1. An image processing apparatus comprising: a division unit configured to divide an input image into a plurality of color regions based on a color difference; a color gradient information calculation unit configured to calculate color gradient information at a boundary between the divided color regions from color information of the input image; an attribute determination unit configured to determine a gradation attribute representing a characteristic of a color gradient at the boundary using the color gradient information; a gradation cluster information generation unit configured to generate a gradation cluster by determining color regions belonging to the same gradation among the plurality of color regions using the gradation attribute of the boundary, and to generate gradation cluster information including information about a color region belonging to the generated gradation cluster and a gradation type of the gradation cluster; a gradation parameter generation unit configured to generate a gradation parameter for each gradation region using the color region, the color gradient information, and the gradation cluster information; a color region integration unit configured to integrate color regions belonging to the same gradation using the gradation cluster information; and a contour vector description generation unit configured to generate a contour vector description based on a color region after integration by said color region integration unit, and to generate vector data of the gradation region based on the contour vector description and the gradation parameter.