# An Overview on the Latest Nature-Inspired and Metaheuristics-Based Image Registration Algorithms
## 5 Analysis and Discussion  
We can see how the number of publications facing the IR problem by means of NI&M keeps raising in the last decade with more than one hundred and fifty in total (see Figure 3). Nevertheless, what about the quality of such amount of contributions? The overview carried out in this contribution can be considered a comprehensive one, in a way that several aspects covering broadly the quality of the proposal can be followed in order to measure this parameter. We schematized the (around 16 percent) twenty-four contributions revised in Table 1 according to relevant design parameters which can be considered for quality measurement, such as coding scheme of solutions (i.e., real, integer, or binary), search space (i.e., parameters vs. matching), and tuning of control parameters (i.e., hand-made vs. self-adaptive), among others. Specifically, the first column in Table 1 identifies the publication year of the algorithm. Next, the second column refers to the coding scheme of solutions, i.e., R (real coding), B (binary coding), and I (integer coding). The third column concerns the IR approach followed (see Section 2): P (parameter-based) and M (matching-based). The NI&M-based optimization technique considered is shown in the fourth column. Moreover, the next two columns are devoted to revealing both those methods recently addressing the optimization problem from a multi-objective view-point and also considering self-tuning of the control parameters. According to the latter, S stands for those methods using a self-tuning approach, and M the canonical hand-made tuning one. Finally, the main application, the image modality, and the computer architecture (S and P stands for serial and parallel, respectively) are comprised in columns seven, eight, and nine, respectively. Figure 4 depicts the statistics regarding the level of impact caused by a subset of these quality parameters extracted from Table 1, in which \(QP\#\)? Refers to the level of usage of the real-coding scheme (\(QP\#1\)), the parameters-based search space (\(QP\#2\)), the multi-objective approach (\(QP\#3\)), the self-tuning strategy (\(QP\#4\)), and the parallel computing paradigm (\(QP\#5\)). The considered subset (\(QP\#1\) to \(QP\#5\)) allows us to provide a more comprehensive analysis of the maturity of the field, as well as how healthy it has become as we argue next. It has been proven by the early NI&M-based IR methods that the usage of real-coding schemes provided a better performance [3], and this trend can also be shown in the last decade due to more than ninety percent (see \(QP\#1\) in Figure 4) of the overviewed contributions that make use of this kind of coding scheme (Other contributions also echoing this fact [71; 72; 73]). Only two of them did not adopt such design. In similar manner, it is remarkable the preference (87.5%) for the usage of the search space based on parameters (\(QP\#2\)). The latter fact confirms the previous results shown in [35], in which those methods using such approach outperformed the results of those based on the point-matching scheme. Moreover, the last decade is also distinguished by the increased diversity of NI&M-based algorithms proposed for addressing the IR problem. This is a feature which exhibits the good health of the field. Moreover, several of the selected contributions provided extensive experiments comparing several of the best NI&M-based IR methods in the SoTA, which brings the opportunity of deepen on the enhancement of both the understanding of the behavior of the new NI&M-based optimization techniques addressing the IR problem and the performance of the best methods in the field. Additionally, it is clear that the number of contributions using novel designs based on self-tuning of the control parameters (\(QP\#4\)) represent the twenty-nine percent of the methods, which depicts the rise in the interest in adding this feature. Specifically, two of the best methods dealing with the 3D reconstruction of range images made use of this particular design [15; 49].