Fringe projection profilometry (FPP) has been widely used in dynamic three-dimensional (3D) morphology measurement. Nevertheless, its application in transient and complex dynamic scenes is weakened by the low measurement efficiency and poor robustness of the traditional methods. As one of the simplest approaches to eliminating phase ambiguity, the Gray code-based temporal phase unwrapping method codes fringe orders f with serial binary Gray-code patterns on the time axis. In static scenes, the Gray code-based method is able to analyze highly discontinuous objects because each spatial pixel of the measured object is unwrapped independently. Another advantage of this method is that noisy pixels remain isolated and do not spread to ruin the entire unwrapped phase. However, since M patterns can be used to code 2^M phase orders at most. Consequently, the entire duration of data acquisition is significantly prolonged, rendering the Gray code-based method inefficient in some time-critical situations such as online inspection and real-time scanning. Furthermore, blurred pattern edges caused by optical defocusing are also a source of additional errors. Pixels incorrectly unwrapped at the partial boundary between adjacent Gray-coded image areas are common. The intrinsic superiority of Gray codes in robustness and anti-noise ability appears to be no longer obvious in dynamic scenes. This study proposes an efficient and robust 3D measurement method based on time multiplexing structured light coding.

The basic idea of time multiplexing light coding in the present study is reorganizing pattern arrangement and reusing fringe patterns and Gray-code patterns on the time axis. The coding process starts by reordering the four-step phase-shifting fringe patterns according to the initial phase order: π/2, 0, π, and 3π/2. Subsequently, M Gray-code patterns are inserted into the gaps among the phase-shifting fringe groups, resulting in a phase shift value of π between two nonadjacent fringe patterns. Then, the combination of one Gray-code pattern and two phase-shifting patterns are called a sequence unit, and it has a corresponding number n on the time axis. Finally, considering the odevity of M, the cyclic sequence units shall be looped end to end to make full use of the projected pattern and obtain as many 3D reconstruction results as possible. The decoding process starts by calculating the average of the fringe patterns from two adjacent units respectively to obtain the background light intensity. Then, the truncated phase is calculated in each unit with the background light intensity. Furthermore, the phase order is calculated with the closest four Gray-code patterns on the time axis, and the unwrapped phase is obtained with the truncated phase and the phase order. The proposed method reduces the number of required patterns and improves coding efficiency. As a result, the number of projected sequential patterns required for updating a new 3D result is reduced to three. With the help of a high-speed projector and a synchronous camera in the aspect of hardware and the generalized tripartite phase unwrapping method in the aspect of algorithm, the blur caused by motion and defocusing can be suppressed greatly in principle. The combination of the above parts all together paves the way for high-robustness and high-speed 3D morphology measurement.

The results of the experiments of striking a badminton ball and pinching a rubber ball with hands prove that the proposed method can efficiently reconstruct 3D morphologies in complex dynamic scenes. The comparison and analysis of the results of measuring different objects with the binary Gray code and the quaternary Gray code confirm that the binary Gray code-based method, with greater robustness and higher anti-noise ability, is more suitable for complex high-noise dynamic scenes where the shooting speed is much higher than the speed of the objects. They also prove that the multi-grayscale Gray codes represented by the quaternary Gray code are more resistant to motion-induced blur and are thus more applicable for reconstruction in dynamic scenes featuring fast speed and lower environmental noise.The following observations can be made from the experimental results: 1) the proposed method can update a new 3D result with every three more patterns in dynamic measurement and finally reconstruct 3D morphology at 3174 frame·s^-1; 2) compared with multi-grayscale Gray codes, the binary Gray code-based method offers a longer decoding sequence, a higher anti-noise ability, and lower resistance to motion-induced blur. Therefore, it is more suitable for complex dynamic scenes featuring measuring speeds much higher than the objects' speeds.

This study proposes a 3D measurement method based on the time-multiplexed Gray-code coding technique assisting phase-shifting fringes. Through multiplexing phase-shifting fringe and Gray-code patterns in time, the study calculates the background light intensity and the phase order successively. Then, the truncated phase is calculated in a pattern sequence unit n with the background light intensity, and the continuous phase is further obtained by decoding the phase order. Breaking through the application limitation of low measurement efficiency and weak robustness on traditional structured-light coding methods in dynamic scenes, the proposed method provides a feasible technical scheme for efficient and robust 3D morphology measurement in complex dynamic scenes.