1. An image processing apparatus comprising: a contour line obtaining unit configured to obtain a contour line of a similar color region from an input image; a feature-point setting unit configured to set a plurality of feature points based upon the contour line; a pixel-value obtaining unit configured to divide the contour line to produce divided contour lines based on the set feature points to obtain pixel values of plural sampling points inside each of the divided contour lines; and a generating unit configured to generate a vector expression by defining a direction and an attribute of gradation as a daub color of the vector expression for each of the divided contour lines based upon the obtained pixel values of the plural sampling points.