# Research on a Measurement Method for the Ocean Wave Field Based on Stereo Vision
## 4 Discussion  
Based on the results that were achieved by utilizing stereo vision in wave-height measurement, the following conclusions have been drawn:  
1. Relying on the actual sea trial measurement results, it can be confirmed that the stereo vision wave measurement method can enable the accurate measurement of wave parameters (wave height, period, and wave direction) [30]. This is because the stereo vision technology uses the parallax formula to calculate the distance to the measured object, while the height of the object can be measured through corresponding coordinate conversion. Through the calculation of the elevation value of a fixed area, the wave height and the period of the sea wave can be analyzed. Furthermore, the wave direction can be detected using the normal vector of the wave crest line in the image, therefore it is necessary to recognize and fit the line features. 2. In order to realize a real-time operation of the whole process algorithm, the richness of the wave field's features, and the accuracy of the measurement results, it is necessary  
Figure 13: Illustration: (**a**) wave field 3D reconstruction of the first frame image; (**b**) wave field 3D reconstruction of the second frame image; (**c**) wave field 3D reconstruction of the third frame image; (**d**) wave field 3D reconstruction of the fourth frame image. to use a high-resolution camera for the measurement. However, this results in a too long matching time and very high computer memory costs. In this regard, the gridding siftGPU algorithm that is proposed in this paper considers that the common area of the left and right images can be gridded without down-sampling and the number of feature pairs can be matched using GPU multithreading processing, hence the matching is faster. At present, radar is used for onboard wave measurements, however radar measurement is expensive and it has a low level of accuracy [31]. The fixed-point measurement methods mainly include ADCP and the wave height meter, both of which have high requirements for the water's depth and they are also troublesome to place and recover. ADCP needs to be installed on the ground, while the wave height meter requires mooring line fixation [32]. In addition, both of these methods can only realize single point measurement and the measurement results will degrade due to the process of wave breaking under strong wave conditions. In contrast, stereo vision wave measurement technology can realize a non-contact and high-precision measurement. The main limitation of the approach that is presented in this paper is the high requirements of computer memory and GPU. Besides this, the internal and external parameters of the camera need to be calibrated manually before the measurement is undertaken. The calibration parameters also need to be kept unchanged throughout the actual application. Binocular stereo wave measurement technology is a promising field, nevertheless there are still some problems that need to be solved in order to fully enable this technology for future engineering application [33]. These include the system's motion compensation and applicability to complex lighting conditions (rainy and foggy days). The pre-calibration part can form a standard library for the internal and external parameters of the camera, which can be called directly in the practical application, so as to adapt to different measurement distances. Under complex lighting conditions, the illumination adaptability of the algorithm needs to be further studied. The relationship between the illumination intensity and the best aperture and focal lengths can be trained by deep learning in order to realize all-weather automatic focusing [34]. The system needs to consider the motion compensation algorithm and hardware for the motion of the platform. Taking a ship as an example, rolling and surge will not affect the measurement accuracy of the system and the heave direction needs to be compensated for by the integration of the results of the acceleration measurement, such as rolling and pitch.