# Research on a Measurement Method for the Ocean Wave Field Based on Stereo Vision
## 3 Results  
### Laboratory Verification  
The relevant experiment was carried out in the China Shipbuilding Research Center. The experiment used two cameras with a resolution of 6280 \(\times\) 3158 in order to obtain the impact pair (shown in the Figure 3). The image shooting time was 17 January 2022. Moreover, a calibration box was used to check the system algorithm in order to measure its accuracy and the specific parameters are as follows:  
1. Calibration Results  
The calibration results of the internal and external parameters of the camera are in Table 1:  
2. Gridding siftGPU Parameter Calculation  
For the left picture, u\({}_{\text{lmin}}\) = 580 and v\({}_{\text{lmin}}\) = 158; whereas, for the right picture, u\({}_{\text{max}}\) = 5700 and v\({}_{\text{rmax}}\) = 3000. Meanwhile, the gridding matching block size was 300 \(\times\) 300 and the number of matching blocks was 190. Here, we chose the left picture (580, 158) and the right image (0, 0) for verification and the matching results are provided in the following Figure 3.  
Figure 3: Illustration: (a) calibration box verification picture 1; (b) calibration box verification picture 2.  
It can be seen from Figures 4-7 that the matching number of feature points that are based on the gridding siftGPU algorithm is obviously better than that of the points that are based on the siftGPU algorithm. The calculation results are shown in the Table 2.  
By comparison, the matching time of the gridding siftGPU was 1/6 of that of the siftGPU, whereas the match quantity that was based on the gridding siftGPU was 8 times that which was based on siftGPU.  
3. Calibration Results for the Base Plane  
Twenty feature points were selected in image 2 in order to calculate the base plane. The selected area is the ground feature in image 2, which is shown in the Figure 8:  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Code No.** & **siftGPU** & **Gridding siftGPU** \\ \hline Threshold & 0.5 & 0.5 \\ Image 1 matching time (s) & 31.7 & 6 \\ Image 1 matching no. & 3003 & 24,424 \\ Image 2 matching time (s) & 29.9 & 5.8 \\ Image 2 matching no. & 2996 & 24,321 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Matching time and feature points numbers.  
Figure 6: Image 2 matching numbers based on siftGPU.  
Figure 7: Image 2 matching numbers based on gridding siftGPU.  
The code numbers and coordinates of the selected feature points are shown in Table 3.  
The calculation parameters of the sea surface base plane are as follows:  
\[\text{a}=2.3715,\,\text{b}=-1.5122,\,\text{c}=-8428.2437\]  
The calculation parameters of \(\text{R}_{\text{w}}\) and \(\text{T}_{\text{w}}\) are:  
\[\text{R}_{\text{w}}=\begin{bmatrix}-0.9983&-0.059&0\\ -0.042&0.717&-0.696\\ 0.041&-0.695&0.718\end{bmatrix}\]  
\[\text{T}_{\text{w}}=\begin{bmatrix}0\\ 5868.045\\ 6049.904\end{bmatrix}\]  
Figure 8: Illustration: (**a**) sea surface base plane calibration image \(1\); (**b**) sea surface base plane calibration image \(2\).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Code No.** & **ul (mm)** & **vl (mm)** & **ur (mm)** & **vr (mm)** \\ \hline
561 & 462.869 & 1400.559 & 117.233 & 1557.912 \\
523 & 602.897 & 1604.144 & 244.895 & 1763.765 \\
757 & 569.487 & 2124.842 & 181  
Additionally, the scatter diagram is shown in Figure 8.  
As shown in Figure 9, the original 3D point images of image 1 and image 2 are consistent with the original image. The blue points represent the ground features and the yellow features represent the calibration box features. The white area is the unrecognized area, due to the strong light.  
Next, 14 points were selected in order to verify the measurement accuracy of the system. The obtained measurement results were compared with the tape (the minimum scale was 1 mm) and the error was basically controlled within 1%. The results are shown in Tables 4 and 5.