A Discrete Cosine Transform (DCT) flattening method is used to precondition a constrained flattening method. In this method, the linear step of a Gauss-Newton flattening method with geological constraints is solved with preconditioned conjugate gradients. The preconditioner utilizes DCTs to invert a Laplacian operator. Memory and computational cost savings from the use of the DCT make this method the most efficient constrained flattenening algorithm to date. A 3D faulted field data example is presented.