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# Constructing 3D Geometric Fish Models

The geometric fish models are constructed manually using the Alias 3D modeler. We model the shape of any given fish's body in accordance with the shape evident in digitized color pictures of the animal (Fig. ). We employ two juxtaposed NURBS surfaces, one for the left half and the other for the right half of the fish body. The NURBS surfaces are of order 3 (or of degree 4) along both the and axes ( and represent the two axes of the material coordinates of a surface). Each NURBS surface has control points which, when connected, form a surface mesh as shown in Fig. . We will refer to this mesh model as the control-point mesh. The control points must be updated at each display time step such that the geometric fish model moves and deforms in accordance with the underlying physics-based fish model (the coming Section gives the details).

We choose to use a moderate number of control points (u=9; v=21) for all the geometric fish models in order to achieve satisfactory rendering speed while capturing the characteristic shape of different species of fishes. The NURBS surface generated from the control-point mesh of Fig.  is shown in Fig. .

The dorsal and ventral fins are also NURBS surfaces each of which has , control points and is of order 1 along the axis and order 3 along the axis. Note that the lower boundary of the dorsal fin coincides with the upper edge of the fish body and the same relationship holds between the upper boundary of the ventral fin and the lower edge of the fish body. This is achieved by simply making the corresponding control points coincide. The left and right pectoral fins are modeled as polygonal surfaces each with five vertices. Fig.  shows the geometric fish model with dorsal and ventral fins (top) and with all fins (bottom). Finally, Fig.  shows the complete geometric models of four different kinds of fishes patterned after the four kinds of natural fishes shown in Fig. .

Figure: The shaded NURBS surface of the left half of a fish body.

Figure: Control-point mesh fish models.

Next: Obtaining Texture Coordinates Up: Modeling the Form and Appearance Previous: Modeling the Form and Appearance
 Xiaoyuan Tu January 1996