Beam-Domain Image Mosaic of Forward-Looking Sonar Using Expression Domain Mapping Model

Since the existing forward-looking sonar (FLS) image mosaic algorithm is completed in the image-domain with reference to the optical image, and the characteristics of FLS imaging are not fully considered, obvious edge-seams and pseudo-target values are prone to generate after mosaic, resulting in poor visual effect and low operation efficiency. To avoid the aforementioned problems, the mosaic operation of beam-domain FLS images is proposed. Combined with the imaging principle of FLS, this study takes advantage of the variation of the sampling beams and sampling points of the same target point during sonar movement to analyze the deformation law of the target in beam-domain which is scanned in image-domain, and then proposes the concept of expression domain mapping model (EDMM). Considering the need of aligning acoustic origin, the target is deformed according to this model in the process of beam-domain mosaic, and the target alignment is completed. Subsequently, the Laplace pyramid (LP) layered algorithm is used for fusion and mosaic, and finally mapped to image-domain for display. Experimental results illustrate that this kind of algorithm can remove the mosaic edge-seams generated during mosaicking in the image-domain because it uses the sonar raw image data for mosaic in the beam-domain and avoids the appearance of sector edges in the image-domain; the actual pixel points sampled by sonar are used for mosaic, which effectively eliminates the pseudo-target value generated by the mosaic of the inter-beam interpolation in the image-domain; moreover, the number of image pixels in the beam-domain is small, and the mosaic time consuming of adjacent frames and continuous frames is more than half shorter than that of the image domain, which significantly improves the mosaic efficiency and has good mosaic performance.