1. A linear image filtering method for linear filtering a graphics signal according to an N-order filter matrix, where N is an odd integer, comprising: subsequently receiving an (N+1).times.(N+1) graphics window of the graphics signal; adding a row of zeros at the topmost/bottommost of the N-order filter matrix and a column of zeros at the leftmost/rightmost of the N-order filter matrix to obtain four (N+1)-order filter matrixes; and reconstructing four central pixels by linearly combining all pixels in the (N+1).times.(N+1) graphics window according to elements in the four (N+1)-order filter matrices.