

Physics in Javascript Using Box2D - bundy
http://somethingcoded.net/post/35794507334/physics-in-javascript-using-box2d

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arikrak
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.

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a1k0n
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.

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bundy
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.

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a1k0n
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.

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kevingadd
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.

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a1k0n
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.

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daredevildave
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.

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

