so say i was going to photon blast my way the light side of the model
all the way to the visible side, how does the light get there collision
to collision. i figure ive got to understand this at least or i wont
have a chance at it ever at all.
I realize it would be similar to diffuse reflection, like a collision of
a ray would produce many rays - and maybe some reversed randomized path
tracing method from visible side to the light might be a better
implementtion - which would give a correct result after averaging some
high enough sample count.
I dont even know how to start, I realize it definitely has a vectoral
property, as it has before its even glanced off the surface to begin
with. First thing I worked out that draw me to the conclusion I know
nothing, was what do I even do to begin with? Just taking the diffuse
reflectance and starting with that as a factor sounds completely wrong,
Im guessing colliding to begin with and colliding ever after could be
the exact same method, its got to be some dependancy of how far apart
you make your collisions, like youd think absorption and angle change
would have to do with how far you make your collisions apart as an
Id like it detailed enough a method, so you could put non scattering
substances inside, or even take into account an uneven medium.
Im thinking possibly it could be simple but there may be some strange
Anyway, if anyone could help id be very happy… im tired of having
basic gi and no scattering to go with it - my lux 3d renderer is never
going to happen at this rate.
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I just had a maybe cool thought. is it like just passing through and
eventually all of it reflects off a collection of randomly directioned
minute faces due to mixture thickness?
another cool thing, is actually its a bit like completely chaotic global
illumination path tracing, add light when you hit a lit portion for so
many chaos samples. sorta like a form of micro path tracing.
You’ve got the right intuition; it does have to do with how far apart
the collisions occur. Technically this is called the mean free
path. The shorter the mean
free path, the more the light gets scattered and the denser the medium
appears. If you read anything about volume scattering they’ll talk about
“scattering coefficients” - that’s basically 1.0 / the mean free path.
Red, green, and blue (or really all wavelengths) could all have a
different mean free path.
So the light goes through and gets randomly scattered with a certain
probability per unit length. Each time it’s scattered it ends up going
in a random other direction. Just as there’s a BRDF for a surface that
describes the probability of getting scattered in different directions,
likewise there’s a thing called the phase function that describes the
probability of scattering in different directions for a volume. The
function is a common one.
For subsurface I’d imagine you could just use isotropic scattering (all
goes in directions equally) as a good approximation though.
thanks man, that extra info is really good… this is looking a whole
lot less mystified to me now… whenever something looks complex, you
know its kindof a lie, all that maths i dont know was always getting in
the way of me understanding the papers. i just had to finally use my
Socratic mind and realize it really is a landslide of simple, everything
cheers man *clink*
I think you can use
this image for
reference (right part shows exact ray path in media). Each turn point is
something like Rayleigh
scattering (you have
to account polarization to do in right).