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摘要:
针对Miguel等人提出的质量图引导相位解包裹算法中串行运算效率较低的缺点，构造了一种多个低可靠度区块并行合并的改进算法。在满足原始算法设计思想的前提下，对解包裹路径进行重新定义，并根据原始算法的解包裹路径非连续的特性，构建了一种低可靠度区块乱序合并的策略，使得多个低可靠度区块的合并任务可以同时进行。改进算法采用多线程软件架构，主线程负责循环遍历未处理的区块，子线程接收待处理的区块执行合并任务。实验结果表明，改进方法与原始算法的处理结果完全一致，而并行改进策略可有效利用计算机多核资源，使得相位解包裹算法的运行效率提高了50%以上。
Abstract:Aiming at the shortcoming of low serial operational efficiency in the quality-map-guided phase-unwrapping algorithm proposed by Miguel, 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%.
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Key words:
- phase unwrapping /
- quality guidance /
- path dependent /
- parallel computing /
- phase measurement
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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%.
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图 2 改进算法的目的。(a)顺序执行合并操作; (b)并行执行合并操作
Figure 2. The idea of the proposed algorithm. (a) The merging order of serial algorithm; (b) The merging order of parallel algorithm
图 6 对低阶像差解包效果。(a)组合像差；(b)包裹图像；(c)原算法解包裹结果；(d)改进算法解包裹结果
Figure 6. Unwrapping of low-order aberrations. (a) The true unwrapped phase; (b) The wrapped phase; (c) Result of the original algorithm; (d) Result of the parallel algorithm
图 7 对高阶像差解包效果。(a)组合像差；(b)包裹图像；(c)原算法解包裹结果；(d)改进算法解包裹结果
Figure 7. Unwrapping of high-order aberrations. (a) The true unwrapped phase; (b) The wrapped phase; (c) Result of the original algorithm; (d) Result of the parallel algorithm
图 8 解包裹过程中的分组示意图。(a)原算法；(b)改进算法
Figure 8. Group diagram in unwrapping process. (a) Group diagram of original algorithm; (b) Group diagram of parallel algorithm
表 1 两种算法的耗时比较
Table 1. Time-consuming comparison of the two algorithms
解包裹算法 平均耗时/ms 加速比 低阶像差 原算法 344.1 1.52 改进算法 226.6 高阶像差 原算法 348.4 1.53 改进算法 227.8 
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