An image processing method is disclosed. The image processing method includes setting n reference pixels around a pixel of interest, where n is an integer, sequentially comparing a pixel value of the pixel of interest with a pixel value of each reference pixel, counting a number of the reference pixels whose pixel values are less than or equal to the pixel value of the pixel of interest, incrementing a histogram value of each reference pixel by 1 to compare with a preset inclination limit value, counting a number of the reference pixels satisfying both a condition that the reference pixel value is less than or equal to the pixel value of the pixel of interest and a condition that the histogram value is less than or equal to the inclination limit value so as to obtain a true-in-both count value, and proportionally distributing the true-in-both count value for output.