1. An interpolation method for digital pictures, comprising the steps of: dividing a blank region among pixels equally into a plurality of squares; determining an interpolation parameter for each of the squares relative to its sampling point according to the position of the square; compressing and storing the interpolation parameters of all the squares relative to their sampling points; zooming the picture and making sure that the inerpolating point falls into a target square; de-compressing the interpolation parameters according to the position of the target square and computing the interpolation parameter corresponding to the square; and using the interpolation parameters and the brightness of the sampling points to compute the brightness of the interpolating point.