Image processing

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In this chpater, image processing means detecting and discriminating the location of the lights on the dock and estimating relative position and distance between the AUV and the dock.

Figure. 16 shows a developed dock and the arrangement of the lights in the entrance of the dock. The diameter of the rim was 1m. Five lights were installed in the circular rim. The locations and brightness of the lights were adjustable. Before the image processing, it was necessary to adjust the intensity of the lights. If the lights is too strong, two or more lights may be misidentified as one light because of scattering. Proper intensity was determined through trial-and-error.

Figure 17 shows the coordinates of the vision system. The origin of the camera coordinate is at the center of the nose. The image coordinate is coincident with the camera coordinate except for the x-directional shift f (focus length).

The processing consists of 4 stages. (1) Image grabbing, (2) Binarization of the grabbed images, (3) Elimination of noisy luminaries and discrimination of the dock lights and (4) Estimation of the position and a distance of the dock center.

Stage (1) Raw images were grabbed. Lights were scattered, and this scattering made finding the exact positions of the light sources difficult.

Stage (2) To discriminate the lights installed around the dock entrance, the image processing unit classified each pixel of the raw images into two groups (a bright group and a dark group) using a pre-specified threshold value. Namely, the grabbed image was converted into a binary image. The lights of the dock were classified as the bright group, and the background became the dark group. In this process, salt-and-pepper noise was produced. Salt-and-pepper noise is the presence of single dark pixels in bright regions, or single bright pixels in dark regions. This is the natural result of creating a binary image via thresholding (Shapiro, 2001). This noise was removed using 3pixel by 3pixel masks. Appropriate size for the mask was determined by trial-and-error. After a pixel was classified in the bright group, the distance between that pixel and the nearest white pixel was investigated to identify each of the lights. If the distance was shorter than a pre-specified range, it was supposed that those two pixels belonged to one light region. If not, it was supposed that those two pixels belonged to two separate lights

Stage (3) Underwater, there were noisy luminaries that had to be eliminated. The luminaries are shown in Fig. 18. Some of the luminaries emitted light with an intensity similar to that of the dock lights. In such cases, ISiMI could be confused. Two particular problems were the presence of several lamps outside of the basin, and the fact that the dock lights were reflected back down from the surface of the water. Because these luminaries were interfered with the upper portion of the dock, they could be eliminated by processing the image frame from the lower-right pixel to the upper-left pixel. The processing was ended after five bright regions were acquired. All of the five lights had to be detected, i.e. the image processing required detection of all five lights. If one or more lights could not be detected, the image processing would fail. This was a defect of the developed processing. Stage (2) and Stage (3) were executed simultaneously. Due to Stage (2) and Stage (3), the dock lights were identified. Once the five dock lights were detected, a local searching area was generated and the image processor considered only this area in the next step.

Stage (4) From positions of the lights identified in the image coordinates, ISiMI estimated the position and the distance to the dock center. The center of the dock was calculated by averaging the coordinates of the five lights. To estimate the distance, the number of white pixels in Light #5 was counted (Fig. 16). Because Light #5 was located relatively far from others, interference from scattering was relatively small. The distance information was used as one threshold to decide whether vision-guidance was valid or not. If the coordinate of the n-th light is Pn then the center of the dock is given by

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