For texture artists, understanding how light rays interact with surface matter is important because the job is to create textures that describe a surface. The textures and materials you author interact with light in virtual worlds. The more you understand how light behaves, the better your textures will look.
Everything you need to know about physically based rendering.
Understanding physically based rendering.
Physically based rendering (PBR), sometimes known as physically based shading (PBS), is a method of shading and rendering that provides a more accurate representation of how light interacts with material properties. Depending on which aspect of the 3D modelling workflow is being discussed, PBS is usually specific to shading concepts while PBR refers to rendering and lighting. Both terms describe the process of representing assets from a physically accurate standpoint.
Whether you work with a real-time rendering system in computer graphics or film production, employing a physically based rendering method of shading will improve your workflow.
What you can achieve with PBR.
- Realistic-looking assets. PBR takes the guesswork out of authoring surface attributes like transparency since its methodology and algorithms are based on physically accurate formulas and resemble real-world materials.
- Cohesive environments. No matter the lighting systems in use, other assets will always work in a PBR environment.
- Sustainable workflow. PBR is a widely adopted workflow for creating consistent artwork, even between different artists. “It cuts down on production time,” says Wes McDermott, creative producer at Adobe, so “you can focus more on the creativity of what you’re doing versus the science of what you’re doing.”
PBR and photorealism.
The art genre of photorealism emphasises creating images that look like photographs. Similarly, the goal of PBR is to provide an accurate representation of how light interacts with objects. This can make or break the viewing experience.
When you create something that looks like it exists in the real world, this creates a more immersive experience. “It falls into the background and the viewer is focusing on the story,” says McDermott. When viewers see something that looks fake, it pulls them out of the story.
Diffusion, translucency and transparency.
When working in a PBR workflow, an artist must note the base reflectivity or minimum amount of colour and light reflected.
“Specular reflection” refers to light that has been reflected off the surface. The light ray is reflected off the surface and travels in a different direction. It follows the law of reflection, which states that on a perfectly planar surface, the angle of reflection is equal to the angle of incidence.
However, most surfaces are irregular and the reflected direction will vary based on the roughness of the surface. This changes light direction, but the light intensity remains constant.
Rougher surfaces will have highlights that are larger and appear dimmer. Smoother surfaces will keep specular reflections focused and they will appear to look brighter or more intense when viewed from the proper angle.
The terms diffusion, diffuse light or subsurface scattering all describe the effect of light that has been absorbed or scattered internally. When light is scattered, the ray direction changes randomly and the amount of deviation depends on the surface roughness of the material, as rough surfaces scatter light. Scattering randomises the direction of light but doesn’t change its intensity. At times the scattered light may reappear on the surface, making itself visible once more.
Materials that have both high scattering and low absorption are sometimes referred to as participating media or translucent materials. Examples of these are smoke, milk, skin, jade and marble.
When passing through translucent material, light can be absorbed or scattered. When light is absorbed, the light intensity decreases as it transforms into another form of energy, like heat. These colour changes depend on the wavelength, but the ray’s direction won’t change.
If there is no scattering and absorption is low, rays can pass directly through the surface, which is true of glass. Imagine swimming in a clean pool. You could open your eyes and see through the clear water. However, if that same pool was fairly dirty, the dirt particles would scatter the light, lowering the water’s clarity and how far you could see.
The further light travels in such a material, the more it is absorbed and/or scattered. That’s why object thickness plays a key role in how much the light is absorbed or scattered.
The Fresnel Effect.
The Fresnel Effect, as observed by French physicist Augustin-Jean Fresnel and cited by graphics professor Wenzel Jakob, states that the amount of light reflected from a surface depends on the viewing angle at which it’s perceived.
Again, imagine a pool of water. If you looked straight down, perpendicular to the surface, you could see right to the bottom. Viewing the surface in this manner would be at zero degrees or normal incidence, normal being the surface normal. If you looked at the pool of water at a grazing incidence, more parallel to the surface, you would see the specular reflections on the water surface become more intense and you may not be able to see below the surface at all.
As you continue to work with textures, you’ll want to expand your knowledge of these key 3D lighting concepts. They will deepen your understanding of how PBR works from a technical perspective and help you to appreciate it from an artistic perspective as well. “It does a lot of heavy lifting for you,” adds McDermott, who frequently turns to PBR methodology. “I can spend more time being creative and making things that look cool.”
For more information on PBR, take a look at The PBR Guide written by Wes McDermott and published by Allegorithmic.