Identification of Underwater Man- Made Object Using Colour

With the development of autonomous underwater vehicles, the identification of underwater objects continues as a major issue. Because of specific properties of light in water particularly the absorption phenomenon, underwater imaging suffers from low contrast, from diminished colours and often from prominent green or blue colour. Today, object identification algorithms are usually based on shape, and the colour is rarely used for underwater operations contrary to terrestrial ones. In this paper we present a new method for underwater object identification exclusively based on colour. The proposed method follows the Beer-Lambert law which describes the exponential decrease of the light intensity with the distance. According to this law, we can assume that each colour has a set of compatible colours depending of the travelling distance of the light through the water. We show that in the logarithm domain these colours belong to a line, we called " attenuation line ". Our algorithm is based on this observation. The segmentation step, splits the image into two classes: the pixels whose colours are compatible with the object and the others. In practice we compute a PCA (principal components analysis) on pieces of pixels (of underwater images) which belong to the object in order to approximate the direction of this attenuation line. Then, each pixel is projected in the 2D plan perpendicular to this line and is labelled as a pixel of the object class or not. In this plan, for any other image of the sequence we decide if the object is present or not, given the number of its projected pixels which fall in the previous class. 1 INTRODUCTION 1.1 Colour perception of an object The colour perception of an object depends on physical, physiological and psychological components. We will just define physical components because the latter two are properties of the visual system of the observer. The first component is the spectral composition of the light which illuminates the coloured body. A light source can be characterised by its spectral distribution 1 and it is the spectrum of light reaching the object which determines the colour reflected. The same object will appear with different colours whether it is illuminates by the sun, an incandescent lamp, or a spot of green light for example. The second component is the spectral reflectance of the object 2. Indeed, the perception of colour depends on the special characteristics of the object itself in particular properties …