An apparatus having an input for inputting a plurality of line drawings indicating changed shapes of a stroke and a quantizer for quantizing corresponding strokes between the plural line drawings input by the input into unit vectors each having a predetermined length. A portion between the plural line drawings is interpolated by obtaining an interpolating vector for interpolating a portion between the corresponding unit vectors in accordance with an amount of change between the corresponding unit vectors of the plural line drawings obtained by the quantization performed by the quantizer.