Quote:Original post by cignox1
Actually, the OP question has little to do with the specific light transport algorithm used. The key point is wich surface description you are using: physically based BRDF's would give a better description of the different materials.

That's true, but if you are to implement any such algorithm for learning transport theory, path tracers has a nice property of being intuitive and they require a relatively small amount of probabilistic calculations and such.

Thankyou. I found the "diffuse" thing difficult to grasp and couldn't figure out why that if both "specular" and "diffuse" reflect off the surface, why diffuse would also be changing the colour. When you talk about "diffuse" being sub-surface scattering, in that the rays actually enter the surface and then re-emit, that explains why the ray would change colour.

So is it correct to say that theres

Specular Reflection - Ray reflects off surface striked, and is exactly the same as it was before it struck, as it is after, only in a different direction. Such as a mirror

Sub-surface-scattering - Ray strikes the surface, but enters it, and reflects multiple times beneath, changing colour with each bounce before finally re-emiting in some direction that could be direction in the hemisphere above the surface. (why do you say roughly in the direction of the normal?) such as skin or was

Transmitance - Where the ray passes through a material with no affect as to its direction or colour. Such as glass

Refraction - Where the ray follows the same approach as Transmitance, only that the direction changes or is bent. such as water

Im still confused as to the situation, (which I believe is all situations), when more than one of these properties applies to some surface. Take for example a glass cup.



You can see refraction, transmitance, and reflection here. But its not 100% of one. So how does that work? What does a surface that is slightly reflective mean?

Whats happens when a ray stikes a surface that is partially reflective, as opposed to a ray that strikes a surface that is 100% reflective? The answer I come up with is that there is more of some other process. so. Keeping things simple

A surface that is 50% reflective, would perform a lot of sub surface scattering, changeing the ray colour a fair bit.

A surface that is 90% reflective, would peform a small amount of sub surfacing scattering, changeing the ray colour only slightly.

But how can a surface perform less or more SSS? surely the amount of bounces is purely random, and how quick the ray re-emits from the surface depends on the random factor that it manages to bounce of a sub surface that points back out?

Couldnt there be the chance that some 2 rays that strike a surface where SSS takes place. And where the first ray happens to emit striaght back out due to it striking a sub surface that points back out. Where the second ray could be highly "unlucky" and be bouncing around by some factor more, becuase it keeps striking sub surfaces that dont point back out.

Or am i thinking into this way too much? :P

I think I understand each process. I just am having trouble understanding when a surface property represents more than one process, such as above

Quote:
Specular Reflection - Ray reflects off surface striked, and is exactly the same as it was before it struck, as it is after, only in a different direction. Such as a mirror

Not really - it's more useful to assume that a ray is split into two whenever it hits a surface, and that only some of it is reflected. The rest is refracted or absorbed.

The proportions of reflection/refraction can be determined with Fresnel's equations for dielectrics, or Schlick's approximations for metals.
http://en.wikipedia.org/wiki/Fresnel_reflection
http://en.wikipedia.org/wiki/Schlick%27s_approximation
...except that we usually ignore polarisation or use a 50/50 blend of perpendicular & parallel.

Quote:
Sub-surface-scattering - Ray strikes the surface, but enters it, and reflects multiple times beneath, changing colour with each bounce before finally re-emiting in some direction that could be direction in the hemisphere above the surface. (why do you say roughly in the direction of the normal?)

Any ray leaving a translucent substance is subject to Fresnel reflection again on its way out. The closer its exit vector to the normal, the less energy it loses to internal reflection. That just shapes the 'lobe' of this shading term so it's weaker when viewed tangentially, but it's not especially important - Lambert's cosine law will usually do the trick.

Oh - and for a more translucent look in this shading term, it pays to use a version of the normal that's unmodified or only weakly affected by bump or normal maps.

Quote:
Transmitance - Where the ray passes through a material with no affect as to its direction or colour. Such as glass

Nope - glass is always refractive, always reflective, and always absorbs a little light. Same as water.

A thin layer of glass (like a window) only appears to leave rays unchanged because the front & back surfaces are parallel, so the refraction on entry is reversed on exit. So for realtime uses, you can skip refraction for thin layers of transparent materials (and for things like mesh curtains, of course) without sacrificing realism.

Quote:
Whats happens when a ray stikes a surface that is partially reflective, as opposed to a ray that strikes a surface that is 100% reflective?

If you prefer to think of rays as indivisible things (like photons), then there's a chance that it'll be reflected, which depends on the angle of incidence.

You can also think of light as waves, which will certainly help improve your understanding, but it's kinda useless from a programming POV.

For our purposes, it makes much more sense to think of a ray is a bundle of infinitely many photons, so a portion of a ray can be reflected, with the proportions determined by the Fresnel equations.

It also helps to remember that nothing is 100% reflective, and that with every shading operation (except for glowy magical stuff), you should be losing 15% to 95% of the light energy you started with.

Ah yes. I was always thinking in terms of rays. And that if one ray stikes only one ray can leave. This way it easier for me to follow the rays path to get an understanding. When you talk about the splitting of rays. Doesnt this typically, from our point of view of a simulation, create an infinite number of rays within the scene? As not all surfaces will create a 100% pure reflection. Meaning that it will split rays, and its descendants will split on their collisions and so on? That is unless we allow these rays to lose all their enery and effectivly be removed from the simulation?

So if a light ray is instead modeled as a wave. what does it mean to split a wave?

Not a direct answer to your question, but:
If you want to dive into physics coming from computer graphics, you may want to check out the book 'Optics' by Eugene Hecht.
While it leaves many open questions (I think it's an undergradute physics book), I found it quite illuminating.

Quote:Original post by maya18222
When you talk about the splitting of rays. Doesnt this typically, from our point of view of a simulation, create an infinite number of rays within the scene?...That is unless we allow these rays to lose all their enery and effectivly be removed from the simulation?

You don't need to create & manage an infinite number of rays all at once, if that's what you're asking. Typically you're scanning the scene with rays fired from the camera, just a few at a time. Those rays may still branch off as they bounce around, but with each bounce they lose energy, and with each branching they lose statistical significance, so when a ray is weak, it's easy to just drop it and move on.

You can still do interesting things by hurling a million rays around all at once - that's essentially what photon mapping is all about - but it's entirely optional.

Quote:
So if a light ray is instead modeled as a wave. what does it mean to split a wave?

For a proper understanding of the wave nature of light, you should start with a proper understanding of waves in general. This isn't hard-core physics - a high-school physics textbook or something like (don't take this the wrong way) 'Physics for Dummies' will explain what waves can do other than just propagating in circles.

Fortunately, for our purposes, the dual wave/particle nature of light lets us get away with ignoring its wave nature altogether and simply approximating its effects. We can treat light as particles that travel instantaneously in straight lines, use tried & tested equations (from Fresnel, Snell, Lambert etc.) and have a much nicer time.