1. An image processing apparatus comprising: a separation unit adapted to determine an attribute of data contained in input document image data and separating the document image data into areas by attributes; an extraction unit adapted to extract, from the separated areas, an area of a graphics image as a target of vectorization processing; an edge pixel count unit adapted to extract an edge strength of each pixel of the area of the graphics image, and count the number of pixels having the edge strength higher than a predetermined first threshold; an identification unit adapted to divide the area of the graphics image into color clusters based on color information of pixels of the area of the graphics image, and identify a background area and an area other than the background area based on the color clusters to which pixels of four surrounding sides belong; a pixel-except-background count unit adapted to count the number of pixels included in the area other than the background area identified by said identification unit; an edge ratio calculation unit adapted to calculate an edge ratio representing a ratio of the number of pixels counted by said edge pixel count unit to the number of pixels included in the area other than the background area counted by said pixel-except-background counting unit; a determination unit adapted to determine that an attribute of the area of the graphics image is a clipart area when the edge ratio is not more than a predetermined second threshold, and is a line drawing area including a line drawing when the edge ratio is more than the predetermined second threshold; and a vector conversion unit adapted to perform vectorization processing corresponding to the attribute of the graphics image based on a determination result of said determination unit.