1. An image processing system comprising: a buffer unit configured to store a predetermined pixel and peripheral pixels neighboring the predetermined pixel in image data in a first direction and a second direction perpendicular to the first direction; a first edge-detecting circuit configured to calculate a maximum value and a minimum value of a set of pixels selected from the peripheral pixels that have the same color as the predetermined pixel to calculate a difference value between the maximum value and the minimum value; an average brightness value calculator configured to calculate an average brightness value for the peripheral pixels; an average peripheral brightness value calculator configured to select a combination of pixels having the three primary colors from among the peripheral pixels and to calculate an average peripheral brightness value for the pixels forming the combination; and a second edge-detecting circuit configured to calculate a sum of absolute values each obtained by subtracting the average brightness value from a corresponding average peripheral brightness value.