High-accuracy 3D Shape Measurement Research Articles

High-Speed Phase Structured Light Integrated Architecture on FPGA

Phase-shifting fringe projection profilometry (FPP) in three-dimension (3D) structured light measurement is widely applied in high-accuracy 3D shape measurement scenarios. But motion objects or other dynamic scenarios make the traditional phase-shifting FPP hard to deal with due to its sizable CPU-based execution time. Therefore, an embedded high-speed and high-accuracy phase-shifting FPP system is designed and implemented on FPGAs. The system implements a three-frequency four-step phase-shifting hardware algorithm with RTL-level caching, multi-frame image ping-pong buffering, and data handling, etc. The system exhibits fully-parallel and pipeline processing while behaving with lower resource and power consumption. Measurement experiments indicate that the proposed method can reach high-accuracy real-time 3D shape measurement with a 3D decoding speed of 40.15 fps when the clock frequency is 200MHz, and the image size is 2432×2048.

High-performance phase measuring profilometry architecture based on Zynq SoC.

This paper presents a novel high-performance heterogeneous computation architecture, to the best of our knowledge, for stereo structure light using the phase measuring profilometry (PMP) algorithm based on a Zynq UltraScale+ system on chip (SoC). The proposed architecture aims to achieve real-time and high-accuracy 3D shape measurement. The experiment results indicate that the calculation time of a standard four-step PMP algorithm with a resolution of 1280×1024 is 14.11ms. It is nearly 51 times faster than the well-optimized software implementation running on a Raspberry Pi and nearly three times faster than a high-end PC, with 15 times less power consumption. Consequently, the proposed architecture is deemed suitable for real-time 3D measurements in embedded applications.

High-efficiency and robust binary fringe optimization for superfast 3D shape measurement.

By utilizing 1-bit binary fringe patterns instead of conventional 8-bit sinusoidal patterns, binary defocusing techniques have been successfully applied for high-speed 3D shape measurement. However, simultaneously achieving high accuracy and high speed remains challenging. To overcome this limitation, we propose a high-efficiency and robust binary fringe optimization method for superfast 3D shape measurement, which consists of 1D optimization and 2D modulation. Specifically, for 1D optimization, the three-level OPWM technique is introduced for high-order harmonics elimination, and an optimization framework is presented for generating the 'best' three-level OPWM pattern especially for large fringe periods. For 2D modulation, a single-pattern three-level OPWM strategy is proposed by utilizing all the dimensions for intensity modulation to decrease the required projection patterns. Thus, the proposed method essentially belongs to the 2D modulation technique, yet iterative optimization is carried out along one dimension, which drastically improves the computational efficiency while ensuring high accuracy. With only one set of optimized patterns, both simulations and experiments demonstrate that high-quality phase maps can be consistently generated for a wide range of fringe periods (e.g., from 18 to 1140 pixels) and different amounts of defocusing, and it can achieve superfast and high-accuracy 3D shape measurement.

A convenient 3D reconstruction model based on parallel-axis structured light system

Conventional high-accuracy three-dimensional (3D) reconstruction methods usually involve complicated and time-consuming system calibration. This paper presents a novel and simple 3D reconstruction model based on a parallel-axis structured light system. Compared to the traditional structured light system, the proposed system contains a perspective projection unit and a telecentric imaging unit. Instead of obtaining the complicated structured light system parameters, the proposed model only requires the retrieval of a product matrix which could be easily calibrated by using an ordinary white plane. For an arbitrary point A on the object surface, it is mathematically proved that the product matrix of its phase map and height distribution is proportional to the distance between A and projector optical center in XY plane, and independent of the depth in Z direction. Experiments were carried out to measure a step-height object and a cylindrical surface, and the results verify that the proposed method can achieve high-accuracy 3D shape measurement.

Large depth-of-field three-dimensional shape measurement with the focal sweep technique.

