Physically based rendering

Physically based rendering or PBR is a model in computer graphics which seeks to render graphics in a way that more accurately models the flow of light in the real world. Many PBR pipelines (though not all) have the accurate simulation of photorealism as their goal, often in real time computing.

PBR is often characterized by - but not necessarily limited to - an approximation of a real, radiometric bidirectional reflectance distribution function to govern the essential reflections of light, the use of reflection constants such as specular intensity, gloss, and metallicity derived from measurements of real-world sources, accurate modeling of global illumination in which light bounces and/or is emitted from objects other than the primary light sources, conservation of energy which balances the intensity of specular highlights with dark areas of an object, Fresnel conditions that reflect light at the sides of objects perpendicular to the viewer, and accurate modeling of roughness resulting from microsurfaces.

The phrase was invented by Matt Pharr, Greg Humphreys, and Pat Hanrahan in their book of the same name from 2004, a seminal work in modern computer graphics which won its authors a Technical Achievement Academy Award for special effects.

As described by researcher Jeff Russell of Marmoset, a physically based rendering pipeline will focus on the following areas:

PBR is, as Joe Wilson puts it, "more of a concept than a strict set of rules" - but the concept contains several distinctive points of note. One of these is that - unlike many previous models which sought to differentiate surfaces between non-reflective and reflective - PBR recognizes that, in the real world, as John Hable puts it, "everything is shiny". Even "flat" or "matte" surfaces in the real world such as rubber will reflect a small degree of light, and many metals and liquids will reflect a great deal of it. Another thing that PBR models attempt to do is to integrate measurements from photographs of real-world materials regarding constants or models to simulate albedo, gloss, reflectivity, and so on. Finally, PBR puts a great deal of emphasis on microsurfaces, and will often contain additional textures and mathematical models intended to model small-scale specular highlights and cavities resulting from smoothness or roughness in addition to traditional specular or reflectivity maps.

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