Method and device for adsorbing line segment in image, as well as method and device for constructing polygon

The invention discloses a method for adsorbing a line segment in an image. The method comprises the following steps of presetting a line segment, selecting a first part of the line segment to drag the line segment; and when dragging a second part of the line segment near a corresponding second part of a certain line segment in the image, setting the preset line segment to be superposed with the certain line segment, wherein the distance between the second part of the preset line segment and the certain line segment matches a preset rule. According to the technical scheme of the invention, the method can conveniently help a user define side frames of a polygon according to existed line segments in the image so as to conveniently carry out the following image processing.