Bringing Transmittance Function Maps to the Screen: Wrath of the Titans and Prometheus Jose Esteve ∗ MPC Jamie Portsmouth † MPC Pascal Gautron ‡ Technicolor Jean-Colas Prunier § MPC Jean-Eudes Marvie ¶ Technicolor Cyril Delalandre ‖ Technicolor Figure 1: Scenes from Wrath of the Titans (a) and Prometheus (b) involve extremely dense volumetric objects. We adapt the Transmittance Function Mapping algorithm for high quality interactive previsualization and tuning of those media. Images: (a) c 2012 Warner Bros. Entertainment, (b) c 2012 20 th Century Fox. Abstract Participating media are an unavoidable part of todays visual ef- fects. The computation of compelling lighting effects within clouds or smoke remains challenging, both in terms of memory occu- pancy 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 pro- vide 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 march- ing approach, our technique provides significant quality improve- ments while preserving real-time performance. 1 Introduction One of the challenges of digital visual effects is the generation of compelling sequences involving massive, highly detailed volumet- ric elements. Besides the costs of simulation, the accurate estima- tion of light transport within such participating media is crucial for producing lifelike digital images. This estimation is generally ex- pensive, both in terms of memory and rendering time. Therefore, ∗ e-mail:jose-e@moving-picture.com † e-mail:jamie-p@moving-picture.com ‡ e-mail:pascal.gautron@technicolor.com § e-mail: jean-cp@moving-picture.com ¶ e-mail:jean-eudes.marvie@technicolor.com ‖ e-mail: cyril.delalandre@technicolor.com the interactive tuning and arrangement of volumetric objects is also of primary importance to avoid numerous overnight test renders. Several approaches address the problem of real-time rendering of participating media by introducing drastic assumptions, limitations or precomputations. However, in the context of production render- ing the images produced interactively must be reasonably consis- tent with the final renders and not require excessive precomputa- tions. Building upon the existing Transmittance Function Mapping technique [Delalandre et al. 2011], we adapted this algorithm for interactive rendering of the Wrath of the Titans’ pyroclastic clouds and Prometheus’ sand storms. 2 Background This section provides technical background on scattering computa- tion, and discusses related previous work on lighting simulation in participating media. 2.1 Scattering The interaction between light and participating media is fully de- scribed by the radiative transport equation [Chandrasekhar 1950], covering both single and multiple scattering events. For perfor- mance reasons we focus on single scattering in the remainder of this section. A medium is described at each point p by the follow- ing functions: • The absorption coefficient σa(p) represents the fraction of in- coming lighting which gets transformed into other forms of energy, such as heat. • The scattering coefficient σs(p) is the fraction of incoming lighting scattered at p in other directions. • The extinction coefficient σt (p)= σa(p)+ σs(p). • The phase function p(p,ωout,ω in ) describes the amount of light scattered at p from the incoming direction ω in into out- going direction ωout. To define single scattering, let us first consider a point pn within the medium, and a scattering direction ωout. Given a lighting inten-