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Disney's Practical Guide to Path Tracing [video]

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Re: Disney's Practical Guide to Path Tracing [video]

#21

Post by solidangle »

billbail wrote: post #11

For optimised ray tracing you don't beam the light from the camera as that has the same chance to bounce to the sun through indirect illumination as a ray from the sun has if it is going to bounce to the camera. What they are saying here is wrong or rather extremely simplified for a younger audience.

No that's wrong. You do start tracing paths at the camera. The lens usually is extremely small and the rays that do contribute are highly directional, the probability of hitting the lens is extremely low and the probability of the hitting the lens in a direction that actually contributes to the final image is even lower. On the other hand there usually are many lights in a scene with a total area that's much larger than the area of lens and most lights don't have a directional profile. So you're much more likely to hit a light than you are to hit the lens, hence we start at the camera and not at a light.

They do simplify things a bit though. Normally we don't trace one path at a time, but we trace multiple of them. Each time we intersect with an object we do not only create another ray to continue to path, but we also sample a point on a light and we connect the two points by a ray to finish the path. This process is called Next Event Estimation and we can combine both 'accidental' paths and 'connected' paths by using a technique called Multiple Importance Sampling (MIS).

Re: Disney's Practical Guide to Path Tracing [video]

#22

Post by darkpore »

forrestthewoods wrote: post #18

Disney's global illumination tech is spectacular. The quality level since Monster's University is just leaps and bounds better than everything before it. Well, technically their short Partysaurus Rex was it's debut. But Monster's University was the first feature length.

Pixar and Disney are different studios...

The global illumination tech used on monsters university was just standard path tracing with physically-plausible sharers - the first time pixar had used that instead of radiosity caching. Other studios have been using path tracing for years before - pixars lighters are very good however, so the results are very good.

Re: Disney's Practical Guide to Path Tracing [video]

#23

Post by solidangle »

burnte wrote: post #12
billbail wrote: post #11

For optimised ray tracing you don't beam the light from the camera as that has the same chance to bounce to the sun through indirect illumination as a ray from the sun has if it is going to bounce to the camera. What they are saying here is wrong or rather extremely simplified for a younger audience.

I was wondering that. I don't have anything to do with 3D graphics, but that had occurred to me. What method do they use to ensure only the rays that have the camera and sun as end points are rendered?

That's very simple: if we don't end up hitting a light the path won't carry any radiance, so it won't contribute to the final image. If we start on a light and we don't end up hitting the camera the path won't carry any importance and so it won't contribute to the final image either.

Keep in mind that there is a very mathematical foundation to all this, we're not just tracing paths for the fun of it. Basically what we want to solve is a path integral (an integral over all paths), we do this using a technique called Monte Carlo integration (which basically means we use randomness). We first sample a path (using path tracing) and then we calculate the contribution of that path (which basically is the amount of radiance is carries divided by the probability of sampling the path) and then we add that contribution to the right pixel.

Re: Disney's Practical Guide to Path Tracing [video]

#24

Post by davvid »

altrego99 wrote: post #17
AndrewKemendo wrote: post #10

Fabulous. This reminds me very much of one of my favorite Disney videos that I showed my daughter long ago. It's this clip of four very talented cell animators out practicing their Art: https://www.youtube.com/watch?v=9JK9uQNBDxQ The whole thing is whimsical while also being very educating. Really glad that Disney is keeping this kind of stuff up.

I actually found it disappointing, compared to some other similar Disney material I've seen earlier. It is perhaps too simple. Certain things are so grossly oversimplified, they are misleading. For example, how does sorting rays following similar direction help? The history book analogy is appalling. It might give the misguided interpretation to a kid that it is always better to sort items - e.g. before summing a few…

how does sorting rays following similar direction help?

They're probably alluding to Disney's Hyperion renderer[1]. Sorting rays helps with cache coherency.

[1] https://disney-animation.s3.amazonaws.com/uploads/production...

Re: Disney's Practical Guide to Path Tracing [video]

#25

Post by solidangle »

Tuxer wrote: post #13
burnte wrote: post #12

Earlier quoted context omitted.

I was wondering that. I don't have anything to do with 3D graphics, but that had occurred to me. What method do they use to ensure only the rays that have the camera and sun as end points are rendered?

Bidirectional path tracing. You send rays from the sun, rays from the camera, and try to connect them. The ones that connect are the ones that get computed for illumination.

Bidirectional path tracing is one way of sampling paths (actually it combines many techniques for sampling paths and weights those using something called Multiple Importance Sampling). It's not the only way of doing it. Disney most likely uses path tracing with next event estimation. This means that they start a path as explained in the video and they end a path by sampling a point on a light and then connecting the start of the path with the point to form a full path. This is one the techniques used by bidirectional path tracing (bdpt), this technique uses n vertices on the camera path and 1 vertex on the light path, but bdpt also uses techniques with s vertices on the camera path and t vertices on the light path. This means that there are multiple ways to sample the same path, so these techniques need to be weighted using something called Multiple Importance Sampling.

