1. An interpolation method used by a microprocessor for enlarging an image by inserting pixels into the image, wherein the image is composed of plural scanning lines, the method comprising the steps of: (A) selecting four continuously adjacent 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 scanning line; (B) determining three linear equations L k−1 , L k , and L k+1 according to every two continuously 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 ); (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, wherein the pixel value f(x) of the pixel x is f(x)=w 1 ×[slope 1 ×(1−s)+f(x k−1 )]+w 2 ×[slope 2 ×(1−s)+f(x k )]+w 3 ×[f(x k+1 )−slope 3 ×s], where slope 1 , slope 2 , and slope 3 are slopes of the linear equations L k−1 , L k , and L k+1 , respectively, s is a ratio of a distance from the pixel x k to the pixel x to a distance from the pixel x k to the pixel x k+1 , 1−s is a ratio of a distance from the pixel x to the pixel x k+1 to a distance from the pixel x k , to the pixel x k+1 , and w 1 , w 2 , and w 3 are the weighting values.