1. A method for magnifying images and videos, applicable to an image processing device so that, upon inputting of a digital image thereinto, the image processing device outputs a magnified digital image according to a predetermined magnification ratio, the method comprising steps of: providing a preprocess module for executing a high-pass filtering process on the input digital image so as to extract a high-frequency portion thereof, wherein the high-frequency portion is used to perform high-frequency compensation on a magnification result of the input digital image; and for executing an image decomposition process on the input digital image so as to extract an image gradient of the input digital image by using a gradient operator and decomposing the input digital image into plane regions and edge regions according to a predetermined fixed threshold, wherein the plane regions and the edge regions are labeled on the input digital image with labels; providing a composite up-scaling module for executing a magnification process on the original input digital image and the plane regions based on a simple interpolation; for executing the magnification process on the edge regions and the high-frequency portion based on both a smart interpolation and the simple interpolation; for subsequently executing a confidence process on a result of the smart interpolation, wherein the result of the smart interpolation has higher confidence if an edge direction is clearer, and lower confidence if the edge direction is less clear; and for finally executing a weighted summation on the result of the smart interpolation and a result of the simple interpolation according to said confidence; and executing a fusion process by which the magnification results of the original input digital image, the plane regions, the edge regions and the high-frequency portion are fused to form the output magnified digital image; wherein the preprocess module and the composite up-scaling module are implemented by a program code which is executed by a graphic processing unit (GPU), wherein the GPU supports parallel computing so as to upgrade execution efficiency of the program code.