It's odd that people in the comments seem to think this is for kids. It surely isn't intended to be for children, right?
Great lessons.
Pixar in a Box – Khan Academy
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Re: Pixar in a Box – Khan Academy
#32Earlier quoted context omitted.
The CG by UC Berkeley on EdX is a great starting point. One of the exercises involves building a simple ray tracer. A slightly more advanced source on ray tracing is http://www.scratchapixel.com/ . Of course the best way to learn about ray tracing is by reading Physically Based Rendering: From Theory to Implementation by Pharr and Humphreys. Pixar has ditched RSL in favor of BSDFs written in C++, so there is no real…
Yes, you're right about RSL. What I've meant is that it is (was) a good learning tool. Maybe something like shadertoy today with an appropriate course would be apt? PBRT is a great resource too. I don't know if they opted out of literate programming? I can't stand that for some reason. I've leafed through PBRT though. I've learned (back in the day, hah) from Shirley's Ray Tracing book and bibles that Andrew Glassner…
Yeah it was a great learning tool. pbrt is a good learning tool, but sadly it already implements most of the basics. Courses at Cornell and ETH Zürich used a small rendering framework called Nori [1], which is a pretty good learning tool. Shadertoy is nice, but perhaps a bit annoying to work in (as it involves a lot of tricks). I guess the best way to learn is to write your own ray tracer, but that takes a lot of time.
The current edition of Physically Based Rendering still uses literate programming, but most of the maths are separated from the code, so you can skip over most of the code. Even if you can't stand I can really recommend reading it, as it's a great book. Shirley's Realistic Ray Tracing is still very useful these days, but obviously it's lacking the state of the art. If you really want to avoid any code you should read Veach's PhD thesis [2] and then some papers on BSDFs [3], shapes [4] and ray tracing acceleration [5].
[1] http://wjakob.github.io/nori/
[2] http://graphics.stanford.edu/papers/veach_thesis/
[3] http://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.html
[4] http://www.graphics.cornell.edu/pubs/1997/MT97.html
[5] http://www.nvidia.com/object/nvidia_research_pub_012.html
Re: Pixar in a Box – Khan Academy
#33It's odd that people in the comments seem to think this is for kids. It surely isn't intended to be for children, right? Great lessons.
I think this is aimed at middle/high school students. It probably can be interesting for adults, but I'd really recommend picking up a book such as Fundamentals of Computer Graphics by Shirley and Marschner [1].
Re: Pixar in a Box – Khan Academy
#34Earlier quoted context omitted.
I love Khan Academy, I used it extensively to practice calculus and I think also discrete mathematics back in college. I just watched the videos of the first few sections of this math course and they're all very nice and I feel I really understand how to draw parabolas now, but the section I just watched ends with a proof. I'm not convinced by the proof at all, it goes very fast through the basics and is very quick t…
The proof starts with two of the "string-art" lines, a line L1 (line Q'R' in the video) with parameter s and a line L2 (line QR in the video) with parameter t, and computes the intersection point P of these two lines. Keeping line L2 fixed, one gets a series of intersection points P as one varies the parameter s. One then notices that the point of intersection of line L2 and the parabola (the "touching point") is the…
These are great points. You'll notice that video is actually a "bonus" step which isn't required to complete the final exercise in that lesson. We are still experimenting with how to go about "bonus" steps. In other lessons you'll see they are done in a multi-step article style. Such as in Animation:
https://www.khanacademy.org/partner-content/pixar/animate/pa...
We also do this for the bonus step in Character Modeling (my favorite lesson!)
Our plan is to continue collecting more feedback on these different styles and find out what works best.
Re: Pixar in a Box – Khan Academy
#35Earlier quoted context omitted.
Isn't that complimentary to what I said though? You could presumably get the same result by just path tracing the whole thing, so all you've gained is speed at the cost memory. I didn't mean speed hack in a pejorative way. I understand why you might want to do this in practice, but it still doesn't seem to be that important to teach as an introduction.
I guess I didn't fully understand your post then, I thought you meant that the Reyes algorithm is an "inferior estimation" and that it might be beneficial to add it to a renderer, but neither is the case. But it isn't an inferior estimation (just a different way of computing things) and you don't want to use the Reyes algorithm in practice as scenes either barely fit in memory or they don't fit in memory at all (and…
Am I missing something?
In "regular" Reyes (and I may just have this wrong - I was unaware that the hybrid technique you describe was common) I though you were a) locally approximating surfaces and b) applying local shaders. These shaders are typically inferior estimates of the BSDF at that location compared to other techniques, but can be very memory efficient.
In the hybrid approach you describe, you avoid at least (b) above by doing the shaders by path tracing, say, but localized to where your rasterizer has discovered. So you aren't benefiting in anything but speed, so this seem more an implementation detail than anything fundamental.
