1. A method for tracing edges of an image using hysteresis thresholding, the method comprising: (a) receiving an edge map of the image; (b) scanning a row of the edge map; (c) assigning a label to each edge pixel in the row based at least in part on the presence or absence of a neighboring edge pixel, wherein the label having a minimum value among all assigned numbers is reserved for the strong-edge pixels; (d) repeating steps (b) and (c) for each row in the edge map; (e) grouping contiguous labels; and (f) identifying groups belonging to strong-edge pixels.