Physics Based Rendering

WHAT IS PBR?

According to Wilson (2015), “Physically based rendering (PBR) refers to the concept of using realistic shading/lighting models along with measured surface values to accurately represent real-world materials.”

traditionalvspbr01Understanding how light works in the real world is essential for creating realistic and believable textures. PBR models, and programs such as Quixel and Substance Painter, calculate the mathematics and physics of how light will interact with different materials and help apply this to the model. This means that the assets will behaviour like the real thing and should look realistic under all different lighting conditions.

IN THEORY:

PBR utilises the physics and mathematical model known as the Light Ray Model to help prescribe different attributes to different materials.

The Light Ray Model is used to predict the trajectory of light rays. The first rule to note is that a light ray that is travelling through a homogeneous transparent media (i.e. air) has the trajectory of a straight line. When light hits a surface it is either, or possibly both, reflected or refracted.

Reflected light is when a light ray bounces off the surface of the material. An example of this is polished metal. The following rules apply to reflection:

  • A “light ray that hits a surface is called the Incident Ray” and the light ray that bounces off the surface is called the Reflected Ray (McDermott, 2015).
  • The Law of Reflection states that the Angle of Incidence is equal to the Angle of Reflection

UntitledRefracted light occurs when the light ray passes through a material in the trajectory of a straight line. An example of this is clear glass.

PrintUnfortunately, materials are rarely this simple as they are rarely homogeneous. Where the Light Ray Model gets more complicated (and where using a PBR program becomes more handy) is when we look at in-homogeneous or translucent materials.

In such materials, the following factors will affect the movement of light rays and therefore its physical appearance:

  • Absorption – light rays lose energy and intensity and the colour changes
  • Scattering – light rays directions changes randomly
Untitled

This is an example of how both absorption and scattering affect the movement of light through a material such as skin. This is referred to as sub-surface scattering.

  • The roughness of the material – this will affect the materials over specularity
  • The Law of Energy Conservation which states that the total amount of light re-emitted by a surface is less than the initial amount it received

Additional theories, such as Microfacet Theory and the Fresnel Effect, are included in PBR models to add extra depth to surface irregularities and realism when viewing an object from different angles.

Untitled

Microfracet Theory calculates the light diffusion caused by surface irregularities on the microscopic level.

This is all extremely complicated and would take an extremely long time to figure out on your own for every different material. This is why so many artists and companies are turning to PBR.

IN PRACTICE:

In practice, PBR programs produce texture maps that utilise this theory to give a realistic result. They contain many different presets that are based upon real world materials and how light would interact with them.

For an example I will use the barrel that I textured in Quixel during Thursday’s class:

screen01

I decided on an old, weathered look for the barrel with some sort of toxic green liquid. For the metal bands I chose ‘Weathered Brass’. Quixel worked out the base colour, specularity and weathering using its algorithms and scanned material. I was able to tweak the ‘grunginess’ of the material – to make it dirtier and more cut up – and Quixel tweaked its specularity to match. What would have taken me hours to do by hand, just to texture those bands, literally took a minute to do. Additionally, I decided to make the ‘toxic’ green liquid glow. I had no baked in a glow / emissive map but was able to create and edit one in Quixel.

EVALUATION:

In order to fully evaluate PBR we must consider its pros and cons:

Pros:

  • Very time efficient
  • Creates realistic textures quickly
  • Physically accurate so the asset will look great under different lighting conditions
  • Quixel and Substance Painter are both extremely cost efficient
  • Allows different team members to produce consistently styled textures

Cons:

  • Doesn’t really work for super stylised textures
  • Programs can be buggy or heavy to run
  • Less unique than ‘handcrafted’ textures
  • May not suit the project

PBR not only saves time (so much time!) when it comes to texturing but also creates physically accurate textures that will look great under different lighting conditions. Additionally, programs such as Quixel and Substance Painter are both extremely cost efficient for both small and large teams. Overall, this makes PBR the sensible choice for creating realistic textures.

This is becoming apparent to the industry, as video game companies have started to incorporate PBR. Several high profile games have already been using it: Remember Me, Ryse: Son of Rome, Battlefield 4 and, most famously, Star Citizen. For good reason, PBR may be more than just a trend but rather the direction the industry is moving into.

REFERENCES

McDermott, W. (2015). The Comprehensive PBR Guide by Allegorithmic – vol. 1. Retrieved from
https://www.allegorithmic.com/system/files/software/download/build/PBR_Guide_Vol.1.pdf

McDermott, W. (2015). The Comprehensive PBR Guide by Allegorithmic – vol. 2. Retrieved from
https://www.allegorithmic.com/system/files/software/download/build/PBR_Guide_Vol.2.pdf

Wilson, J. (2015). PBR In Practice. Retrieved from
http://www.marmoset.co/toolbag/learn/pbr-practice

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