Posts Tagged ‘physics testbed’


continuous collision detection

October 18, 2008

Do you people think it’s about time?  HUH?  Well, I do.  So, several things are planned for addition soon.  This will involve a lot of lists:

  • 0 degrees of freedom constraint: pivot constraints (the anchor point on each object are forced to share a common point in global space.)  Basically a bar constraint, but with a length of zero.  This is needed because a length of zero creates a singularity in bar constraints.
  • 2 degrees of freedom constraint: groove constraints (probably.)
  • Continuous Collision Detection by means of sweep test + a time of impact advancement method.

In the future, I will probably add force fields, but not just yet, all we have is the global gravity constant for now.  My current collision algorithm is completely discrete, and is as follows:

  1. Update positions by delta time.
  2. Cache AABBs and world transforms.
  3. Check for intersections.
  4. Generate collision contacts and update arbiters.
  5. Do response.

Continuous collision detection is trickier:

  1. Cache swept AABBs and swept world transforms using the Minkowski Sum method or gift wrap method.  With the Minkowski Sum you could do an iterative deformation for orientation movement, but only if you wanted to handle potentially concave hulls.
  2. Check for sweep intersections.
  3. For each sweep intersection calculate the time of impact of the two hulls.  The best method in my opinion is a convex cast, but there are more accurate ways out there.  The time of impact should have a slight slop to it.
  4. Then take each of those and update positions by time of impact and accumulate lost time as delta time minus time of impact, and update the rest by delta time plus time lost in any previous TOI (time of impact) collisions.
  5. Cache world transforms.
  6. Check for intersections.
  7. Generate collision contacts and update arbiters.
  8. Do response.

As you can clearly see, the only complex part is updating of the position based on advanced timing.  After that, it’s just like the discrete method, as none of the actual collision handling is different.  We’re just making sure there is no tunneling, that is, nothing can pass through anything else.  This is a huge undertaking.  As such, this is a big milestone.  I’ll keep people in the loop about progress as much as I feel like it.

Peace  🙂



October 16, 2008

So, I’ve been hard at work, and here’s what I’ve done.  First off, on campus, Dr. Liow REALLY wanted me to do a python project.  Why?  Because in some strange way it made me a freeloader to hang around in a club where the point was messing with python without actually doing Python.  I was really just there to talk to other people, mostly students there.  So, what I did to make him happy, was Python bindings for my physics engine and  a small demo.  YAY!  Here are some shots of knocking over a tower.  Remember, running in Python, but using my physics engine, which as you probably know is in C.  Check this shit:

oh, yeah, it's a tower.

oh, yeah, it's a tower

let's get ready to rumble, bitches!

let's get ready to rumble, bitches!

steady now...

steady now...

we're down, son!

we're down, son!

It wasn’t that hard.  I used SWIG based on the apparently valid opinions on it from this guy and this guy.  I was really pleased with the results! 🙂  The one thing I had trouble coming to terms with though, was callbacks.  I have world_something_callback(rat_world *world,whateverargs) calls, and setting up a callback system was hard  and ultimately something I failed at.  Here’s why:

I have a function used to set the callback: rat_world_set_body_left_world_callback(rat_world *world,body_left_world_callback *cb).   What I wanted to do was pass a C function that could call a python callable object and convert the arguments properly.  The store the PyObject somewhere I could get back to it.  All went according to plan, until I tried to set up the arguments.  I found I simply didn’t know how I could convert the C (rat_world *) pointer in to the PyObject rat_world class set up by SWIG.  No matter how I diced it, I just couldn’t find a way to pull the whole thing together.  I messed with the idea of SWIG_NewPointerObj, but even with that external runtime header, I could access the type information, because it only defined functions.  Still, the rest of it looks fine.  I’ll be in touch.  And on the C front, I’ve fixed all that sloppy inlining and some really major memory problems with constraints.  So, back to work.

Bye. :3