A method for calculating the parameters of a resist model of an IC manufacturing process is provided. Accordingly, a function representative of the target design convoluted throughout the whole target design with a kernel function compounded with a deformation function with a shift angle. The deformation function is replaced by its Fourier series development, the order of which is selected so that the product of convolution is invariant through rotations within a tolerance of the corrections to be applied to the target design. Alternatively, the product of convolution may be decomposed into basic kernel functions selected varying by angles determined so that a deformation function for a value of the shift angle can be projected onto a couple of basic kernel functions the angles of which are proximate to the shift angle.