Patent ID: 8480200

Claim:
A method of processing image data for an image forming apparatus for forming an image on a recording medium using a recording head including multiple nozzles for jetting ink droplets, the method comprising: a conversion step of converting multilevel data of the image into a dot pattern using a multilevel error diffusion process supporting n gradation levels (n≧2), wherein the conversion step includes the steps of: determining a pixel corresponding to an abnormal nozzle incapable of correctly jetting droplets onto the recording medium or incapable of jetting droplets of a particular droplet size based on abnormal nozzle information provided for each of droplet sizes supported by the nozzles; preventing jetting of a droplet onto the determined pixel corresponding to the abnormal nozzle; distributing a quantization error of the determined pixel calculated in the multilevel error diffusion process to neighboring pixels; and if extra-large droplets with a droplet size greater than a droplet size of a full-size droplet capable of filling a pixel are to be formed in the neighboring pixels as a result of distributing the quantization error, removing one or more of the extra-large droplets or reducing the droplet size of one or more of the extra-large droplets.