Simulating the Physics World

Simulating the physics scene

Now, we’ve only had some theory so far, but haven’t seen any simulation yet. To simulate, we will need to keep calling the quickStep(stepSize) function on the OdeWorld instance. stepSize is how much time should be simulated in one step. To get the most stable simulation, it is recommended that the stepSize be kept constant.

The problem with using the delta time of a task to step the simulation is that the time between tasks might not be consistent. To get around this, a deltaTime accumulator is used to figure out how many steps must be taken. When a step is performed, the world is iterated a few times, you can specify how much times the world is being iterated by calling the setQuickStepNumIterations(num) function on the OdeWorld instance.

Here’s a small example showing a simple simulation showing an iron ball falling from a ridge:

// To keep the C++ samples short, we assume a running Panda environment,
// with "framework", "window", "camera" and "taskMgr" variables in the
// global scope. Likewise, only the includes relevant to this chapter
// are shown. Check the beginning of the manual for a tutorial on making
// a full Panda3D C++ app.
// Sample entry point: simulation()

#include "odeWorld.h"
#include "odeBody.h"
#include "odeMass.h"

OdeBody *body;
OdeWorld world;
NodePath sphere;
PT(ClockObject) globalClock = ClockObject::get_global_clock();

// Create an accumulator to track the time since the sim
// has been running
float deltaTimeAccumulator = 0.0f;

// This stepSize makes the simulation run at 90 frames per second
float stepSize = 1.0f / 90.0f;

AsyncTask::DoneStatus simulationTask(GenericAsyncTask *task, void *data);

void simulation() {
  // Load the cube where the ball will fall from
  NodePath cube window->load_model(framework.get_models(), "models/box");
  cube.reparent_to(window->get_render());
  cube.set_scale(0.25, 0.25, 0.25);
  cube.set_pos(0, 0, 0);

  // Load the smiley model which will act as our iron ball
  sphere = window->load_model(framework.get_models(), "models/smiley");
  sphere.reparent_to(window->get_render());
  sphere.set_scale(0.25, 0.25, 0.25);
  sphere.set_pos(0, 0, 1);

  // Setup our physics world and the body
  world.set_gravity(0, 0, -9.81);
  body = new OdeBody(world);
  OdeMass M;
  M.set_sphere(7874, 1.0);
  body->set_mass(M);
  body->set_position(sphere.get_pos(window->get_render()));
  body->set_quaternion(sphere.get_quat(window->get_render()));

  // Set the camera position
  camera.set_pos(80, -20, 40);
  camera.look_at(0, 0, 0);

  PT(GenericAsyncTask) simulationTaskObject =
    new GenericAsyncTask("startup task", &simulationTask, nullptr);
  simulationTaskObject->set_delay(2);
  taskMgr->add(simulationTaskObject);
}

// The task for our simulation
AsyncTask::DoneStatus simulationTask (GenericAsyncTask *task, void *data) {
  // Set the force on the body to push it off the ridge
  body->set_force(0, min(pow(task->get_elapsed_time(),4) * 500000 - 500000, 0), 0);
  // Add the deltaTime for the task to the accumulator
  deltaTimeAccumulator += globalClock->get_dt();
  while (deltaTimeAccumulator > stepSize ) {
    // Remove a stepSize from the accumulator until
    // the accumulated time is less than the stepsize
    deltaTimeAccumulator -= stepSize;
    // Step the simulation
    world.quick_step(stepSize);
  }
  // set the new positions
  sphere.set_pos_quat(window->get_render(),
    body->get_position(), body->get_quaternion());
  return AsyncTask::DS_cont;
}