Improved fast phase unwrapping algorithm based on parallel acceleration

Overview: In the phase measurement technology, the phase directly obtained is usually folded into a range of wavelengths, so that the phase pattern appears as a stripe pattern. Generally, this stripe pattern is not the final result required for phase measurement. There is a phase unwrapping operation needed to obtain a continuous phase map.

The main problem facing by the phase unwrapping algorithms is how to balance the robustness and the computational efficiency. At present, there have been a lot of researches on unwrapping algorithms. They are mainly divided into two categories, including the minimum norm method and the path tracking method. The minimum norm algorithm is a global algorithm that transforms the process of phase expansion into a process of minimizing an objective function of the full graph. In the least norm method, the least squares algorithm is a commonly used unwrapping algorithm for two-dimensional phase wrapping images. Because global algorithms usually need a large amount of calculations, they require high computing power. The idea of the path tracking algorithm is to choose a suitable path for expansion, so as to avoid that the areas affected by noise appear prematurely in the path and cause errors and continue to be transmitted along the path. Quality map guidance algorithm is a common type of path tracking algorithm. This algorithm first generates a quality map describing the impact of noise. The quality map guides the unwrapping path through high-quality areas, so that the errors generated in low-quality areas will not be propagated. Thus, quality map guidance algorithm has good noise immunity.

The quality map-guided unwrapping algorithm proposed by Miguel needs a small amount of calculations and has strong noise immunity, but its serial calculation method has low operating efficiency. To solve this problem, an improved algorithm for parallel merging of multiple low-reliability blocks is proposed. Under the condition that the original algorithm design idea is satisfied, the unwrapping path is redefined as the largest reliable edge of the block. In addition, based on the non-continuous characteristic of the unwrapping path of the original algorithm, a low-reliability block out-of-order merging strategy is proposed to make multiple merging tasks can be performed simultaneously. The improved algorithm uses a multi-threaded software architecture. The main thread is responsible for looping through the unprocessed blocks to check whether they meet the requirements of merging, and the child threads receive and perform the merge tasks. The experimental results show that the improved method is completely consistent with the processing results of the original algorithm, and the parallel improvement strategy can effectively use the computer's multi-core resources, so that the operational efficiency of the phase unwrapping algorithm is improved by more than 50%.