US Patent 7502140 Image processing apparatus

An error adding step adds together input multi-tone image data and peripheral error information. An adding/subtracting step adds to or subtracts from multi-tone image data calculated by the error adding step, a random number in a matrix prepared for each set of pixels of the input multi-tone image data. A quantizing step quantizes multi-tone image data calculated by the adding/subtracting step into image data, the number of bits of which image data being smaller than the number of bits of the multi-tone image data calculated by the adding/subtracting step. An error calculating step calculates an error based on the multi-tone image data calculated by the error adding step and the image data quantized by the quantizing step. An error storing step stores the error calculated by the error calculating step for each peripheral pixel. An error operation step calculates peripheral error information based on the error stored by the error storing step for each peripheral pixel, and provides the peripheral error information to the error adding step.