Participating media are an unavoidable part of todays visual effects. The computation of compelling lighting effects within clouds or smoke remains challenging, both in terms of memory occupancy and computational power. Also, the fine tuning and layout of production-quality scenes requires efficient techniques for fast pre-visualization of the results. The Transmittance Function Maps provide an efficient solution for real-time previsualization of relatively wispy media such as clouds. However, this technique cannot sup port the extremely high densities encountered within the pyroclastic clouds of Wrath of the Titans, or in the sandstorm of Prometheus. We propose an adaptation of the Transmittance Function Mapping technique for the interactive previsualization of extremely dense, production-quality participating media. Based on a dual ray marching approach, our technique provides significant quality improvements while preserving real-time performance.