A mechanism is provided for simultaneous optical proximity correction (OPC) and decomposition for double exposure lithography. The mechanism begins with two masks that are equal to each other and to the target. The mechanism simultaneously optimizes both masks to obtain a wafer image that both matches the target and is robust to process variations. The mechanism develops a lithographic cost function that optimizes for contour fidelity as well as robustness to variation. The mechanism minimizes the cost function using gradient descent. The gradient descent works on analytically evaluating the derivative of the cost function with respect to mask movement for both masks. It then moves the masks by a fraction of the derivative.