To avoid losing edge information in an image in scaling of the image and optimally process with taking account of other channels than RGB ones, if provided.

The method comprises determining dynamically a plurality of thresholds from the size and scale factor of input image data in scaling of an image and applying variable weighting filters with the thresholds, applying an interpolation algorithm, thus changing the resolution, without losing edge information. If the input image data have other channels than RGB ones, the method comprises changing the resolution for the other channels and determining the thresholds of the RGB channels from each pixel value in the image after processing for the other channels and the size and the scale factor of the image data. Thus the values resulting from the resolution changing process are added to judging materials to determine the threshold of the RGB channels.