Mosaicking of the ocean floor in the presence of three-dimensional occlusions in visual and side-scan sonar images

We describe how an autonomous underwater vehicle (AUV) engaged in the task of mosaicking an unknown region of the ocean floor recognizes "essential" occlusions in its sensor image. The sensor on board the AUV is either a downward pointing camera or a side-scan sonar. Essential occlusions, which are created by irregularities in bathymetry, force the AUV to make diversions from its regular mosaicking path in order to prevent gaps in the mosaic. We classify features on the ocean floor which give rise to visual occlusions into three categories: knolls, furrows and dimples. Geometric conditions are derived that enable the AUV's camera to distinguish between these occlusions. We show that only furrows and dimples give rise to essential occlusions. We discuss extensions of the geometric conditions to shadows in side-scan records. There, however, the conditions are not as straightforward, and their implementation constitutes work in progress. An example is provided, which exhibits the modifications required in a visual mosaicking procedure in a complex environment.