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; an image processor to determine whether the bitmap data comprises a transparency pattern which hands over color information of an object whose transparency has been set, as a pattern format, to the image processor, and when it is determined that the bitmap data comprises the transparency pattern, to perform smoothing processing with respect to the bitmap data; and an output unit to output the bitmap data processed by the image processor, characterized in that 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 determiner to determine whether a corresponding pixel area comprises a 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. 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; an image processor to determine whether the bitmap data comprises a transparency pattern which hands over color information of an object whose transparency has been set, as a pattern format, to the image processor, and when it is determined that the bitmap data comprises the transparency pattern, to perform smoothing processing with respect to the bitmap data; and an output unit to output the bitmap data processed by the image processor, characterized in that 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 determiner to determine whether a corresponding pixel area comprises a 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. 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 determiner to determine whether a corresponding pixel area comprises a 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.