The BRDF is ‘bidirectional’ because it can also evaluate all outgoing directions v given an incoming (light) direction l. Given a view direction v, the BRDF defines the relative contributions of each incoming (light) direction l. The Bidirectional Reflectance Distribution Function (BRDF) represents the relationship between the light and view vectors and describes the light response at the shading point based on those two variables. In shading, we assume that the local illumination at a surface point depends on only two vectors – the view direction and the light direction (though there may be multiple lights as we will see later). THE BRDF AND MICROFACET THEORYīefore I describe the individual models implemented, I want to go over some basic concepts at work. There are quite a few additional features I plan to implement over the coming weeks and months, including AOVs, BTDF refraction, transluscency, emission, other BRDF models, and smooth integration into Slim (currently not practical since RPS 16 is not integrated into RenderMan for Maya). A secondary goal of this project is to spend enough time learning and recalling the relevant computer graphics math so that I can gain a general understanding of a shading-related technical paper from a quick reading.
In the several times I have attended SIGGRAPH, the technical papers were always miles over my head. Once I have a basic understanding of the maths for the various components, I will move on to creating the class-based shader and structs required to work with RPS 16’s MIS features. MIS-aware shaders follow a somewhat strict structure utilizing RSL 2.0 structs and shader objects. RenderMan Pro Server 16 introduced a number of new features for physically-plausible shading, particularly Multiple Importance Sampling (MIS).
Next, I will add reflection functionality, but by importance sampling the specular BRDF instead of the standard cone around the reflection vector. First, I plan to implement a number of diffuse and specular models to better understand the underlying maths and practice implementation of a microfacet-based BRDF.
There are several stages to implementing a shader of this type. As I have continued my explorations with shading, I have discovered that this is not particularly physically-based, nor is it efficient.įor my independent project, I plan to build on these concepts and create a physically-plausible material along the lines of the mia_material_x (mental ray) material. My blurry reflection and refraction was just due to a large number of samples in a widening cone around the reflection/transmission direction, with all samples weighed equally. The dielectric shader I created earlier in the quarter afforded me an opportunity to re-learn the relevant computer graphics maths, but was not particularly concerned with physical correctness. This post is a complete copy and paste directly from “Renderman Portfolio” All credits go to Kevin George.
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