1. An image forming apparatus for forming a color image by using a plurality of color component materials, comprising: an area setting unit configured to set a reference area around a pixel of interest in an image to be processed; a determination unit configured to determine whether a pixel not to be referred exists in the reference area; a transformation unit configured to, if the pixel not to be referred exists, transform the shape of the reference area based on the pixel not to be referred; a decision unit configured to specify at least a first group and a second group of peripheral pixels, each of which has a color component that corresponds to one of a plurality of color component materials and that is not included in the pixel of interest, around the pixel of interest included in the reference area with the transformed shape, and configured to decide a trapping color for the pixel of interest based on a single peripheral pixel determined by checking distances from the pixel of interest to the first and second groups of peripheral pixels; a density calculation unit configured to calculate a density of the trapping color based on at least the first or second groups of peripheral pixels; a correction unit configured to correct a pixel value of the pixel of interest using the trapping color having the calculated density in the image to be processed; and an image forming unit configured to form an image having pixels corrected by the correction unit, wherein the area setting unit sets a first reference area around the pixel of interest, and wherein, in a first case where the pixel of interest is included in a character or thin line, and in a second case where the pixel of interest is not included in the character or thin line but the first reference area includes the character or thin line, the area setting unit determines whether or not to change the reference area from the first reference area to a second reference area smaller than the first reference area for making a trap width smaller, and changes the reference area to the second reference area based on the determination results.