1. An image processing method comprising: performing, by an image processing apparatus, volume rendering of volume data which relate to an object and acquiring volume-rendered image data; acquiring information which relates to a virtual illumination position; determining at least one conversion function from a plurality of conversion functions stored in a database, based on the virtual illumination position which relates to the object; and correcting the acquired image data by using the virtual illumination position and the at least one conversion function, wherein the correcting comprises performing a tone mapping with respect to the acquired image data by using a second conversion function using a difference between coordinates of the virtual illumination position and coordinates of a reference position, and a distribution of the virtual illumination position.