Glue: A Physically Based Renderer

Most of the materials in real life can be modeled more precisely by using layers of different materials. Some layered surfaces can be analytically solved and converted into a single reflection model. However, vast majority of them cannot be represented this way. Therefore, representing arbitrarily layered materials is quiet challenging. The most obvious way is to simulate all the interactions between layers. Although this is straightforward to implement, it might cause higher variance and also it is not possible to find pdf and bsdf values for $(w_i, w_o)$ pairs, obviously. Arbitrarily Layered Micro-Facet Surfaces [WW2007] presents a method that unifies arbitrarily layered microfacet surfaces into a single surface model. However, there are some issues with this paper: When sampling a direction between layers, a direction is chosen according to the individual brdfs of the surfaces. So, if a layer is rough, the transmitted or reflected direction is chosen accordingly. How

Although it is possible to mimic real world lighting with various area light sources and analytical skylights to some extent, it is pretty hard to achieve what can be represented by image based lights (environment lights). Using an image based light source adds a great level of realism to scenes without having to create an entire environment just for lighting purposes. Because images of these light sources are stored as one of the hdr file formats, high dynamic range of scenes is well preserved so that the correct radiance distribution can be used. Image based lights should not be distinguished from other light sources in order to keep a generic light interface. So, an integrator must be able to do the followings at some surface: Sample a direction according to the pdf of this light source. Get pdf value for a certain direction, that is, probability of choosing this direction according to the pdf of this light source. Get radiance value for a certain direction, that is, how mu

Importance sampling is proven to be very practical variance reduction technique since exact sampling routines can be found analytically for terms in the rendering equation most of the time. However, finding a single sampling routine for whole integrand analytically is not trivial. Therefore, it is important to combine various strategies focused on different terms. For the integrand with indirect lighting, using more than one sampling strategy is not feasible since following more than one ray degrades performance due to recursive nature of the algorithm. For the integrand with direct lighting, however, different sampling strategies can be used since each sampled direction is tested against lights to understand if anything exists between the surface and lights. This intersection test procedure is costly but not recursive. Two of the terms of the integrand cause most of the variance: bsdf and incoming radiance. If just one of them is used following cases might occur: If a direc