An apparatus comprises: a contour extraction unit configured to extract contours of color regions, each of which includes continuous pixels having the same pixel value, in image data; an intersection detection unit configured to detect, as an intersection, a point in each of contours extracted by the contour extraction unit; a division boundary line storage unit configured to store, in association with a contour in which the intersection is detected by the intersection detection unit, contours divided at the intersection as boundary lines; a non-division boundary line storage unit configured to store, in association with a contour in which the intersection is not detected by the intersection detection unit, the contour as a boundary line without dividing the contour; a function approximation unit configured to execute function approximation for each of boundary lines stored by the division boundary line storage unit and the non-division boundary line storage unit.