基于FFT配准和多尺度融合的声纳图像拼接算法

Sonar image mosaic based on FFT registration and multi-scale fusion

针对声纳图像视野窄,无法一次性反映大体积目标或同时呈现若干目标完整视觉信息的问题,提出一种声纳图像拼接算法.该算法主要步骤分为:声纳图像配准和声纳图像融合.根据声纳图像高噪声和低分辨率的特点,采用FFT图像配准方法配准声纳图像.对于准确配准后的声纳图像,提出一种基于NSCT多尺度分解的声纳图像融合算法.首先,对源图像进行NSCT多尺度分解,得到一系列不同尺度和不同方向上子带分解系数;然后,根据声纳图像特殊性,构建声纳图像融合规则:分别对低频子带采用门限判别法,高频子带计算局部区域对比度指导融合规则,产生融合图像在相应尺度和方向的融合系数.最后,对融合系数进行NSCT多尺度逆变换,生成融合后的拼接图像.通过声纳图像拼接实验验证提出方法有效性.

Image mosaic is a technique of image processing that stitches multiple overlapping images in order to produce a high resolution and global view composition.To widen the view field of sonar image,a novel mosaic method is proposed in this paper.The proposed method mainly includes two steps：sonar image registration and sonar image fusion.It is hard to extract the features from and match the correspondences exactly between sonar images,which have high-level noise and low resolution.Therefore,the FFT method,which is independent on the features of image,is used for sonar image registration.After registration,image fusion is applied to improve the quality of the mosaic result.Image fusion can be categorized into two groups：spatial-based methods and multi-scale transform based methods.The spatial-based fusion techniques are likely to produce blocky artefacts due to the complexities of sonar image.Accordingly,the multi-scale transform based method is more suitable to sonar image fusion.In the fusion step,a multi-scale transform based fusion method is proposed using Non-subsampled Contourlet Transform（NSCT）.First,the source images are decomposed into sub-bands of multi-scale and multi-direction by NSCT.Then,the fusion rules are established to deal with the inherent challenging properties of sonar images,such as featureless,high level noiseand low resolution.A threshold-based method is used for merging the coefficients of low-frequency sub-bands while the local contrast is applied to conduct the fusion of high-frequency sub-bands.The fused coefficients of the corresponding sub-bands can be generated by the proposed fusion rules.Finally,the inverse NSCT is implemented on the fused coefficients to generate the fused image.The effectiveness of the proposed method for sonar image mosaic is demonstrated in the experiment.