Due to the subwavelength lithography, manufacturing sub-90-nm feature sizes require intensive use of resolution-enhancement techniques, among which optical proximity correction (OPC) is the most popular technique in industry. Considering the OPC effects during routing can significantly alleviate the cost of postlayout OPC operations. In this paper, we present an efficient, accurate, and economical analytical formula for intensity computation and develop the first modeling of postlayout OPC based on a quasi-inverse lithography technique. The technique provides key insights into a new direction for postlayout OPC modeling during routing. Extensive simulations with SPLAT, the golden lithography simulator in academia and industry, show that our intensity formula has high fidelity. Incorporating the OPC costs computed by the quasi-inverse lithography technique for our postlayout OPC modeling into a router, the router can be guided to maximize the effects of the correction. Compared with a rule-based OPC method, the experimental results show that our approach can achieve 15% and 16% reductions in the maximum and average layout distortions, respectively.