The automatic measurement of fiber orientation in composite materials is usually performed using imaging techniques on multiple parallel sections. Measurements on a single section are ambiguous because one can not distinguish two fibers which are symmetric regarding the section plane. We propose to estimate orientations on a single section without ambiguity, using simple a priori information about material structure. The method applies to anisotropic materials with several main fiber directions and with cylindrical fibers bundled in threads. A thread-based labeling algorithm has been developed which allows to get rid of the direction ambiguity for each fiber and to estimate the directional fiber volume ratio.