An image forming apparatus having a recording head with a plurality of printing elements comprising: a first quantization unit that performs a first quantization process on input image data to calculate N-level data by using a screen parameter corresponding to a screen condition of a target device; a conversion unit that converts the N-level data to M-level data (M>N); a configuring unit that configures printing density for each of the printing elements for each scan, using the M-level data; a second quantization unit that performs a second quantization process on the printing density for the scan of interest, using constraining information for the scan of interest; and a generating unit that generates the constraining information for the scan of interest, from the printing density configured for the preceding scan and the result of the second quantization process for the preceding scan.