According to paper: “Segmentation and tracking of live cells in phase-contrast images using DGVF for snakes About We report a simple algorithm for automated cell segmentation in high magnification phase-contrast images, which takes advantage of the characteristic directionality of the local image intensity gradient at cellular boundaries due to the "halo-effect". We employ a two-step algorithm in which a gradient vector flow (GVF) field is first used to direct active contours to an approximate cell boundary.  A directional gradient vector flow (DGVF) field is then calculated by considering only edges for which the image intensity gradient is directed outward with respect to the approximate cell contour. Subsequently, the DGVF field is used to refine the cell contour, by directing active contours to edges with the desired gradient directionality. This method allows  accurate segmentation of cells in an image series, as well as tracking the dynamics of cell shape over time in a time-lapse movie in an automated fashion.