Physics in Javascript Using Box2D | Hacker News Reader

Physics in Javascript Using Box2D

somethingcoded.net

29 pointsbundy10 comments

Looks cool, but I couldn't get the page to load. I was once worked on a demo of a similar kind of game to help teach physics.

ah, it might be that we only tested on chrome. the project only allowed for 48 hours of development, so we can't really say that we made the best effort to keep it cross browser compatible. once the voting period is done and the code freeze is over, we're thinking of implementing a lot of bug fixes and changes that hopefully would address that.

I'm using box2d (there are at least three box2d javascript implementations; the most impressive is the emscripten port from the actual C++ code: https://github.com/kripken/box2d.js but I'm using an older one for this). I wanted to prototype a simple balancing robot controller, and I hacked this up on the bus to work earlier this week in fact: http://www.a1k0n.net/code/balance.html Click to drop junk.

(Sorry, this is totally off-topic, but...) It's surprisingly easy to make a segway-like balancing robot with perfect sensors. My plan is to add noise in the sensors and limited motor torque, etc. Ideally it would estimate its own kinematic parameters and do an A* search for approximate optimal control.

hey a1k0n, we'll definitely have to take a look at the port you dropped in your post. we didn't do a ton of research before hand, but we were debating between box2djs and box2dweb. in the end we choose box2dweb since it's self contained within a single file and was supposed to be more up to date.

i don't think what you're saying is totally off topic. box2d is pretty awesome and it's surprisingly easy to get physics simulations up and running using the library.

The thing about the emscripten version is that their own demos seem to GC-pause constantly on Chrome. I'm assuming the emscripten-generated code is making tons of incidental garbage, but I haven't profiled it or anything; it's all Closure-compiled and impossible to read. I'm still using the old old old actionscript port of the old version, and wish I had seen box2dweb sooner.

btw disasteroids is hilariously awesome. Great work.

Emscripten generally speaking does not produce garbage because it uses a virtual heap. If you profile the demos using a heap profiler like Chrome's you will be able to observe this. The pauses are likely due to bugs in V8 that cause it to recompile a page's JavaScript constantly (it has a lot of them). The cost of those bugs ends up being higher than that of equivalent bugs in Firefox's JS engine because of the way V8 is designed.

Either way, though, if you have a reproduction case for those pauses you should definitely file a bug about it on the appropriate tracker - emscripten's if something is wrong with the generated code, or chromium's.

Yeah -- I knew about the virtual heap arrays, so the GC explanation didn't make sense. I'll dig into it; thanks for the pointers.

I think you're right though. Once it's been running a while, it seems OK; so I think it was JIT compilation pauses.

We've been testing out ammo.js (Emscripten compiled Bullet) for 3D physics. In this demo http://apps.playcanvas.com/will/ammo/crates after the first couple of resets, it runs smoothly.

It does indeed seem to be the JIT warming up. The good news is that Chrome Canary is significantly smoother, so 6-8 weeks until that hits stable.

Canary takes 0-6 weeks til Dev, then 6 weeks til Beta, then 6 weeks til Stable, so it's 12-18 weeks.

Wow, yeah. That's a much more dramatic demonstration. Looks great though, once it warms up.

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