A pixel value array is provided, which includes an examined pixel value. Several reference pixel values are each subtracted from the examined pixel value to obtain several differences. The sampling pixels of these reference pixel values are spatially adjacent to the sampling pixel of the examined pixel value. The reference and examined pixel values represent the same color. The differences are compared with a reference value. When the absolute values of the differences are all greater than the reference value, the examined pixel value is determined as being wrong.