Image forming apparatus and control method thereof

A method for controlling an image forming apparatus having a recording head with a plurality of printing elements, wherein the recording head scans the same location on a print medium repeatedly in order to form an image, comprising:

• a first quantization step of performing a first quantization process on input image data to calculate N-level data (where N is an integer of at least 2) by using a screen parameter corresponding to a screen condition of a target device;
• a conversion step of converting the N-level data to M-level data (M>N);
• a configuring step of configuring printing density for each of the printing elements for each scan, using the M-level data;
• a second quantization step of performing a second quantization process on the printing density for the scan of interest, using constraining information for the scan of interest;
• and
• a generating step of generating 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.