So I don't see how dropping this sort of detail from any introductory course is anything but sensible. There should probably be a discussion of space/speed/distributional issues in general, but at a higher level.
Again, I'm not questioning why anyone would want to implement it, just why anyone would find it odd to leave out from an introductory course.
Re: Pixar in a Box – Khan Academy
#36Earlier quoted context omitted.
The REYES algorithm is an algorithm that can very quickly produce batches of shading points (with the required information to shade them such as the viewing direction). It's not really a hack and you can still use path tracing (with BSDFs) to correctly shade these points. In fact this is what Pixar did for Monster's University; they used the REYES algorithm to determine shading points visible from the camera and then…
Isn't that complimentary to what I said though? You could presumably get the same result by just path tracing the whole thing, so all you've gained is speed at the cost memory. I didn't mean speed hack in a pejorative way. I understand why you might want to do this in practice, but it still doesn't seem to be that important to teach as an introduction.
The difference between Reyes and path tracing is this:
- Reyes batches camera-visible geometry into grids of micropolygons and microvoxels. These micro-elements are then shaded using any method you please (which, in the case of Pixar movies since Monsters University, has been raytraced physically based illumination). The micropolygons are projected to the image plane and stochastically sampled, much more akin to traditional rasterization. There is no raytracing for camera-visible geometry, and all projections must be linear.
This style of doing things was to exploit locality in screen space. It assumes that the image can be broken into "buckets", and the information outside the bucket can be destroyed when the bucket is finished. This used to be true when RenderMan was first designed, but now that everybody is raytracing their illumination, it is less of a useful optimization.
This also means that shading happens at discrete times. Usually everything is shaded at the shutter-open instant, so specular highlights will be smeared in an unphysical way, because their motion is not properly correlated with the geometry motion.
I should also point out that Renderman keeps an internal radiosity cache in Reyes mode, which is basically just stored colors of illuminated geometry. This cache is queried by raytracing when computing indirect illumination, and a cache of the whole scene is kept for each bounce. Pixar only uses one or two bounces in Reyes mode due to the heavy memory cost. Note that view-dependent indirect illumination is not possible with this scheme-- Second bounces (and deeper) are always diffuse only. (It is possible to avoid this, but it tends to be slow because Reyes makes all kinds of generally-untrue assumptions about its ability to batch shading which fall apart in second+ bounce shading).
- Path tracing fires rays for camera-visible geometry. In most path tracers, including RenderMan RIS, each ray hit generates a new shading exec. This means that time-jittering for camera motion blur also trivially produces time-jittered shading, so (among other things) moving specular highlights will be correct, and view-dependent indirect illumination is possible. Because of this live-shading aspect there is no radiosity cache, and so much deeper levels of indirect illumination bounces can be used (compare 8 bounces to Reyes' typical 2, for example).
Also, because camera-visible geometry is raytraced, it is possible to do interesting things like nonlinear projections.
Re: Pixar in a Box – Khan Academy
#37Hey HN! My name is Brit Cruise and I'm the lead content developer on the Khan Academy side of this equation (working alongside Tony DeRose from Pixar...plus a village of others), if you have any specific questions about this project I'll be happy to answer as best I can.
This is such an awesome resource!!!! {Edit: I say this as a writer and not as a 3D animator / designer. I apologize for sounding very fan-boy about it, but I do really appreciate this.}
Re: Pixar in a Box – Khan Academy
#38I brought a computer graphics textbook into my high-school math class and asked my teacher if he could help me figure it out. His response was that I needed to pay attention to the current lesson plan, and would need to get through trig, then calculus before we could approach the book.
What's amazing about this video series is that they have actually captured the elements that _can_ be taught to kids of various ages and skill levels. Man how I wish there were a resource like this when I was younger.
Re: Pixar in a Box – Khan Academy
#39I brought a computer graphics textbook into my high-school math class and asked my teacher if he could help me figure it out. His response was that I needed to pay attention to the current lesson plan, and would need to get through trig, then calculus before we could approach the book. What's amazing about this video series is that they have actually captured the elements that _can_ be taught to kids of various ages…
You are younger than you will be two years from now.
Re: Pixar in a Box – Khan Academy
#40It's interesting that the Rendering chapter only covers ray tracing, while most classic Pixar movies were actually mainly rendered using the Reyes algorithm, which is a form of rasterization.
Pixar movies from Finding Dory onward will be exclusively raytracing, FWIW.
You sound pretty authoritative here, so I'll ask a semi-topical question: are the creative teams the ones driving this kind of development? "We need better fidelity/IQ because I would like this film to look more impressive that previous ones." Or is it more along the lines of Pixar demonstrating industry leadership?