1. An image processing apparatus comprising: a classifying unit constructed to classify pixels whose values are the same or similar in an input image into a same group, and classify pixels whose values are neither same nor similar in the input image into different groups; a color gradient information calculation unit constructed to calculate color gradient information at pixels of a boundary between groups classified as different groups, wherein the color gradient information calculation unit does not calculate color gradient information at pixels except the boundary; an attribute determination unit constructed to determine a gradation attribute representing a characteristic of a color gradient at the boundary using the calculated color gradient information; a gradation cluster information generation unit constructed 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 constructed 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 constructed to integrate color regions belonging to the same gradation using the gradation cluster information; and a contour vector description generation unit constructed 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.