1. An image processing apparatus which performs reduction processing for rendering data in accordance with a reduction ratio, the apparatus comprising at least one processor which functions as: an edge extraction unit configured to extract edges from a contour of an object in the rendering data; a saved line specification unit configured to specify, out of lines including the extracted edges, lines each having an outer shape feature of the object as saved lines which are lines not subjected to thinning processing; and a thinning processing unit configured to determine lines out of lines other than the saved lines in the object and thin out the determined lines, thereby performing the reduction processing for the rendering data, wherein the saved line specification unit specifies saved lines by determining the outer shape feature of the object in accordance with characteristics of the extracted edges.