Physically Based Rendering Theories

The main aim of a Physically Based Rendering system, and what makes it different to other methods of texturing, is that it strives to create realism. Using lighting techniques that ‘treat light the way it behaves in the world’ (Gamer's Nexus 2015) and using key principles of physics, and how light interacts with matter, objects are given a ‘photo-realistic’ appearance.

Light Rays and Ray-Trace

Light is generated along ray-traces, which are generated from the camera viewing position, onto the scene. How the ray-traces interact with geometry and objects, determines how light behaves and what properties that object displays visually.

Distribution

When light hits an object, how that light reacts to the object denotes characteristics like, opacity, reflectiveness etc. The light is either reflected from the object, or refracted, meaning it passes through it to some degree.

Absorption and Scattering

Absorption

When the light is absorbed by an object the intensity changes into another form of energy, such as heat. In a PBR Shader, the rays themselves do not change, as they are just a representation of the path that the light must follow.

Scattering

Scattering refers to light entering an object, and ‘scattering’ before it is emitted back out. The intensity of the light doesn’t change, but the thickness denotes how much of the light is scattered inside the object.

Diffuse and Specular Reflections

Specular

Specular is the light that is reflected off a surface, and follows the law of reflection, as per the laws of physics. This means that the angle of incidence, the angle that the light hits the object, is equal to the angle of reflection. The light intensity remains the same, even on uneven surfaces.

Diffuse

A diffuse material allows light to enter an object, scattering the light directions before it is refracted out again, however, diffuse is absorbent, meaning that refracted light has a chance to be completely absorbed if it travels for too long. An object that has high light scattering properties, but low absorption, allowing light to pass through it, gives a translucent effect, like frosted glass.

Microfacet Theory

This theory suggests that a surface is made of small, scaled planar surfaces that reflect light, meaning that if these are irregular due to an uneven surface, the light that would’ve normally been reflected, is instead diffused. This gives a surface its rough, or smooth texture.

Colour

Colour is determined by light reflections, and which wavelengths are emitted by the object, and reflected.

BRDF

Bidirectional Reflectance Distribution Function or BRDF describes the reflectance properties of a Surface. For a BRDF to work, it must conserve energy, as per the laws of physics, and exhibit reciprocity.

Reciprocity Principle

Helmholtz reciprocity principle states that incoming and outgoing rays are reversals of each other, without affecting the outcome of the BRDF.

Energy Conservation

Following the physics of light in the real world, physically based rendering uses energy conservation, where the light reemitted from an object is less than what is received. Energy has been converted into other forms. Allegorithmic cites that this is handled in their PBR Shader, and isn’t necessary to adjust.

Fresnel Effect

This is the effect where the amount of light that is reflected off an object is dependent on the viewing angle. For example, viewing a pool of water from a perpendicular angle, the viewer would see straight through the water, depending on if it was clean or not. But as the viewer puts distance between themselves and the pool, their viewing angle changes, and so does the way that light is reflected at their eyes.

F0 (Fresnel reflectance at 0 degrees)

F0 is the amount of light that is reflected at a 0degree viewing angle, that is, at a straight on angle. In a PBR Shader, this value is given to non-metals (Dielectrics/Insulators) as opposed to metals (Conductors)

Conductors and Insulators (Metals and Non-Metals)

For Physically based rendering, there are set guidelines for metal and non-metal surfaces, with values that correspond to real life values.

Metals

Metals are good conductor of heat and electricity, and so light that hits a metal surface is reflected. All refracted light is absorbed. Any painted metals or rusted metals are treated as dielectrics, as they do not behave as metals do.

Non-Metals

Dielectrics are poor conductors of electricity, and so they reflect a smaller amount of light. Common dielectrics have an F0 value of between 2-5%

Linear Space Rendering

This means that intensity values are not corrected as post production volumes and so are proportionate to how they are perceived. This means that light behaves how it does in the real world, and so contributes to a photo-realistic look.

Sources

Gamers Nexus. (2015). What is PBR? Physically-Based Rendering Explained. [Online Video]. 13 March 2015. Available from: https://www.youtube.com/watch?v=7NjGETJMZvY. [Accessed: 18 October 2017].