The Brigade Renderer: A Path Tracer for Real-Time Games

We present the Brigade renderer: an efficient system that uses the path tracing algorithm to produce images for real-time games. We describe the architecture of the Brigade renderer, and provide implementation details. We describe two games that have been created using Brigade. 1. Background Historically, games have been an important driving force in the advance of graphics hardware and rendering algorithms. Effort has evolved from striving for abstract, visually pleasing results, to more plausible realistic rendering. In the former, a distinct visual style is chosen, which does not necessarily require realism. Instead, over-the-top animation styles and matching graphics are used. Examples of this approach are most early 2D computer games, but there are also more recent titles such as Super Mario Galaxy [1] and Okami [2] (Figure 1). Figure 1: Two examples of modern games that use a nonrealistic visual style. (a) Super Mario Galaxy, (b) Okami. Many modern games strive for realistic graphics, where the goal is to convince the player that the result is (or could be) realistic. Examples are racing games such as the Gran Turismo series [3] and flight simulators such as Tom Clancy’s H.A.W.X. [4] (Figure 2), which use rasterization-based renderers, augmented with various algorithms to add secondary effects such as shadows, reflections, and indirect illumination. Figure 2: Two examples of modern games that aim for a high level of realism. (a) Tom Clancy’s H.A.W.X., (b) Gran Turismo 5. Recently, efforts are being made towards physically correct results. For static scenery and a static light configuration, this can be achieved by precalculating global illumination, or by coarsely calculating radiosity. Examples of this are games based on the Unreal 3 engine [5] (Figure 3). Games using the Frostbite 2 engine [6] support ray tracing of coarse level geometry for glossy reflections. The Unreal 4 engine [7] supports approximate global illumination using cone tracing [8]. Figure 3: Precalculated global illumination, calculated using Unreal technology. (a) Mirror’s edge, lit by Be？st. (b) Scene lit by Lightmass. Physically based rendering of virtual worlds has strong advantages. The obvious advantage is image fidelity (Figure 4). Perhaps of equal importance, however, is production efficiency. Whereas lighting for a scene in a rasterization-based engine typically requires a designer to work around technical limitations of the renderer to make the lighting look right, physically based rendering naturally leads to correct lighting. This limits the design effort to a