This is the case because many of the BRDFs rely on lobe-shaped distributions that could go under the horizon for grazing view angles.

University of British Columbia, Vancouver, BC, Canada, Canada. Making Shaders More Physically Plausible. Our 3Delight Material provides this possibility. As an example, many available materials provide a choice of BRDFs and an extended choice of parameters. These materials are not suited to render some of the most common real-life objects such as a table or a car. These types of materials are usually coated and it takes a two-layer material and special care to properly render them. Realism in look development is achieved by a mix of sound science, intelligent parameter design and a clear understanding of what is being simulated. It's possible to design physically a plausible BRDF, respecting the three rules sited above, but still produce images that do not render real life phenomena. In a sense, physical plausibility is not a guarantee of a realistic material. So it is often more accurate to talk about visually plausible materials. In reality, it is very difficult to design materials that are physically plausible since many useful BRDFs are not even energy conservative. The expression 'physically plausible material' is used in the VFX industry, somewhat loosely, to indicate that the material is based on such BRDFs. Below is a simple exploration of each choice and how they are designed to be used. Layering is also possible and we simplify this approach with specialized nodes for layering PxrSurface. In simpler terms, it just means that the BRDF acts as a real life surface would act. RenderMan presents the PxrSurface Material as a complete choice for shading most solid objects ( hair and volumes have their own solutions). Doesn't return negative values (this last point is not always mentioned in literature).Is reciprocal (respects the Helmholtz reciprocity principle). This will maximize the space available, although this will distort proportions, but if you are painting your texture maps in Zbrush or any 3D paint package, it.doesn't emit energy that it doesn't receive). In technical terms, a physically plausible BRDF: My PBR Textures to Maya Arnold, Redshift, Vray, Renderman Materials Tutorial. 'Physically plausible' terminology has been introduced in the computer graphics literature to described a certain category of BRDFs. Resources - PBRtoMaya1.1: PBR Textures to Maya Arnold, Vray, Renderman.

Renderman materials tutorial