Three-dimensional (3D) shape measurement based on the fringe projection technique has been extensively used for scientific discoveries and industrial practices. Yet, one of the most challenging issues is its limited depth of field (DOF). This paper presents a method to drastically increase DOF of 3D shape measurement technique by employing the focal sweep method. The proposed method employs an electrically tunable lens (ETL) to rapidly sweep the focal plane during image integration and the post deconvolution algorithm to reconstruct focused images for 3D reconstruction. Experimental results demonstrated that our proposed method can achieve high-resolution and high-accuracy 3D shape measurement with greatly improved DOF in real time.

Multilevel symmetric pattern design and optimization for high-speed and high-accuracy 3D shape measurement

The binary defocusing technique has enabled speed breakthroughs for 3D shape measurement, yet simultaneously achieving high accuracy and high speed remains difficult. To overcome this limitation, we propose to utilize multilevel symmetric pattern for high-speed and high-accuracy 3D shape measurement. Compared to conventional binary patterns, multilevel pattern could bring more flexibility for eliminating undesired high-frequency harmonics, thus has the potential to greatly enhance the phase quality and measurement accuracy. In this paper, the symmetric pattern design principle and related optimization procedure are presented in order to find the “best” multilevel fringe patterns. Both simulation and experiments verify that with respect to conventional methods, the proposed method could consistently generate better fringe patterns for a wide range of fringe periods. Furthermore, we developed an absolute 3D shape measurement system with the speed of 667 Hz, verifying that the proposed method is applicable for high-speed, high-accuracy applications.

High-accuracy 3D shape measurement of translucent objects by fringe projection profilometry.

Acquiring complete and accurate 3D shape measurement results of translucent objects by fringe projection profilometry (FPP) is difficult because of the subsurface scattering effect. The phase offset introduces geometric errors, and the degraded image contrast leads to incomplete measurement results and random errors. In this research, a high-accuracy 3D shape measurement method for translucent objects based on phase-shifting FPP is proposed. The relationship between fringe period and phase error is investigated to determine the fringe periods. Random errors are suppressed by temporal noise reduction, and the robustness of multi-frequency heterodyne phase unwrapping is improved by increasing the interval of fringe periods along with temporal noise reduction. Geometric errors are compensated for by projecting multi-frequency fringe patterns to establish the relationship between fringe period and depth offset. Experimental results show that the proposed method can acquire complete measurement results and significantly reduce the overall error for measuring translucent objects.

Three-dimensional shape measurement of complex surfaces based on defocusing and phase-shifting coding

Binary fringe defocused projection is a promising method for high-speed, high-accuracy three-dimensional (3D) shape measurement because of the advantages of superfast projection and capability to overcome the nonlinear gamma of the projector. However, the application of this approach for complex surface measurement remains a challenge. Thus, a method that combines binary fringe and phase coding fringe based on defocusing is proposed to achieve fast-speed, high-accuracy 3D shape measurement of complex surfaces. However, in the case of defocusing, as the number of phase coding fringe cycles increases, the fringe order k(x, y) becomes difficult to judge, errors occur in the jump area, and the periodic dislocation problem becomes worse, ultimately leading to phase unwrapping error. A phase-shifting coding method is thus presented to resolve these issues. This method not only alleviates the difficulty in judging the fringe order as a result of defocusing but also resolves the correction problem, and in turn, the periodic dislocation problem. Thus, measurement accuracy is improved. Experimental results demonstrate the feasibility and superiority of the proposed method.

High-speed, high-accuracy 3D shape measurement based on binary color fringe defocused projection

A widely used method in high-speed 3D shape measurement, color-code fringe projection requires the projection of only one image. In traditional color-code method, the projected fringe is compounded by sinusoidal fringes, grayscale value distribution ranges from 0–255, and projection speed is limited to 120 frames/s. Consequently, measurement speed is restricted, and a nonlinear gamma of the projector exists, as well as high harmonics, which have a great influence on measurement accuracy. Binary color-code fringe defocused projection is proposed to solve the abovementioned problems. With the proposed method, projection speed can switch to tens of K frames/s because it only has two grayscale values (0 and 255). A standard sinusoidal color-code fringe can be generated by properly defocusing the binary color-code fringe, thereby overcoming the influence of nonlinear gamma and ultimately improving measurement accuracy. Experiment results verify the feasibility and superiority of the proposed method.