The present invention accurately reproduces an image pattern by referring
to neighboring pixels upon determining the threshold value of a target
pixel so that a delay in dot generation can be prevented while
emphasizing an image transition portion so that both graininess and
sharpness are improved in the image. To this end, a neighboring pixel
density difference detection part obtains an absolute value of the
density difference between a target pixel and its neighboring pixels and
selects the density of a neighboring pixel with a large absolute value as
a reference pixel density. Further, a threshold value quantity
determination part determines a threshold value corresponding to the
density selected by the neighboring pixel density difference detection
part. Then, a quantization part outputs an output value based on a
comparison between the threshold value and a corrected value obtained by
adding a pre-calculated error value of neighboring pixels to an input
value.