The impact of this on the final lit result is that GGX has a brighter highlight, followed by a trailing halo as shown below, which gives a more realistic appearance.
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Comparison between GGX and conventional normalized phong.[/caption]
In academics, physically based BRDFs use roughness as the parameter to control the microfacet distribution function. Academic roughness is defined as the root mean square slope of the profile. A common misunderstanding is that roughness maps in CG are the same as academic roughness, which is not the case. The reason academic roughness is not used for texture maps or sliders is because the "blur levels" are not evenly distributed, which is both very difficult to work with, but also leverages the limited bit precision of a texture map poorly. To avoid confusion, Unity uses smoothness instead of roughness maps, where smoothness is converted into academic roughness in the shader, using the formula (1-smoothness)^2. Distribution wise this is equivalent to Burley's roughness, but reversed such that the most blurry response maps to 0.0 and perfect mirror reflection maps to 1.0, which we find more intuitive.
The significance to such a standardized distribution is that it allows you to import content into Unity made with external tools and achieve similar results. Most CG painting tools today support smoothness maps. To be clear, an identical match is not guaranteed, but proportionality between diffuse, specular brightness and overall blurriness of the specular reflection should be close. The following comparison shot between Unity 5 and Substance Painter, was kindly provided by Wes McDermott from Allegorithmic.
As we see the visuals are very similar. I would also like to thank Wes and Allegorithmic for their collaboration and helpful iteration on this. For more details on the subject people are encouraged to check out their detailed course on PBR and Unity 5.
In Unity 5.4 we have focused on improving the speed of cube map convolution and getting exceptionally clean visuals for the image based lighting (IBL). Below we see a comparison between a sphere lit in Unity 5.4 vs. a conventional path tracer at 50000 rays per pixel.
As we see, there is a significant amount of noise using the conventional path tracer on the right, even at 50000 rays per pixel. The reason for this is because a basic path tracer (BRDF importance sampling) struggles with environment maps, which contain hot singularities such as a sun at physically proportional intensity. Unity 5.4 is now resilient to this problem and the off-line cube map convolution is roughly 2 times faster than Unity 5.2.