Three-Dimensional Measurement of Objects in Water by Using

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Three-Dimensional Measurement of Objects in Water by Using Space Encoding Method Ryohei Kawai, Atsushi Yamashita and Toru Kaneko

Abstract—In this paper, a new method for 3-D measurement of objects in water is proposed. When observing objects in water through a camera contained in a waterproof housing or observing objects in an aquarium tank filled with preserving liquid, we should solve a problem of light refraction at the boundary surfaces of refractive index discontinuity which gives image distortion. The proposed method uses a space encoding method which does not have a problem of corresponding point detection as a stereo vision system has, and is faster than spot light projection or slit light projection methods. A ray tracing technique solves the problem of image distortion caused by refractive index discontinuity. It should be noted that monochromatic light projection onto objects gives more accurate measurement than white light projection because the refractive index depends on the wavelength of the light. Then, in order to measure colored objects, we should project red, green and blue light patterns onto them separately. Experimental results show the validity of the proposed method.

water surface

(a) Image without water. (b) Image with water. Fig. 1 Influence of light refraction.

I. INTRODUCTION Underwater imagery is essential to marine environment investigation, and three-dimensional measurement of objects in water gives important information [1][2]. This technique is also available when measuring the shape of specimens in a transparent container filled with preserving liquid. Photographic 3-D measurement methods have the advantage that it is possible to measure objects without contact, and they are well used in atmospheric environments. However, measurement of an object in water by these methods has a problem of light refraction [3][4]. Figure 1 shows that an object in a jar filled with air (Fig. 1(a)) and an object in a jar filled half with water (Fig. 1(b)) look different. Accurate results can not be obtained from the measurement using the distorted image without considering the influence of light refraction. This problem occurs not only when the object in a container filled with water is observed by using the camera outside of water but also when the camera is put into water with waterproof housing. This is because the housing case for waterproof is filled with air and light refraction occurs at the boundary of air and water.

This work was in part supported by the Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research (C), 19560422. R. Kawai., A. Yamashita. and T. Kaneko are with Department of Mechanical Engineering, Shizuoka University, 3–5–1 Johoku, Naka-ku, Hamamatsu-shi, Shizuoka 432–8561, Japan (phone: 053-478-1604; fax: 053-478-1604; e-mail: {f0830022, tayamas, tmtkane}@ipc.shizuoka.ac.jp)

Ray tracing solves the problem of light refraction. Accurate results considering the influence of light refraction can be obtained by calculating the direction of rays from the camera before and after light refraction. Methods for 3-D measurement of objects in water with camera are possible to measure objects which are relatively near to the camera more accurately than methods with supersonic waves [5][6] do, and there have been proposed a variety of methods such as using a stereo camera system [7]-[9], using a motion stereo system [10] and using spot light projection [11]-[13] or slit light projection [14]. However, the methods using a stereo camera system have a problem that it is difficult to detect the corresponding points when the object does not have a texture nor a feature point, and the methods using a motion stereo system have a problem that the relationship between the camera and the surface of the water is difficult to estimate accurately because the camera moves. The methods using spot light projection or slit light projection also have a...
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