A linearly convergent iterative algorithm that approximates the
rank-1 convex envelope $f^{rc}$ of a given function $f:\mathbb{R}^{n\times m} \to \mathbb{R}$,
i.e. the largest function below f which is convex along all rank-1 lines, is
established. The proposed algorithm is a modified version of an approximation
scheme due to Dolzmann and Walkington.