Simple Bouncing Sphere

Introduction

This article describes how to make a sphere that falls onto a plane and bounces. This is about as simple as it gets, but it shows the general structure of a dynamic simulation using ODE.

Sample Code

This code is written in the style of the ODE demos, and uses drawstuff to show the simulation.

First the objects for the simulation must be declared. You must have a world, a space to put everything in, a body with associated geometry and mass, and a joint group to store the contact joints that are created during a collision.

#include <ode/ode.h>
#include <drawstuff/drawstuff.h>
// dynamics and collision objects
static dWorldID world;
static dSpaceID space;
static dBodyID body;	
static dGeomID geom;	
static dMass m;
static dJointGroupID contactgroup;

When the collision system detects that two objects are colliding, it calls this routine which determines the points of contact and creates temporary joints. The surface parameters of the joint (friction, bounce velocity, CFM, etc) are also set here.

// this is called by dSpaceCollide when two objects in space are
// potentially colliding.
static void nearCallback (void *data, dGeomID o1, dGeomID o2)
{
   dBodyID b1 = dGeomGetBody(o1);
   dBodyID b2 = dGeomGetBody(o2);
   dContact contact;  
   contact.surface.mode = dContactBounce | dContactSoftCFM;
   // friction parameter
   contact.surface.mu = dInfinity;
   // bounce is the amount of "bouncyness".
   contact.surface.bounce = 0.9;
   // bounce_vel is the minimum incoming velocity to cause a bounce
   contact.surface.bounce_vel = 0.1;
   // constraint force mixing parameter
   contact.surface.soft_cfm = 0.001;  
   if (int numc = dCollide (o1,o2,1,&contact.geom,sizeof(dContact))) {
       dJointID c = dJointCreateContact (world,contactgroup,&contact);
       dJointAttach (c,b1,b2);
   }
}

This function is called at the start of the simulation to set up the point of view of the camera.

// start simulation - set viewpoint
static void start()
{
   static float xyz[3] = {2.0f,-2.0f,1.7600f};
   static float hpr[3] = {140.000f,-17.0000f,0.0000f};
   dsSetViewpoint (xyz,hpr);
}

This is the main simulation loop that calls the collision detection function, steps the simulation, resets the temporary contact joint group, and redraws the objects at their new position.

// simulation loop
static void simLoop (int pause)
{
   const dReal *pos;
   const dReal *R;
   // find collisions and add contact joints
   dSpaceCollide (space,0,&nearCallback);
   // step the simulation
   dWorldQuickStep (world,0.01);  
   // remove all contact joints
   dJointGroupEmpty (contactgroup);
   // redraw sphere at new location
   pos = dGeomGetPosition (geom);
   R = dGeomGetRotation (geom);
   dsDrawSphere (pos,R,dGeomSphereGetRadius (geom));
}

When the program starts, the callbacks are set up, everything is initialized, and then the simulation is started.

int main (int argc, char **argv)
{
   // setup pointers to drawstuff callback functions
   dsFunctions fn;
   fn.version = DS_VERSION;
   fn.start = &start;
   fn.step = &simLoop;
   fn.stop = 0;
   fn.command = 0;
   fn.path_to_textures = "../../drawstuff/textures";

   dInitODE ();
   // create world
   world = dWorldCreate ();
   space = dHashSpaceCreate (0);
   dWorldSetGravity (world,0,0,-0.2);
   dWorldSetCFM (world,1e-5);
   dCreatePlane (space,0,0,1,0);
   contactgroup = dJointGroupCreate (0);
   // create object
   body = dBodyCreate (world);
   geom = dCreateSphere (space,0.5);
   dMassSetSphere (&m,1,0.5);
   dBodySetMass (body,&m);
   dGeomSetBody (geom,body);
   // set initial position
   dBodySetPosition (body,0,0,3);
   // run simulation
   dsSimulationLoop (argc,argv,352,288,&fn);
   // clean up
   dJointGroupDestroy (contactgroup);
   dSpaceDestroy (space);
   dWorldDestroy (world);
   dCloseODE();
   return 0;
}