1. A color image forming apparatus 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 to correct registration shifts by performing a subscan direction line shifting according to an amount of registration shifts; a means to decide whether or not to correct grayscale levels for interpolation of less than one pixel of image data, and use a threshold determined by a result of the decision in quantizing multivalued image data; a means to generate a reference window comprising a group of the quantized image data; and a means, when the reference window matches trailing phenomenon prevention patterns, to thin a pixel located at the pixel of interest in the reference window.