1. An interpolation method for enlarging an image, wherein the image is composed of plural scanning lines, the method comprising the steps of: (A) selecting four adjacent continuous pixels x k−1 , x k , x k+1 , and x k+2 which have the pixel values f(x k−1 ), f(x k ), f(x k+1 ), and f(x k+2 ) respectively on each of the scanning lines; (B) determining three linear equations L k−1 , L k , and L k+1 according to every two adjacent pixel values f(x k−1 ) and f(x k ), f(x k ) and f(x k+1 ), and f(x k+1 ) and f(x k+ 2 ), respectively; (C) selecting a pixel x to be interpolated between the pixel x k and the pixel x k+1 ; and (D) applying the pixel x to the three linear equations L k−1 , L k , and L k+1 to determine three candidate pixel values, respectively, which are weighted and combined to obtain a pixel value f(x) of the pixel x.