Julien has been working at e-on software for more than 15 years. He's worked on many aspects of VUE, but specializes in offline rendering technologies. He’s been enhancing our standard renderer throughout the years, adding global illumination, surface scattering, and more. More recently, he's implemented our interactive path tracer and PBR materials for additional support in both renderers.
e-on software: What does PBR stand for and can you define what PBR materials are in one sentence?
Julien Chauvier: Physically based rendering of materials. It can also be called physically based shading (PBS). PBR Materials allow you to simulate almost any existing material - and even a mix of several ones - with a single unified and compact format.
e: What are the key benefits of PBR materials?
JC: There are several benefits including:
1. A single material type can now be used to describe all usual ones (diffuse, glossy and specular dielectrics, as well as metals).
2. The format is very compact, meaning only a few parameters are necessary to fully describe all possible materials, making it very well suited for real-time applications.
3. Each parameter can vary over the textured object (generally through the use of UV-mapped textures, which are well suited for real-time engines). This allows you to simulate a mix of several material types using a single PBR material.
4. Although generally based on texture maps, the format allows more complex descriptions of the parameters (like procedural functions, for instance), which makes it scalable in terms of complexity and flexibility.
5. Being physically based, they nicely, and naturally fit within more general physically based rendering work-flows without any restriction.
6. It provides a consistent workflow for artists, since PBR materials should look the same in all compliant offline or real-time renderers.
e: How are PBR Materials different from other shading methods?
JC: Interestingly enough, each material type that PBR materials can describe can already be simulated with the same quality using specific models. The true difference is about how data is organized to allow all of these models to be merged into a single one, while keeping full simulation quality for all of them.
"PBR materials offer a very high shading quality using a simple and compact format."
e: What are the basic principles behind how physically-based shaders calculate the realistic aspect of materials?
JC: PBR materials are based on the micro-facet theory, describing a surface as a collection of tiny, perfectly reflective facets whose orientation will define how "rough" the surface will look at larger scales. If facet orientations are mostly random, the surface will have a diffuse look, while if facets are mostly aligned, the surface will have a smooth specular look. This "roughness" parameter is the main and most important one of the PBR material model.
Combined with an accurate Fresnel effect simulation (describing how much light gets reflected depending on the viewing angle), both dielectric and metallic surfaces can be fully described with this model.
e: Is PBR new to our industry?
JC: The micro-facet theory is very old (early 80's), and shading models based on it have existed for quite some time. However, the PBR material model description is much more recent, and has gained popularity only within the past few years. This is probably due to the increasing power and flexibility of modern graphics hardware, which now allow efficient implementations of such models in real-time engines.
e: Why do we use PBR materials?
JC: There are many different reasons to use PBR materials including;
(1) Quality: PBR materials offer a very high shading quality using a simple and compact format. The ability to describe most material types and even a mix of them using a single material is extremely powerful and user-friendly.
(2) Compatibility: While polygonal geometry description is nowadays somewhat standard (vertices, normals, faces, UV coordinates, etc.), material descriptions are still far from it, and it is notoriously difficult to support external material descriptions other than very basic texture mapped ones. Since PBR materials are basically described using texture maps, they are a very good candidate for a standard exchange format. It seems to be the first time that such high quality material descriptions can be shared so easily between applications. Besides, several software's now offer the ability to create and export such PBR materials, such as the products from Substance. A lot of online PBR material collections can also be found over the internet, providing a very large amount of ready-to-use, high quality materials.
(3) Scalability: While basically simple, their description can be enriched at will, even procedurally.
e: Does the usage of PBR materials reduce creativity and innovation?
JC: I don't think so. It's definitely even profitable for artists who can now benefit from a consistent workflow (for both offline AND real-time rendering) to define physically based materials. They are indeed designed to simulate realistic surfaces, so other techniques will need to be used for more artistic effects (like cartoon shading, for instance), but simplifying the task of defining realistic surfaces, as needed, will definitely spare time for other aspects of creation, like modeling, lighting, animation, compositing, and more!
For more information about PBR materials within VUE, check out this Tips & Tricks with Barry Marshall!