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Lecture 13: Global Illumination and Path Tracing (11)

Why is the moon retro-reflective? Doesn't it reflect the light from the sun in a Glossy-specular way?


@Zyy I think it is most accurately modelled as retro-reflective. If the moon really was glossy specular, I imagine it'd look much more like if you shined a bright flashlight at a plastic ball. Looking at some diagrams of the phases of the moon, it also seems like it reflects the light back towards the source, I imagine these diagrams would look a bit different if the moon was indeed Glossy-specular.


@Zyy7390 In the real world, the moon is retro-reflective according to this article: , A full moon looks intensely bright, much more so than a waxing crescent, because the moon’s surface is “retro-reflective.”and as we try to mimic the real world, it's reasonable to use retro-reflective function.


How would ideal diffuse work with non flat surfaces? Would the reflection extend equally to wherever the other side of the object is? (i.e. Would the reflection extend to more than the hemishpere shown in the images?)


I wonder if what type of atomic characteristic, if any, creates these types of reflective properties. It seems that all 4 types of reflection functions can have a variety of colors, so maybe there's a certain amount of energy that different atoms are able to absorb and re-emit? Perhaps these reflection functions are a result of the photoelectric effect?


The statement that the moon is retro-reflective still doesn't make much sense to me... If the moon is retro-reflective, wouldn't the entire moon be barely visible any time other than the full moon phase? For example, at the phase of crescent, given retro-reflectiveness, the moon will reflect the sunlight back to the direction of the sun, but the earth is on the other side of the moon, and thus can't see the moon at all. I might miss something, but idk for now...


Those retro-reflectors on the bottom are actually sort of interesting. It's as simple as placing two reflective surfaces at a right angle to each other. (Well a whole array of tiny pairs of reflective right angle cavities) If a ray shoots into the angle, it will reflect off both surfaces according to the law of specular reflection and return at the same angle.


Photons are reflected because the surface they hit is "elastic" in an electromagnetic sense. When a material has a soup of non-localized electrons sort of permeating its ion lattice structure (metals), they can respond very quickly and proportionally to electro-mag waves, and thus are seen as a sort of "elastic barrier" which "stretches", bouncing the wave. Nonmetals do not have electron soups, I am Speculating (hah) about this, since the electrons can't work together to form a group response against the light, the waves travel right on through into the material, then quickly becoming damped by the individual lattice sites until it has all been dissipated into heat (ion vibration). A few ranges of frequencies may be absorbed by the materials constituent atom's/molecule's specific electron excitation frequencies, then being emitted.


I'm really curious how people accurately model these reflection functions. The renders of many of the objects in the next slides seem extraordinarily realistic, so a lot of time and effort must have been spent making these brdfs.


The glossy specular BRDF function is not explicitly mentioned in these lectures slides. If you are looking for it for your cheatsheet visit lecture 14 the slide about microfacet materials. Microfacet materials can act as glossy specular when the bumps on the surface are small or act as ideal diffuse when the bumps are large.

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