| Summary: | International audience In computer vision and image processing, motion estimation is of increasing interest because of the large number of applications: object tracking (military, video-surveillance, robotics), complex behavioral analysis (modeling of human body motions, meteorology), medical analysis (cardiac contraction follow-up, infarction detection) [1][2]. In biology, tracking the motion of animals sometimes poses technical problems, related to the characteristics of species and stages of development. For example, the European glass eel(Anguilla anguilla) has a complex life cycle, with reproduction in the sea of Sargasso, a larval phase that crosses the Atlantic Ocean and a juvenile stage, the glass eel, which goes up the estuaries to grow in the river [3]. To study the estuarine migration of glass eel, it is possible to reproduce the tidal currents in the laboratory and observe the swimming behavior of individuals [4]. The major difficulties concern the animal itself, which is transparent, and moves mainly at night or at very low light intensity. To follow glass eels, each individual is tagged with VIE Tag (Visible Implant Elastomer) [4]. This marking consists in implanting under the skin a tip of colored elastomer, visible under UV. Tracking individuals is done on video recordings but it is a tedious job because currently not automated. The parameters that interest biologists are mainly the motion direction of glass eels (with or against the current) and their velocity. Any measure to assess energy expenditure is also sought, as glass eels do not eat during migration, and their energy status could play an important role in the migration potential. In this work, we have chosen differential methods for their many advantages. These methods are at first robust and precise, while being easy to implement. Because of its differential nature, the optical flow equation also allows a sub-pixellic estimation of the motion [5,6]. The advantages of these methods are: firstly, robustness and precision, the equation ...
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