A wavelet transform based contrast enhancement method for underwater acoustic images

Dredging the surface of the ocean to identify both living and non living things nowadays has become an unproblematic task with the help of the acoustic instruments. Side scan sonar is one of such instruments used for far-reaching the seafloor. The sonar captures the scene of the sea bed by releasing fan shaped sound signal which is then converted to images. These images are normally gray scale low contrast images where the objects cannot be viewed clearly. The proposed method uses the Stationary Wavelet Transform (SWT) to decompose the input image into four components such as Low–Low, Low–High, High–Low and High–High components. The low frequency component is sharpened using Laplacian filter and a mask is created by subtracting the LL component with the filtered image. Then the enhanced LL component is obtained by adding the mask to the input image. The high contrast image is reconstructed by applying inverse stationary wavelet transform which combines the enhanced LL component and the other sub-bands. The results have been compared by replacing the SWT with the Discrete Wavelet Transform by interpolating the frequency components. The quantitative and visual results show that the proposed method using SWT outperforms the state of art techniques in terms of contrast.