors in a small region do not affect other regions. To verify the effectiveness of our method, we performed simulations and experiments. Simulation and experimental results show that our method is effective for objects with different sizes and isolated objects.

Intensity-Averaged Double Three-Step Phase-Shifting Algorithm with Color-Encoded Fringe Projection

Fringe projection profilometry (FPP) has been broadly employed for three-dimensional shape measurements. However, the measurement accuracy suffers from gamma nonlinearity. This paper proposes an intensity-averaged double three-step phase-shifting (IDTP) algorithm making use of color-encoded fringe projection, which does not require complex calibration processes or extra fringe patterns. Specifically, two phase maps with π/2 phase shift are encoded into the red and blue channels of color fringe patterns. The average fringe patterns of the red and blue channels are approximately in sinusoidal waveform with little harmonics, thus can be directly used for accurate phase recovery. Additionally, an adaptive weight is also estimated for average operation to suppress the effect of color crosstalk. Both simulations and experiments demonstrate that the proposed IDTP algorithm can effectively eliminate nonlinear phase errors.

Fringe-width encoded patterns for 3D surface profilometry

This paper presents a new fringe projection method for surface-shape measurement that uses novel fringe-width encoded fringe patterns. Specifically, the projection patterns are adjusted with the width of the fringe as the codeword. The wrapped phase with coding information is obtained by using the conventional wrapped phase calculation method, and the fringe order can be identified from the wrapped phase. After the fringe order is corrected based on the region growing algorithm, the fringe order and the wrapped phase can be directly used to reconstruct the surface. Static and dynamic measurements demonstrated the ability of the method to perform 3D shape measurement with only three projected patterns, single camera and projector in the least case.

Three-Dimensional Morphology and Size Measurement of High-Temperature Metal Components Based on Machine Vision Technology: A Review

The three-dimensional (3D) size and morphology of high-temperature metal components need to be measured in real time during manufacturing processes, such as forging and rolling. Since the surface temperature of a metal component is very high during the forming and manufacturing process, manually measuring the size of a metal component at a close distance is difficult; hence, a non-contact measurement technology is required to complete the measurement. Recently, machine vision technology has been developed, which is a non-contact measurement technology that only needs to capture multiple images of a measured object to obtain the 3D size and morphology information, and this technology can be used in some extreme conditions. Machine vision technology has been widely used in industrial, agricultural, military and other fields, especially fields involving various high-temperature metal components. This paper provides a comprehensive review of the application of machine vision technology in measuring the 3D size and morphology of high-temperature metal components. Furthermore, according to the principle and method of measuring equipment structures, this review highlights two aspects in detail: laser scanning measurement and multi-view stereo vision technology. Special attention is paid to each method through comparisons and analyses to provide essential technical references for subsequent researchers.

Improved spatial-shifting two-wavelength algorithm for 3D shape measurement with a look-up table

Conventional two-wavelength algorithms have been broadly used for three-dimensional shape measurement. However, the maximum unambiguous range of phase unwrapping depends on the least-common multiple of two wavelengths, and thus coprime wavelengths are commonly selected. The recently proposed spatial-shifting two-wavelength (SSTW) algorithm can achieve the maximum unambiguous range with two non-coprime wavelengths, but this algorithm tends to fail for some wavelength selections. To address this problem, this paper presents a general look-up-table-based SSTW (LUT-SSTW) algorithm with arbitrary wavelength selection. The paper also analyzes the phase unwrapping robustness in terms of phase errors and provides guidance for wavelength selection. In addition, an improved LUT-SSTW algorithm is developed to enhance the phase unwrapping robustness, and further relax wavelength selection. Some experiments have been conducted, and their results verify the efficiency of the proposed method.

Absolute phase retrieval for colored objects based on three phase-shifting amount codes

We propose an absolute phase retrieval method based on three phase-shifting amount codes (3-PSA-codes) to measure the colored object with one additional pattern. 3-PSA-codes adopt the coding concept of 3-digit-codes, in which the code elements of three consecutive periods are treated as a unique code word for one period. However, to measure the colored object more effectively in the proposed method, each code element is embedded into the PSA domain and retrieved from the phase difference. Fringe patterns for the wrapped phase are artfully employed in the code element retrieval. Hence, for the first time, to the best of our knowledge, the code element related to the phase can be determined by one additional pattern. It breaks the constraint that temporal methods require multiple additional patterns to overcome the adverse effect of the surface color of objects on absolute phase retrieval. Experimental results demonstrate that the proposed 3-PSA-codes have strong robustness in the measurement of the colored object.

Encoding technology of an asymmetric combined structured light for 3D measurement

Sinusoidal phase-shifting symmetrically combined with cyclic code is one of the most important encoding methods in the field of 3D measurement. Due to the modulation of the object surface and the influence of the noise of the image acquisition system, the periods of the cyclic code and the sinusoidal phase-shifting in the intensity image do not coincide completely, and they lead to large absolute phase decoding errors near the cycle boundaries, which are called cycle dislocation errors. In order to eliminate these errors in principle, the concept and method of region encoding for four-step sinusoidal phase-shifting are proposed, and the sinusoidal phase-shifting is combined with cyclic code asymmetrically. Under the premise that the cyclic code and the region code change at different times, the cycle dislocation error is reduced from one cycle of cyclic code to one pixel by the dual constraint of cyclic code and region code. The simulation measurement results of 3 ds max and the physical measurement results show that the asymmetric combination encoding method effectively eliminates the cycle dislocation errors; the maximum measurement error is reduced by an order of magnitude, and the root mean square measurement error is reduced by 70%.

Absolute phase measurement with four patterns based on variant shifting phases

Fringe projection profilometry has been proverbially utilized for measuring the shapes of objects. A common challenge in those systems is to accurately obtain a smooth absolute phase. Many new methods have been proposed to address this challenge. In this paper, we discuss a technique based on variant shifting phases. This approach embeds codewords into the shifting phase and only needs four patterns. However, reliable measurement results are difficult to achieve with a large number of codewords because of the phase errors. To address this shortcoming, we present a robust coding method that embeds a specific code sequence into the shifting phase and can generate more than 36 periods. The fringe order is determined using unique three-adjacent-codes combining the current period and its neighbors. An error correction algorithm is also proposed to optimize the codewords. The proposed method is experimentally verified using an established measurement system. The result shows that the proposed method is robust and efficient.

Spatial binary coding method for stripe-wise phase unwrapping

Binary coding methods have been widely used for phase unwrapping. However, traditional temporal binary coding methods require a sequence of binary patterns to encode the fringe order information. This paper presents a spatial binary coding (SBC) method that encodes the fringe order into only one binary pattern. Each stripe of the sinusoidal phase-shifting patterns is corresponding to an N-bit codeword of the binary pattern. A robust stripe-wise decoding scheme is also developed to extract the N-bit codeword, then fringe order can be determined, and stripe-wise phase unwrapping can be performed. Experiment results confirm that the SBC method can correctly recover the absolute phase of measured objects with only one additional binary pattern.

Hilbert transform-based crosstalk compensation for color fringe projection profilometry

This Letter presents an effective crosstalk compensation method for color fringe projection profilometry. The crosstalk-induced phase error has been studied, and we find that the phase error doubles the frequency of the color fringe. Therefore, Hilbert transform is applied on the color fringe to shift the carried phase by $\pi /{2}$π/2. Two phase maps are recovered from the normalized and the transformed patterns, and their average phase is used for three-dimensional (3D) reconstruction. Simulations and experiments confirm that the proposed method can effectively suppress the crosstalk-induced phase error to improve the measurement accuracy.