Modeling Physics in Javascript: Gravity and Drag

http://burakkanber.com/blog/modeling-physics-javascript-gravity-and-drag/?

by bpierre • 14 years ago

91 37 14 years ago

gbrindisi• 14 years ago

This might become my favorite blog ever.

You really want to play with physics/math with ECMA 262? O_O (hint JS knows nothing of integers) Floats are way to slow (and faulty when div/mul are called) compared to int to do anything serious. All 3D engines tricks are about doing complex math with integers (EDIT: or other tricks http://en.wikipedia.org/wiki/Fast_inverse_square_root#Overvi...)

grimboy• 14 years ago

Why would you post a link to an example using floats then? I mean sure, part of the trick involves treating the float as an integer but the number is still meant to be interpreted as a float.

Your assumptions about speed might be true for something like a Nintendo DS which doesn't have a floating point unit but a 3DS does, a vectorised one no less. Also in Javascript integers that can fit into 32 bits are actually integers nowadays.

All in all, I believe your information is outdated.

mrspeaker• 14 years ago

You really want to discourage someone playing with physics/math in their browser?

Tichy• 14 years ago

Aren't the JavaScript engines free to optimize the numbers and treat them as integers as long as they are integers?

mburns• 14 years ago

https://developer.mozilla.org/en-US/demos/detail/bananabread - seems to be quite performant for a javascript 3d fps.

bkanber10• 14 years ago

You might be my favorite reader! Thanks!

hrabago• 14 years ago

The page won't come up for me. Is it already hurting from being HN'ed with only 26 upvotes?

bkanber10• 14 years ago

Yeah it's a tiny box with Wordpress on it. Fixed!

slig• 14 years ago

Also, it would be great to notified about ALL your blog posts, not only this series. :-)

mweibel2• 14 years ago

Very good article and very good explained. I have a hard time really getting physics but if my teachers would have explained it this way I might actually have had fun with it :)

bkanber10• 14 years ago

Thanks! That's what this series is all about. Physics and math really are fun, you just need to figure out an application that excites you and also have someone teach it to you in a manner that keeps you engaged. That's what I'm trying to do here, so if you're interested you can sign up for the mailing list and I'll send you emails when new articles are posted!

mweibel2• 14 years ago

Yes, did this already :) thanks for making these articles!

mbq2• 14 years ago

I know it is educational and it does not matter in presence of dissipation, but it would be nice to mention that Euler method is not only an approximation but also terribly inaccurate one. It won't produce any stable oscillations or orbits for instance.

bkanber10• 14 years ago

I actually mentioned that in a comment lower on this page. I will be talking about ODE solvers a couple of articles from now. The Euler method isn't acceptable for actual simulation but it's the easiest to understand and it gets the job done for now!

xxbondsxx• 14 years ago

The Velocity Verlet integration method isn't much of a jump in complexity (http://en.wikipedia.org/wiki/Verlet_integration#Velocity_Ver...) and works _great_ for kinematic simulators.

As a cool feature, it maintains the energy of the system, so you can slow down, stop, and then rewind time and arrive at the same previous configuration, all by changing the delta T's inside the evaluation.

viraj_shah• 14 years ago

Thanks for putting up the email subscriptions

Simucal• 14 years ago

I wrote something like this a couple years ago[1]. I also attempted to support ball-to-ball collisions. I rarely do any graphics type programming so this was a fun diversion.

However, the simulation would slow down considerably after adding a few dozen balls. This was largely due to the quadratic nature of my naive collision detection. In order to remedy the speed problems I implemented[2] a quadtree I found online to assist me in culling a lot of my expensive collision checks. The bounds of the quadtree sections will appear as you begin to add balls.

Unfortunately the quadtree wasn't the performance savior I expected it to be. I didn't notice a dramatic improvement in the number of balls I could support. I also introduced a bug with the quadtree version where certain collisions are missed entirely.

Perhaps I should revisit this simulation and see where I was off.

[1] http://jsfiddle.net/simucal/RvA9w/33/

[2] http://jsfiddle.net/simucal/RvA9w/

mrhyperpenguin• 14 years ago

Instead of quadtrees take a look at spatial hashing[0]. It's more simple than quadtrees and the performance is similar if not better.

[0] http://www.gamedev.net/page/resources/_/technical/game-progr...

zackmorris• 14 years ago

I've messed around a bit with the Bullet physics library and was wondering if anyone knows how it works under the hood. It's not brute forcing the collisions, because it can handle thousands of objects.

Does anyone know how they do the approximation? There is a fudge factor you can set for say a 10% boundary around an object, where it's almost like it moves all of the objects and then takes a best guess where they should be placed for the next physics frame. But I'm curious what their approach is called, if it's a general solution.

bkanber10• 14 years ago

Look into quadtrees, that's a good way to do collision detection in 2d. Or just partition your world into a fixed grid and only consider collisions with objects in the same grid square. Quadtrees are just a smarter way of doing that.

typpo• 14 years ago

I recently did something somewhat physics related, implementing Kepler's laws of planetary motion in JS to produce a 3d simulation of the solar system: http://www.asterank.com/3d/ ...I plan to generalize it into an open source educational tool that allows people to build arbitrary space simulations on top of its physics engine.

Another interesting JS physics experiment that I found while looking into this stuff is this n-body gravity simulator: http://www.spacegoo.com/solar_system/

bkanber10• 14 years ago

Awesome! I couple years ago I made a simple game that uses orbital mechanics too: http://burakkanber.com/orbit

If it will help at all feel free to poke through the source code!

akldfgj• 14 years ago

Move the Fiddle to the top of the page, and get much more user interest.

Modeling Physics in Javascript: Gravity and Drag

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