Create a vector field out of wood grain

I'm currently looking at ways of extracting information regarding the specific wood grain structure in standard wood components. What I'm particularly interested in is to create a vector field out of an image that describe the "flow" of the grain (see this image as reference). I'm relatively new to computer vision and I would like to know if you have any suggestion/known way on how to solve this task. Thanks!  The picture is taken from the paper "Automatic measurement of wood fiber orientation and knot detection using an optical system based on heating conduction" by Daval, V et al.