# Line Matching Based on Viewpoint-Invariance for Stereo Wide-Baseline Aerial Images
## 4 Experiment and Analysis
For convenience of expression, the line matching method performed directly on the original images is called LJL, whereas the method on the rectified images proposed in this paper is called Perspective-LJL. Relevant parameters and line matching results are shown in Figure 8:0  
For depicting of the difference between LJL and Perspective-LJL intuitively, the correct number, the total number, and the accuracy rate of matched line segments (Table 3 are shown in Figure 15). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Data** & **Method** & \begin{tabular}{c} **Extracted Line** \\ **Segments** \\ \end{tabular} & \
\begin{tabular}{c} **Constructed LJL** \\ **Structures** \\ \end{tabular} &
\begin{tabular}{c} **Matched Line Segments** \\ **(Correct/Total/Accuracy)** \\ \end{tabular} \\ \hline \multirow{2}{*}{Image pair 1} & LJL & 1459,1459 & 1323,1251 & 117/186/62.9\% \\  & Perspective–LJL & 1369,1369 & 1274,1121 & 238/301/79.1\% \\ \hline \multirow{2}{*}{Image pair 2} & LJL & 2795,2310 & 2800,1912 & 130/166/78.3\% \\  & Perspective–LJL & 2640,2143 & 2636,2025 & 520/558/93.2\% \\ \hline \multirow{2}{*}{Image pair 3} & LJL & 2825,2570 & 3221,2046 & 0/14/0\% \\  & Perspective–LJL & 2461,2138 & 2877,1760 & 345/397/86.9\% \\ \hline \multirow{2}{*}{Image pair 4} & LJL & 2825,2879 & 3221,3196 & 2/15/13.3\% \\  & Perspective–LJL & 2461,2779 & 2877,3038 & 332/374/88.8\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The comparison of line matching amongst the four pairs of original and rectified images. Figure 14: The matched line segments after back-projection to the original images. (**a**) The matched result on the original image pair 1; (**b**) The matched result on the original image pair 2; (**c**) The matched result on the original image pair 3; (**d**) The matched result on the original image pair 4. While analyzing and summarizing the data in Table 3 and Figure 15, the following discoveries are made:  
1. There is a large gap between the matching results of the four oblique image pairs with respect to the matching results of conventional images (Table 1). The matching accuracy rate is far less than 97%. There are thousands of lines extracted from the two pairs of down-looking and back-looking images; however, only a small proportion of lines are matched. The number of matched line segments is 186 and 166, whereas the accuracy rate is only 62.9% and 79.1%, respectively. Especially in the last two pairs of side-looking images, the baselines are wider and the difference in viewing angles is larger; therefore, the matched line segments are fewer in number, and the accuracy rate is 0% and 13.3%. The data fully reflects the matching difficulty of images with wide baselines and large viewing angles. 2. The number of line segments extracted from rectified images is less than that of original images. Because rectified images are resampled and their quality is lower than that of original images, the extraction effect will be worse. At the same time, their effective trapezoidal contents are smaller than the original images' rectangular contents. For a line extraction algorithm such as LSD, the line segments smaller than a certain length threshold is not used (the threshold value in this algorithm is set to 20 pixels), so some line segments that are compressed to be shorter are discarded. 3. The number of LJL structures is directly proportional to the number of extracted line segments; that is, the more line segments are extracted, the more LJL structures can be constructed and the greater probability of matched line segments can be obtained. Although the number of line segments and the LJL structures of rectified images is smaller than that of the original images,  
Figure 15: Comparison of the correct number, the total number, and the accuracy rate of the matched line segments for four image pairs. (**a**) the number of correct matched line segments; (**b**) the number of total matched line segments; (**c**) the accuracy of line matching.