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Detecting Borderline between Two Points (x1, y1) and (x2, y2). First find the most possible border point in two dark regions. Then, treat the new point and the left old one as same as (x1, y1) and (x2, y2), and find the next border point. Repeat this procedure until the distance between two points is less than 2s. Connecting all points orderly, we obtain the borderline.
