|Crytek's Sponza atrium rendered in Leadwerks using PBR|
Physically Based Rendering (PBR) is a workflow and rendering method. It couples a set of shaders that approximate physically accurate optical effects with input that is based on the properties of real surfaces.
PBR has become the standard method for rendering surfaces over the last few years. It is the default rendering method for game engines such as UE4, Unity 5, FrostBite, Cryengine and more. It is also being used in film studios such as Disney and Pixar.
This project aimed to implement this rendering method into the Leadwerks game engine to bring the benefits of PBR to it's users. The flexibility of the engine, coupled with recent updates to it's graphics engine such as environment probes and HDRi have allowed this to happen.
PBR is supported in both the C++ and LUA versions of the engine.
Physically based shading model
The system uses a physically based shading model based on EA's Frostbite engine and Unreal engine 4's implementation. The shading model uses Disney's principled BRDF for diffuse lighting, with the frostbite normalisation factors.
Specular highlights are generated using a BRDF consisting of; GGX for the NDF, Shlick's approximation for Fresnel effects and Smiths shadowing for the visibility function.
For more information on the details on physically based shading models I recommend the work done by Sébastian Lagarde for remember me as well as the SIGGRAPH physically based rendering courses.
Metalness / roughness workflowTo create content for the PBR system a roughness/metalness workflow is used. The system has been deigned to match the output from Substance painter/designer as closely as possible.
- Roughness: determines how rough a surface is, black = shiny, white = rough.
- Metalness: determines whether a surface is metal or not, white = metal, black = dielectric(non-metal). this should really be fully white or fully black.
- Specular: Specularity can also be modified, specularity affects the intensity of specular reflections, this value relies on the Index of Refraction of a material.
- For shaders that don't specify specular, the value defaults to 0.04. This value represents an ior of 1.5, most materials have ior's near this value. If a texture is used then the value of the texture is mapped to a 0.001 - 0.08 range. 99% of surfaces have specular values in this range. This precedent is taken from the UE4 system.
Decals allow for areas of extra detail to be added to areas.
Decals modify the underlying surface properties, this includes roughness and metalness.
Terrain / Vegetation support:
Vegetation and terrain are supported.
|Image provided by Jen from the Leadwerks forums - showcasing the vegetation and terrain shaders|
The vegetation shaders all work the same as the built in defaults, with distinctions for static, leaves and ground foliage. For best results I advise using shadows on vegetation objects, if performance allows.
Terrain also works in the same way as default leadwerks. Because of this metalness, roughness and specular cannot be controlled by textures. Therefore terrain is assumed to be dielectric, Roughness uses a combination of the Albedo's value and a set value of 0.05 and specular is set to 0.04 (this is the same value used when there is no specular map).
Also included in the PBR system is a Filmic tonemapping post process shader. This allows artists to take full advantage of the HDR system available inside Leadwerks.
Limitations / improvements
- Requires a gamma-correction post process shader. Not a huge issue, adding a pp is pretty easy but still something to remember.
- Currently the built in environment probes are stored in a low dynamic range. This leads to clamping and precision errors as HDR values move towards extremes. this limits the usefulness of HDR. This is an engine issue I can't fix.
- Probes also use simple mipmapping for different roughness values, PBR often performs a convolution on these values to better match reflection blurring due to roughness. A fix may be possible for this, but would require C++.
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