In positioning assist features on a photomask pattern to improve the image quality of the main features, the method includes deriving an h-function in a first process which represents a contribution of an assist feature with respect to image intensity at a main feature. In a continuation of the method, the position of the assist features are determined in a second process using the h-function derived in the first step. The assist features are then formed on the mask at the positions indicated. Also included is a computer readable medium having instructions for performing the h-function calculations, and the mask apparatus itself with both main and assist features positioned according to the h-function.