I am a computer graphics guy in the area of physically based rendering and recently I've been searching on the net to figure out how physical is Physically Based Rendering (PBR). The nature of the question lies somewhere between pure physics and pure graphics, so I'm hoping I find a someone familiar with graphics here.
For those who don't know, PBR aims to render the different materials in the same way it actually happens in real life, so it's a more physical approach. This lets us render highly realistic images than other approaches.
I want to check if my understanding is correct about how light actually interacts with different materials.
First of all when light first strikes a surface, the Fresnel equations directly come into play and they determine how much light gets reflected and transmitted. This is different for different wavelengths since the index of refraction depends on the wavelength.
The transmitted light may travel farther for refractive surfaces or may get absorbed quickly as in the case for metals. The transmitted light may also re-emerge from different positions. This is what we in graphics call "diffuse reflection". The counterpart being "specular reflection" (light just reflects off the surface. Note that the reflection doesn't necessarily have to be in the mirror direction).
In graphics we usually define the color of the object through RGB triples. I know this is inaccurate however I'll move on with this assumption. I think that the Fresnel equations are indirectly defining this RGB triple for different surfaces? Since Fresnel reflection is different at different wavelengths, then if we sample at the 3 main wavelengths R, G, B, we are actually getting the amount reflected for each wavelength, which in turn is our color of the object? However the color of the object should be independent of the view direction which clearly isn't the case with my assumption. Can anybody clear out this point?
The next difficulty is how much light do rough surfaces like an unpainted wall or wood etc, reflect light diffusely or specularly.
We have 2 parameters which we denote as the reflectivity of the surface. We say a surface might reflect light 60% diffusely ( that is 60% get transmitted then re-emerges) while 40% get reflected specularly. This doesn't need to always add up to 1 since some energy is also lost in the form of heat. We denote both of these parameters as $K_d$ and $K_s$ (diffuse and specular reflectivity).
But doesn't this sound the same as Fresnel reflectance? Basically doesn't $K_d$ seem like the amount of light transmitted (ignoring for the moment that a fraction of transmitted light may get lost in the form of heat) and $K_s$ the amount of light reflected? This implies that these parameters would vary w.r.t viewing direction but We normally set $K_d$ and $K_s$ to fixed value when we are defining the material in graphics or at least that's what I have seen so far.
If $K_d$ and $K_s$ really is the same as Fresnel reflectance, this would mean that rough surfaces such as wood will reflect light specularly when viewed at grazing angles and when viewed head-on, the light coming from the wood into our eye is more that had transmitted into the surface and re-emerged i.e. diffusely. Is this the case in real life as well?
Last but not least is the case of reflection.
In PBR we usually have microfacet based models according to which each surface has micro-bumps. From there comes the first important parameter for PBR that is roughness/smoothness. This parameter governs how much the specular reflection is concentrated in a single direction.
(5a) In point 2 I assumed specular reflection meant light just scattering off the surface not necessarily in the mirror direction. Is this true? Or light always reflects in the mirror direction, it's just that it isn't concentrated due to these microfacets?
(5b) This leads me to believe reflections are governed by 2 parameters. How smooth the surface is and how much the light reflects off the surface (specularly). Are there any other parameters that govern why we see reflections on surfaces of different objects?