1. An image processing system comprising: an input unit for inputting N-bit image data; a pseudo halftone representation unit for converting the N-bit image data into M-bit image data; a first bit conversion unit for converting the N-bit image data into L-bit image data; a control signal generation unit for generating a control signal from the L-bit image data; a first edge detection unit for detecting a first edge from the control signal generated by the control signal generation unit; a second edge detection unit for detecting a second edge from the control signal generated by the control signal generation unit; a second bit conversion unit for converting the L-bit image data into M-bit image data; a smoothing unit for smoothing the L-bit image data and generating M-bit image data; and an output image data switching unit for switching output image data between the image data output by the smoothing unit, the image data output by the second bit conversion unit, and the image data output by the pseudo halftone representation unit, wherein the output image data depends on the first and second edges detected by the first and second edge detection units.