Dear Fellow Scholars, this is Two Minute Papers with Dr. Károly Zsolnai-Fehér.
Whenever we look at these amazing research papers on physical simulations, it is always
a joy seeing people discussing them in the comments section.
However, one thing that caught my attention is that some people comment about how things
look, and not on how things move in these papers.
Which is fair enough, and to this end, I will devote this episode to talk a little about
a few amazing techniques used in light transport simulations.
But first things first: when talking about physics simulations, we are talking about
a technique that computes how things move.
Then, we typically run a light simulation program that computes how things look.
The two are completely independent, which means that it is possible that the physical
behavior of bread breaking here is correct, but the bread itself does not look perfectly
realistic.
This second part depends on the quality of the light simulation and the materials used
there.
We can create such an image by simulating the path of millions and millions of light
rays.
And initially, this image will look noisy, and as we add more and more rays, this image
will slowly clean up over time.
If we don’t have a well-optimized program, this can take from hours to days to compute.
We can speed up this process by carefully choosing where to shoot these rays, and this
is a technique that is called importance sampling.
But then, around 1993, an amazing paper appeared by the name Bidirectional Path Tracing, that
proposed that we don’t just start building light paths from one direction, but two instead
- one from the camera, and one from a light source and then, connect them.
This significantly improved the efficiency of these light simulations, however, it opened
up a new can of worms.
There are many different ways of connecting these paths which leads to mathematical difficulties.
For instance, we have to specify the probability of a light path forming, but what do we do
if there are multiple ways of producing this light path?
There will be multiple probabilities.
What do we do with all this stuff?
To address this, Eric Veach described a magical algorithm in this thesis, and thus, multiple
importance sampling was born.
I can say without exaggeration that this is one of the most powerful techniques in all
photorealistic rendering research.
What multiple importance sampling, or from now on, MIS in short does, is combine these
multiple sampling techniques in a way that accentuates the strengths of each of them.
For instance, you can see the image created by one sampling technique here, and the image
from a different one here.
Both of them are quite noisy, but if we combine them with MIS, we get this instead in the
same amount of time.
A much smoother, less noisy image.
In many cases, this can truly bring down the computation times from several hours to several
minutes.
Absolute witchcraft.
Later, even more advanced techniques appeared to accelerate the speed of these light simulation
programs.
For instance, it is now not only possible to compute light transport between points
in space, but between a point and a beam instead.
You see the evolution of an image using this photon beam-based technique.
This way, we can get rid of the point-based noise and get a much, much more appealing
rendering process.
The lead author of this beam paper is Wojciech Jarosz, who, three years later, ended up being
the head of the Rendering group at the amazing Disney Reseach lab.
Around that time he also hired me to work with him at Disney on a project I can’t
talk about, which was an incredible and life-changing experience and I will be forever grateful
for his kindness.
By the way, he is now a professor at the Darthmouth University and just keeps pumping out one
killer paper after another.
So, as you might have guessed, if it is possible to compute light transport between two points,
a point and a beam, later, it became possible to do this between two beams.
None of these are for the faint of the heart, but it works really well.
But, there is a huge problem.
These techniques work with different dimensionalities, or in other words, they estimate the final
result so differently, that they cannot be combined with multiple importance sampling.
That is indeed a problem, because all of these have completely different strengths and weaknesses.
And now, hold on to your papers, because we have finally arrived to the main paper of
this episode.
It bears the name UPBP, which stands for unifying points, beams and paths, and it formulates
multiple importance sampling between all of these different kinds of light transport simulations.
Basically, what we can do with this is throw every advanced simulation program we can think
of together, and out comes a super powerful version of them that combines all their strengths
and nullifies nearly all of their weaknesses.
It is absolutely unreal.
Here you see four completely different algorithms running, and as you can see, they are noisy
and smooth at very different places.
They are good at computing different kinds of light transport.
And now, hold on to your papers, because the final result with the UPBP technique is this.
Wow!
Light transport on steroids.
While we look at some more results, I will note that in my opinion, this is one of the
best papers ever written in light transport research.
The crazy thing is that I hardly ever hear anybody talk about it.
If any paper would deserve a bit more attention, it is this one, so I hope this video will
help with that.
I would like to dedicate this video to Jaroslav Krivanek, the first author of this absolutely
amazing paper, who has tragically passed away a few months ago.
In my memories, I think of him as the True King of Multiple Importance Sampling and I
hope that now, you do too.
Note that MIS is not limited to light transport algorithms, it is a general concept that can
be used together with a mathematical technique called Monte Carlo integration, which is used
pretty much everywhere, from finding out what an electromagnetic field looks like, to financial
modeling, and much, much more.
If you have anything to do with Monte Carlo integration, please read Eric Veach’s thesis
and this paper, and if you feel that it is a good fit, try to incorporate Multiple Importance
Sampling into your system.
You’ll be glad you did.
Also, we have recorded my lectures of a Master-level course on light transport simulations at the
Technical University of Vienna.
In this course, we write write such a light simulation program from scratch, and it is
available free of charge for everyone, no strings attached, so make sure to click the
link in the video description to get started.
Additionally, I have implemented a small 1 dimensional example of MIS, if you wish to
pick it up and try it, that’s also available in the video description.
While talking about the Technical University of Vienna - we are hiring for a PhD and a
PostDoc position.
The call here about “Lighting Simulation For Architectural Design” is advised by
my PhD advisor, Michael Wimmer, who I highly recommend.
Apply now if you feel qualified, the link is in the video description.
Thanks for watching and for your generous support, and I'll see you next time!