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Comment by britcruise | Hacker News Reader

Hey 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.

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Not a comment about this specifically, but I'm a big fan of your other courses on Khan Academy. They changed the way I think about the world. I can't overstate how grateful I am that you created them.

Everyone should check out these lessons:

Journey into cryptography: https://www.khanacademy.org/computing/computer-science/crypt...

Journey into information theory: https://www.khanacademy.org/computing/computer-science/infor...

I can't believe you just posted this. I had a 5 hour drive this morning and listened to nearly all the cryptography lectures during it.

Interesting I've been doing podcasts during my drives but never thought about Khan Academy. You can get away with not seeing anything they put on the screen or draw on a whiteboard? Have you had success with just listening to all the courses?

Hey. Is there a way to extract the audio so I can listen to it from my phone? The links above point to flash content that I can't access on mobile

There are lots of websites that will extract mp3s from youtube videos which you'll find if you just google "youtube to mp3". There are also browser extensions which do this. Here's one:

http://www.video2mp3.net/extension.php

If its android you can try this http://www.snaptube.in/

I use it for everything including X rated content from adult sites

My side project TubeSock (http://stinkbot.com/index.html) can do this for you. Just open the preferences and set it to download as MP3 or AAC.

youtube-dl can do that.

This service is neat: http://huffduff-video.snarfed.org/

It extracts the audio then "huff-duffs" it; i.e., puts it into an RSS feed you can consume with your podcatcher of choice. Check it out, it's great.

I'm a developer at Khan Academy (currently working on the upcoming Android app), and it's cool to see that there's real interest in that kind of use case! In our hackathon a few weeks ago I wrote an audio-only Khan Academy app that lets you listen to any video or the text-to-speech of any article and is controlled completely through voice commands. Unfortunately it's not quite at a point where other people can use it, but maybe I'll take the time to actually productionize it. :-)

Here's the code if you're curious:

https://github.com/alangpierce/Audicademy

And there's a feature-incomplete web version that lets you navigate to any video and play it (requires a recent version of Chrome):

https://audicademy.appspot.com

Audio only is a big thing for me. Commuting to work is a key learning time, and mostly audio is the only workable option.

That's great, thanks for posting it. And I love Khan academy, thanks for the great work. My son is 3 and I'm chomping at the bit to introduce him to Khan academy.

These videos were the main reason I grokked Diffie-Hellman during my grad CS course in crypto last year. Those are very good resources!

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 to point out the different relations between the ratios and the points. Now I know I've never been very quick with proofs, and I know the whole idea of Khan Academy is that you can pause and replay whenever you like, but I feel just a video isn't enough to explain even a simple geometric proof like this. Did you test this on any students? Could they reproduce the proof?

(p.s.: many events cause a bar to appear on the top of the page that pushes the entire page down, that's very not-done in web ux and causes disorientation and frustration with the user especially if the site is slow to respond (which the site is for me))

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 limit of these intersection points P as s tends toward the value t. The proof finishes by showing that this limit point is just what it was hypothesised to be, namely the point along line L2 that divides it in the same proportions as its endpoints divide the control lines.

I agree that some students will not recognise the crux of the proof, but will simply see something ends up being equal to something else in some identity without understanding how that relates to what was being proved.

Euclid's Elements (an ancient Greek textbook on Geometry) is a good model in this regard. He first states what he is to prove. He then starts from the assumptions and finishes with what he was to prove. Each step in between is justified.

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.

There is an undergrad (I think it's an undergrad) course on Ray Tracing for Global Illumination here https://www.youtube.com/playlist?list=PLslgisHe5tBPckSYyKoU3...

I've written an RSL tutorial that has been online for past 15 years (it's currently offline, I'll put it up again) and I've had people from all over the world from different studios (big and small) thank me.. it seems RSL doesn't have much resources online and considering this is tied to Pixar - do you have plans for RSL?

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 reason for learning RSL anymore. Open Shading Language (OSL) is a new shading language which is getting more and more use in the industry, but it's completely different by design.

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 wrote (Principles of Digital Image Synthesis) which I still consider awesome resource.

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

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.}

I've been lucky to see Tony DeRose, Alvy Ray Smith, and a few others give presentations in small, intimate settings at my local University.

Is there some way we could please SUBSCRIBE to notifications that wonderful speakers like these are coming to our area?

Okay, yes, I know this isn't your job, but please... you might be able to help me! :)

Looks pretty interesting. Was the domain for the content a Khan Academy decision, or was that just personal prerogative?

The initial idea for this content came from Pixar (Tony DeRose had given talks in the past showing connections between math and the movies - see here a video he did with numberphile a while back: https://www.youtube.com/watch?v=2NzTAaYgk4Q). The goal was to turn that sort of thing into a series of interactive lessons.

Are you going to add this content to your iOS app? Also my friend is looking forward to that Android app!!

The non-interactive content should mostly work in the iPad app. iPhone is trickier.

This is a tough area for us in general, unfortunately.

I'm trying out different videos and I can't find ANY instructional content in them - I mean in terms of technical information, something applied that you can learn and use. It feels like watching a TED talk and not a Khan academy lecture!

I just watched the first two sequences of videos and so far they have shown how to compute midpoints of lines and how to compute the coordinates of points on parabolic arcs given three arbitrary control points, including how to prove the identities involved. They then explain how this is used in practice to generate the shape of a blade of grass. Is this not the kind of technical information you were expecting to find?

I guess it would be great if there was some sort of application that let you apply these techniques and actually see the results.

Otherwise, it's like reading the theory section of documentation without getting to use the program.

All of our lessons give you interactive tools to create and experiment with in various ways. For example this one is fun: https://www.khanacademy.org/partner-content/pixar/modeling-c...

Yeah, I take my criticism back.

I didn't go through the whole thing initially. Excellent examples.

Thanks for your hard work. It is indeed an amazing series.

Some text indicating the target audience and any prerequisite knowledge would be great.

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