Abstract: In this paper, we introduce a novel technique for pre-filtering multi-layer shadow maps. The occluders in the scene are stored as variable-length lists of fragments for each texel. We show how this representation can be filtered by progressively merging these lists. In contrast to previous pre-filtering techniques, our method better captures the distribution of depth values, resulting in a much higher shadow quality for overlapping occluders and occluders with different depths. The pre-filtered maps are generated and evaluated directly on the GPU, and provide efficient queries for shadow tests with arbitrary filter sizes. Accurate soft shadows are rendered in real-time even for complex scenes and difficult setups. Our results demonstrate that our pre-filtered maps are general and particularly scalable.