1. An image forming apparatus, comprising: an input unit to receive printing data; a rendering unit to transform the printing data into bitmap data; a storage unit to store a plurality of pieces of preset transparency pattern information in a lookup table (LUT); an image processor to compare the bitmap data with a preset transparency pattern having a plurality of different preset transparency rates to represent various amounts of transparency, and if it is determined that the bitmap data comprises a transparency pattern matching the preset transparency pattern having one of the preset transparency rates, to perform smoothing processing with respect to the bitmap data at the one of the preset transparency rates; and an output unit to output the bitmap data processed by the image processor, wherein the image processor comprises: a transparency pattern matcher to respectively compare pattern formats of predetermined pixel areas of the bitmap data with the plurality of pieces of preset transparency pattern information; a transparency pattern determiner to determine whether a corresponding pixel area comprises the transparency pattern matching the preset transparency pattern among the plurality of pieces of preset transparency pattern information, based on the comparison result; and a smoothing processor to perform the smoothing processing with respect to the corresponding pixel area which has been determined that it comprises the preset transparency pattern.