1. An image processing apparatus comprising: designating means for designating a plurality of non-continuous points for specifying a desired area of an original document, each of the plurality of non-continuous points indicating a corner of the desired area, the desired area being a polygon of which at least one side is parallel to neither a main scan direction nor a sub-scan direction; dividing means for dividing an outline of the desired area defined by connecting the plurality of non-continuous points designated by said designating means into a plurality of line segments, two adjacent line segments being capable of making an arbitrary angle; memory means for storing line segment data indicative of a position and a slope of each of said line segments; area data generating means for calculating, in units of one or more main scan lines, area data indicative of the desired area defined by connecting the plurality of non-continuous points on the basis of the line segment data read out from said memory means; and processing means for performing mutually different processing for an image within the desired area of the original document and an image outside the desired area in accordance with the data calculated by said area data generating means.