PURPOSE:To enable fast processing by selecting candidate picture elements so that the inclination of a straight line connecting two continuous candidate picture elements increases monotonously when it direction is counterclockwise or decreases monotonously when clockwise. CONSTITUTION:When all picture border elements of a figure or picture elements except picture elements which never become envelope points are regarded as candidate picture elements, the candidate picture elements are traced in consideration of the inclinations of straight lines connecting two adjacent candidate picture elements and selected so that the inclination of the straight lines increase monotonously when the direction is counterclockwise or decreases monotonously when clockwise, thereby composing the figure of only the selected candidate picture elements. Consequently, picture elements P3-P6 are ignored at recessed parts P2-P7 of the figure and only P2 and P7 is left, so the figure is deformed into a projection figure.
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