Thursday, May 19, 2011

Pure Python physics?

Recently I tried to get pybox2d compiled and running with PyPy, but it failed for whatever reason. I had it in my head to just give up and port the whole bloody thing to pure Python and see how that would go. Not wanting to waste inspiration, I gave that a shot. Fast forwarding to today, and it's pretty much done.

So, what's in the port? (almost) All features of Box2D up to r175:
  • All shape types (polygons, circles, edges, and loops)
  • All joint types (distance, revolute, friction, prismatic, weld, rope, wheel, mouse, pulley, gear)
  • A dynamic tree class for AABBs
  • Collisions, contacts, shape distance calculations, etc.
  • A mostly Pythonic interface to all of the useful classes
  • The testbed, in simplified form

Before you jump for joy, there are many reasons to not use this:

  • It's slow.
  • It's really slow.
  • It has a bad name (pypybox2d -- changing it is on the TODO list)
  • It's still early, though almost everything has worked well enough in my tests. I'm positive there are plenty of typos and minor bugs.
  • Some source files are much too big, because I haven't gotten around to splitting them up
  • It uses __slots__ for my own debugging purposes. I have no qualms about (almost wholly) removing their usage eventually.
"Why do this, then, if it's so slow and unusable?" Do you really have to ask? Why not?

So, what of PyPy after all that hard work? Unfortunately, in my few tests, it's been slower than CPython **. Perhaps one day it will be sped up by another means. Or maybe I'll figure out a way to get Cython working with it such that it won't totally screw everything up. If you have any ideas, please do get in touch with me.

Find the source here. The testbed requires pygame, but there's a (very) simple graphic-less in examples/ if you simply must run something.

** edit May 20th: But those tests were superficial, only testing for a few iterations. Increase the number of iterations, and PyPy wins, hands-down. See Antonio Cuni's helpful comment below for more information.