1. A method for computing the effect of perturbations to an input pattern layout during an OPC (Optical Proximity Correction) process, the method comprising: receiving a pattern layout; receiving a set of lithography model kernels; obtaining a set of convolved patterns by convolving the pattern layout with each of the set of lithography model kernels; computing a model flash lookup table for each of the lithography model kernels, wherein the model flash lookup table contains precomputed values for a set of convolution functions obtained by convolving a set of basis functions with the lithography model kernel; receiving a perturbation pattern to be added onto the pattern layout; for a query location in a plurality of query locations on the pattern layout, obtaining a set of convolution values at the query location by using the model flash lookup tables to convolve the perturbation pattern with the set of lithography model kernels; updating the set of convolved patterns at the query location to account for the effect of the perturbation pattern by combining the set of convolution values with the set of convolved patterns; and computing an intensity value at the query location.