Patent ID: 7564588

Claim:
An image forming device in which each pixel of an M-level gradation input image is quantized into one of N levels (M>N≧2) through an error diffusion process, comprising: a threshold determination unit determining a threshold value according to a gradation value of a target pixel of the M-level gradation input image before quantization; a specific concentration determination unit determining whether the gradation value of the target pixel is equal to a specific concentration which indicates one of the N levels after quantization; a mask addition unit adding a mask noise value to the gradation value of the target pixel when the gradation value of the target pixel is equal to the specific concentration before quantization, wherein the mask noise value is determined according to a position of the target pixel; and a quantization unit performing the quantization through the error diffusion process by using the threshold value and the gradation value of the target pixel to which the mask noise value is added.