1. An image processing method in an image processing apparatus for converting a document image including a gradation image into vector data, the method comprising: a dividing of region step of dividing the document image into regions; a color change direction detecting step of detecting color change directions of pixels in each of the regions; a gradation type determining step of determining gradation types of the regions based on the color change directions detected in the color change direction detecting step; and a description step of describing the regions using a graphic language in accordance with the determined gradation types.