The epipolar plane is tangent to the curved surface at the frontier point in 3D space, and the epipolar line is tangent to the apparent contour at the projection of the frontier point in images (see Fig. 2). These properties of epipolar geometry on curved surfaces are called epipolar tangency [10].
The epipolar tangency is important for finding the frontier points and the epipolar geometry from images. That is, if the epipolar geometry is known, the frontier point can be extracted as a tangent from an epipole to the apparent contour. If the frontier points are known, the epipolar lines and epipoles are computed from tangent lines at the frontier points on the apparent contours. However, if we do not know both frontier points and the epipolar geometry, it is a chicken and the egg problem.