mental ray Architectural and Design Visualization Shader Library

72 3 Sun and Skylit surfaces). Since this is a large number of rays, the results are cached (for performance) atFG points and the result is interpolated, “smoothing” the result.This all works very well when there is a lot of fairly uniform light that is “seen” by the FGrays. In general, FG gives the best result when the light levels in a scene is fairly uniform; itworks well to illuminate an outdoors scene by the sky (most FG rays “see” the sky), and itworks well to bounce secondary light in a room in which most surfaces are lit by direct lights(most FG rays “see” some already-lit surface).However, a scene of a dark room with no lights, and a single window only exposed to the skyis more problematic:In the image on the right, almost all FG rays will “see” blackness and only a select few willbe able to “escape” through the narrow window to hit the sky. To resolve this accurately oneneed to shoot very many FG rays, which has a negative impact on performance.A further difference is that in the first two scenes (the outdoor scene and the well-lit interior)there are already direct lights which cause high-quality direct shadows, which resolves thedetails in the scene; FG is only used for additional bounce- or sky-lighting, and doesn’t providethe bulk of the lighting. Therefore, any “over-smoothing” caused by the interpolation of FGpoints is drowned out by other lights (or can be resolved with AO in mia material).But the rightmost scene does not have that luxury, all light is indirect skylight. Any oversmoothingdue to interpolation will be clearly visible, which means that one needs both a highFG ray count and a high FG point density to create a pleasing render, which gives longerrender times.A common technique used by many people doing interior renderings is to put an area light inthe window, to provide the sky-lighting rather than rely on FG to “find” the sky. But thisgives rise to the question “how bright and what color should this light be?”.3.8.2 The SolutionTo solve all these issues the concept of a portal light is introduced. The portal light is a(rectangular) area light which is placed in the window, which obtains it’s proper intensity and