1. A method for computing dummy feature density for a CMP (Chemical-Mechanical Polishing) process, wherein the dummy feature density is used to add dummy features to a layout to reduce a post-CMP topography variation, the method comprising: discretizing a layout of an integrated circuit into a plurality of panels; computing a feature density and a slack density for a panel; and computing, by computer, a dummy feature density for the panel by, iteratively, adjusting a characteristic length of a filter function to obtain an adjusted filter function, wherein the filter function models the CMP process; and updating the dummy feature density for the panel using the feature density, slack density, and the adjusted filter function; wherein the computed dummy feature density is used to add dummy features to the layout, thereby reducing the post-CMP topography variation.