1. A color image forming apparatus constructed to form an image by correcting registration shifts during an image forming process by using image data, the color image forming apparatus comprising: a means for correcting registration shifts by performing a subscan direction line shifting according to an amount of registration shifts; a means for deciding whether or not to correct grayscale levels for interpolation of less than one pixel of image data, wherein one of a first threshold and a second threshold is used to quantize multivalued image data based on a result of the decision; a means for generating a reference window comprising a group of the quantized image data; a means for thinning a pixel of the multivalued image data located at a pixel of interest in the reference window, in a case where the reference window matches trailing phenomenon prevention patterns; and a means for correcting, for interpolation of less than one pixel of image data, grayscale levels of the thinned multivalued image data.