Whilst it does have noise, the noise is close to the eventual value rather than being black, as in the previous image. The animated image with explicit light sampling is comparatively boring.Įven at the very start, the majority of the scene is well defined. This is as the majority of light paths never end up at the small light source. The animated image that does not use explicit light sampling begins almost completely black, adding speckled light over time. In the two animated images below, we can see the impact that explicit light sampling has on rendering. When the luminaire is small, the probability that a random light path will intersect with it is highly unlikely, requiring a larger and larger number of light paths to be sampled.īy exploiting our knowledge of the scene, specifically the location of these light sources, we can perform importance sampling towards their locations.īy sampling towards the light and checking if any other objects intervene (a "shadow ray"), we don't need to wait for chance to point us to our strongest light contribution in the scene. In an unbiased Monte Carlo renderer, the light path being sampled must hit a light source (luminaire) before it can make any contribution to the radiance of a given pixel. Importance sampling using explicit light sampling Uniform sampling of the potential bounce directions (blue) versus BRDF sampling (orange). This distribution function dictates how much light ends up reflected in a given direction.īy sampling directions to test directly from this distribution function (importance sampling), we can avoid taking samples that contribute little to the final image.
This is dictated by the material's Bidirectional Reflectance Distribution Function (BRDF). This is commonly related to the roughness of the material.Ī mirror, such as the one on the left, needs far less samples than the diffuse surface on the right. Some materials only reflect light in very specific directions. To achieve a noise free image requires a large number of samples, especially if the scene is complex. The basic idea is light is either produced by the object (emission) or reflected onto it from other objects (reflected).
This is referred to as the light transport equation, described in Kajiya (1986). To simulate the global reflection of light throughout a scene, we need to consider how light moves around the scene. Our results show that the improved sampling methods we use for rendering can give comparable image quality over twenty times faster than naive Monte Carlo methods. We implement an unbiased Monte Carlo renderer as an experimental testbed for evaluating improved sampling strategies. Stochastic ray tracing is one of the fundamental algorithms in computer graphics for generating photo-realistic images.
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