Re: Disney's Practical Guide to Path Tracing [video]

#26

Post by solidangle »

The video was made for this article:

http://www.disneyanimation.com/technology/innovations/hyperi...

If you want to learn more about rendering then here is some more info:

Stanford CS348b course notes:

http://candela.stanford.edu/cs348b/doku.php

Cornell CS6630 course notes:

http://www.cs.cornell.edu/Courses/cs6630/2012sp/schedule.stm

Eric Veach's Phd thesis:

http://graphics.stanford.edu/papers/veach_thesis/

Physically Based Rendering: From Theory to Implementation by Matt Pharr, Greg Humphreys and Wenzel Jakob (book and open source implementation of a state of the art renderer):

http://pbrt.org/

Re: Disney's Practical Guide to Path Tracing [video]

#27

Post by solidangle »

davvid wrote: post #24
altrego99 wrote: post #17

Earlier quoted context omitted.

I actually found it disappointing, compared to some other similar Disney material I've seen earlier. It is perhaps too simple. Certain things are so grossly oversimplified, they are misleading. For example, how does sorting rays following similar direction help? The history book analogy is appalling. It might give the misguided interpretation to a kid that it is always better to sort items - e.g. before summing a few…

how does sorting rays following similar direction help? They're probably alluding to Disney's Hyperion renderer[1]. Sorting rays helps with cache coherency. [1] https://disney-animation.s3.amazonaws.com/uploads/production...

Yeah, I was a bit disappointed that they started focusing on their own renderer instead of just explaining more of the basics of path tracing. Note that it isn't necessary at all to sort rays, although it can make things faster if done correctly. Also note that Disney isn't the first to consider sorting rays and tracing them at the same time, similar research was done in the 90s.

Re: Disney's Practical Guide to Path Tracing [video]

#28

Post by santaclaus »

solidangle wrote: post #26

The video was made for this article: http://www.disneyanimation.com/technology/innovations/hyperi... If you want to learn more about rendering then here is some more info: Stanford CS348b course notes: http://candela.stanford.edu/cs348b/doku.php Cornell CS6630 course notes: http://www.cs.cornell.edu/Courses/cs6630/2012sp/schedule.stm Eric Veach's Phd thesis: http://graphics.stanford.edu/papers/veach_thesis/ Physicall…

Wenzel Jakob's Mitsuba renderer is proper badass.

Re: Disney's Practical Guide to Path Tracing [video]

#29

Post by tgb »

solidangle wrote: post #21
billbail wrote: post #11

For optimised ray tracing you don't beam the light from the camera as that has the same chance to bounce to the sun through indirect illumination as a ray from the sun has if it is going to bounce to the camera. What they are saying here is wrong or rather extremely simplified for a younger audience.

No that's wrong. You do start tracing paths at the camera. The lens usually is extremely small and the rays that do contribute are highly directional, the probability of hitting the lens is extremely low and the probability of the hitting the lens in a direction that actually contributes to the final image is even lower. On the other hand there usually are many lights in a scene with a total area that's much larger t…

One more thing that makes this possible: you actually have some room for "choice" in which way the ray goes, and so at the 'last step' you can just choose that it goes towards the sun. What do I mean? Well, most objects are largely diffuse, meaning that light is reflected in a random direction. When you hit a diffuse object, you can choose to sample light that 'randomly' bounces off in the direction of the sun. Since if it bounces off away from the sun, then it doesn't contribute light.

Lots of caveats here, of course. You do need to also sample light going in other directions, since the sun isn't the only light source (other objects reflect). You can only do this on diffuse surfaces, so you need to keep going until you hit a diffuse surfaces. Most surfaces are partly diffuse partly specular, etc. so you'll actually want to sample both straight towards the light source and off in other angles.

But what does the most simple path tracer look like? You shoot rays from the camera. If it hits a diffuse surface, bounce a ray toward each light, adding that light if that ray isn't obstructed from the light. If it hits a specular surface, bounce off based on the surface and ray orientations, and recurse when you hit another surface. You see how we cheat? If everything is diffuse, then we only ever make one bounce, straight from us to the sun. But that's a great first order approximation, since sunlight is so much brighter than reflected light. Same approach works for more bounces; just end with it trying to hit the sun.

Re: Disney's Practical Guide to Path Tracing [video]

#30

Post by falcolas »

solidangle wrote: post #26

The video was made for this article: http://www.disneyanimation.com/technology/innovations/hyperi... If you want to learn more about rendering then here is some more info: Stanford CS348b course notes: http://candela.stanford.edu/cs348b/doku.php Cornell CS6630 course notes: http://www.cs.cornell.edu/Courses/cs6630/2012sp/schedule.stm Eric Veach's Phd thesis: http://graphics.stanford.edu/papers/veach_thesis/ Physicall…

+1 on PBRT - great textbook, and a full program to boot.

Though they don't use the batching method outlined in the video - it would be interesting to see PBRT modified to do it and compare the resulting efficiency in rendering.

https://github.com/mmp/pbrt-v3/

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