Video-based Automatic Extraction Algorithm Of Whitecap Coverage

Wave breaking is a common phenomenon occurring on the sea surface,which affects the height of the surface wave,speeds up the air-sea flux transfer,generates the sea salt aerosols and produces bubbles changing sea surface optical properties.It has important significance in many aspects such as the sea-air interaction and marine remote sensing.Ocean whitecap is the direct expression of the wave breaking.As its statistical parameter,whitecap coverage can effectively reflect the changes of sea condition.Applying the theoretical basis of wave breaking and sea-air exchange model,with the technical methods of image processing and video analysis,this paper studies the automatic identification algorithm of whitecaps.The main research contents and innovations of this paper are summarized as follows:1.According to the observation requirements of whitecaps,the shipboard wind-wave observation system was designed and constructed.In order to further improve the accuracy of the measurement,calibration experiment was carried out on the installed camera,and then the internal reference matrix which can restore images was obtained.In this paper,the effective area of the sea surface was designed and verified so that the interference factors in the near or far field were excluded.2.According to the characteristics of video taken on the ship,we can obtain the rolling and pitching of the ship by detecting the angle of the sea-sky-line instead of the inertia device.So we propose a sea-sky-line recognition algorithm based on two-layer structure to eliminate the influence of wave's boundary which realizes the extraction of sea-sky-line's angle in the complex sea surface conditions.We can compute camera parameter matrix by the sea-skyline's angle and then restore the sea surface image in the world coordinates in order to reduce the error of whitecap coverage extraction.Aimed on uneven illumination of sea surface image,it is proposed a kind of adaptive illumination correction algorithm using top-hat transform to eliminate negative impact by sunshine reflection and using image enhancement technology to make calculation operation stable.3.According to the features of whitecap pixels,we proposed a region-based automatic whitecap extraction algorithm named RAWE.We apply super pixel segmentation algorithm,simple linear iterative clustering,as the first step,and then use the Otsu algorithm in the area of over-segmentation and under-segmentation as the secondary step.Our algorithm not only focuses on pixel brightness,but also considers the spatial relation of pixels.So it can realize the recognition for different scales of whitecap pixels.In this paper,it is analyzed in detail that the recognition algorithm and application of whitecap coverage.An effective method,which applies image correction,image preprocessing,SLIC super pixel segmentation and Otsu algorithm to realize the automatic calculation of whitecap coverage under complex sea surface conditions,is designed for automatic extraction of whitecaps based on the video of wave observation system.Whitecap coverage can be actually measured by our algorithm in marine observation and has the higher accuracy and applicability through the digital image technology.