When Giants Collide — WGC for short — is one of the “fun” projects I am working on. Once finished, it will be a small in-browser simulator where you can collide giant planets together (with some degree of realism). You can see my progress on my GitHub repo and the series of blog posts under this category.
In this little demo app, you can run an N-body simulation in your browser where you make two spheres made of point masses “collide”. You can tweak various parameters (collision speed, impact parameter, distance and number of particles) to change the outcome of the simulation.
Underneath it all: TreeSPH.js
The app above is powered by the portion of WGC’s code that computes the gravitational force between a set of point particles (a gravitational N-body system). The gravitational force is computed using the Barnes-Hut tree gravity algorithm, and the coordinates of the points are evolved using a third-order embedded Runge-Kutta algorithm. The code is available in the GitHub repo for WGC.
I am now working on writing the hydrodynamical part (via the SPH algorithm), which will let me simulate the collision between two gaseous spheres. The resulting library will be called treesph.js.
- a library to set up initial equilibrium conditions (e.g. Lane-Emden spheres, or N-body spheres with isotropic velocity dispersion);
- a canvas-based library to plot and animate the simulation snapshots;
- a fast library for operating on vectors and matrices that minimizes allocations and copying, and other math routines.
While playing with the app, you may be wondering (a) why there is a “buffering” stage before you can see the evolution of the system, and (b) why so few particles?
Buffering: it's all about delayed gratification
This issue can be ameliorated by running the numerical computation in a separate thread (a web worker), and drawing frames on the main thread as soon as they are computed. This is still not as optimal: while it solves the UI freezing issue, the particle motion will appear very jerky as it will be animated at (typically) less than a frame per second!
More particles, pretty please?
The default setting of the app is to animate 250 particles (125 per sphere). Why so few, when typical number of particles quoted for N-body simulations routinely exceed millions — or even billions! — of particles?
There are three bottlenecks at work. The first is obvious: the code isn’t fully optimized and profiled yet, and I am certain there is room for improvement. I am writing a small math library of common mathematical routines called math.js (also in the same GitHub repo) which will be fully optimized for V8.
The second is also obvious: simulations with lots of particles are usually run at full-speed, on multiple cores, and in the background. These simulations can save their snapshots, to be plotted and animated at the end of the run. An online app (or game) with real-time requirements (or, say, a <1 minute buffering time) doesn’t have this kind of luxury!
You can check out the other demos using code from WGC in this webpage.