Box2D Forums

  private final Vec2  gravity = new Vec2(0.0f, -10.0f);
  private final World world;
  private Fixture     wheel_fixture;
  private Fixture     floor_fixture;
  private Fixture     roof_fixture;
  private Fixture     left_fixture;
  private Fixture     right_fixture;
  private Fixture     box_fixture;
  private LineJoint   joint;

  public Wheel()
  {
    this.world = new World(this.gravity, true);

    /*
     * Ground.
     */

    {
      final BodyDef bd = new BodyDef();
      bd.position.x = 20.0f;
      bd.position.y = 2.0f;
      bd.type = BodyType.STATIC;
      final Body b = this.world.createBody(bd);
      final PolygonShape s = new PolygonShape();

      s.setAsBox(20f, 0.5f, new Vec2(0.0f, 0.0f), 0.0f);
      final FixtureDef floor = new FixtureDef();
      floor.density = 1.0f;
      floor.friction = 0.5f;
      floor.shape = s;
      this.floor_fixture = b.createFixture(floor);

      s.setAsBox(20f, 0.5f, new Vec2(0.0f, 17.0f), 0.0f);
      final FixtureDef roof = new FixtureDef();
      roof.density = 1.0f;
      roof.friction = 0.5f;
      roof.shape = s;
      this.roof_fixture = b.createFixture(roof);

      s.setAsBox(0.5f, 20.0f, new Vec2(-18.0f, 0.0f), 0.0f);
      final FixtureDef left = new FixtureDef();
      left.density = 1.0f;
      left.friction = 0.5f;
      left.shape = s;
      this.left_fixture = b.createFixture(left);

      s.setAsBox(0.5f, 20.0f, new Vec2(18.0f, 0.0f), 0.0f);
      final FixtureDef right = new FixtureDef();
      right.density = 1.0f;
      right.friction = 0.5f;
      right.shape = s;
      this.right_fixture = b.createFixture(right);
    }

    /*
     * Wheel.
     */

    {
      final CircleShape wheel_shape = new CircleShape();
      wheel_shape.m_radius = 2.0f;

      final FixtureDef wheel_fixture_def = new FixtureDef();
      wheel_fixture_def.density = 1.0f;
      wheel_fixture_def.friction = 0.5f;
      wheel_fixture_def.shape = wheel_shape;

      final BodyDef wheel_body = new BodyDef();
      wheel_body.position.x = 20.0f;
      wheel_body.position.y = 10.0f;
      wheel_body.type = BodyType.DYNAMIC;

      final Body b = this.world.createBody(wheel_body);
      this.wheel_fixture = b.createFixture(wheel_fixture_def);
    }

    /*
     * Box above wheel.
     */

    {
      final PolygonShape box_shape = new PolygonShape();
      box_shape.setAsBox(1f, 1f, new Vec2(0.0f, 0.0f), 0.0f);

      final BodyDef box_body = new BodyDef();
      box_body.position.x = 20.0f;
      box_body.position.y = 15.0f;
      box_body.type = BodyType.DYNAMIC;
      box_body.fixedRotation = true;

      final FixtureDef box_fixture_def = new FixtureDef();
      box_fixture_def.density = 1.0f;
      box_fixture_def.friction = 0.5f;
      box_fixture_def.shape = box_shape;

      final Body b = this.world.createBody(box_body);
      this.box_fixture = b.createFixture(box_fixture_def);
    }

    /*
     * Joint.
     */

    {
      final LineJointDef jd = new LineJointDef();
      jd.initialize(
        this.box_fixture.getBody(),
        this.wheel_fixture.getBody(),
        this.wheel_fixture.getBody().getWorldCenter(),
        new Vec2(0.0f, 1.0f));
      jd.enableLimit = true;
      jd.enableMotor = true;
      jd.maxMotorForce = 1000.0f;
      jd.motorSpeed = 1.0f;

      this.joint = (LineJoint) this.world.createJoint(jd);
    }
  }

#define GL_GLEXT_PROTOTYPES 1

#include <assert.h>
#include <stdio.h>

#include <GL/glut.h>
#include <GL/gl.h>
#include <GL/glext.h>
#include <err.h>

#include <Box2D/Box2D.h>

static b2World *world;
static b2Vec2 gravity;
static b2Fixture *floor_fixture;
static b2Fixture *roof_fixture;
static b2Fixture *left_fixture;
static b2Fixture *right_fixture;
static b2Fixture *wheel_fixture;
static b2Fixture *box_fixture;
static b2WheelJoint *joint;

static void
init (void)
{
  gravity.x = 0.0f;
  gravity.y = -10.0f;
  world = new b2World(gravity);

  /*
   * Ground
   */

  {
    b2BodyDef bd;
    b2PolygonShape s;
    b2Vec2 v;

    bd.position.x = 20.0f;
    bd.position.y = 2.0f;
    bd.type = b2_staticBody;

    b2Body *b = world->CreateBody(&bd);

    v.x = 0.0f;
    v.y = 0.0f;
    s.SetAsBox(20.0f, 0.5f, v, 0.0f);
   
    b2FixtureDef floor;
    floor.density = 1.0f;
    floor.friction = 0.5f;
    floor.shape = &s;
    floor_fixture = b->CreateFixture(&floor);

    v.x = 0.0f;
    v.y = 17.0f;
    s.SetAsBox(20.0f, 0.5f, v, 0.0f);
 
    b2FixtureDef roof;
    roof.density = 1.0f;
    roof.friction = 0.5f;
    roof.shape = &s;
    roof_fixture = b->CreateFixture(&roof);

    v.x = -18.0f;
    v.y = 0.0f;
    s.SetAsBox(0.5f, 20.0f, v, 0.0f);
 
    b2FixtureDef left;
    left.density = 1.0f;
    left.friction = 0.5f;
    left.shape = &s;
    left_fixture = b->CreateFixture(&left);

    v.x = 18.0f;
    v.y = 0.0f;
    s.SetAsBox(0.5f, 20.0f, v, 0.0f);
 
    b2FixtureDef right;
    right.density = 1.0f;
    right.friction = 0.5f;
    right.shape = &s;
    right_fixture = b->CreateFixture(&right);
  }

  /*
   * Wheel
   */

  {
    b2CircleShape wheel_shape;
    wheel_shape.m_radius = 2.0f;

    b2FixtureDef wheel_fixture_def;
    wheel_fixture_def.density = 1.0f;
    wheel_fixture_def.friction = 0.5f;
    wheel_fixture_def.shape = &wheel_shape;

    b2BodyDef wheel_body;
    wheel_body.position.x = 20.0f;
    wheel_body.position.y = 10.0f;
    wheel_body.type = b2_dynamicBody;

    b2Body *b = world->CreateBody(&wheel_body);
    wheel_fixture = b->CreateFixture(&wheel_fixture_def);
  }

  /*
   * Box above wheel.
   */

  {
    b2PolygonShape box_shape;
    b2Vec2 v;

    v.x = 0.0f;
    v.y = 0.0f;
    box_shape.SetAsBox(1.0f, 1.0f, v, 0.0f);

    b2BodyDef box_body;
    box_body.position.x = 20.0f;
    box_body.position.y = 15.0f;
    box_body.type = b2_dynamicBody;
    box_body.fixedRotation = true;

    b2FixtureDef box_fixture_def;
    box_fixture_def.density = 1.0f;
    box_fixture_def.friction = 0.5f;
    box_fixture_def.shape = &box_shape;

    b2Body *b = world->CreateBody(&box_body);
    box_fixture = b->CreateFixture(&box_fixture_def);
  }

  /*
   * Joint.
   */

  {
    b2Vec2 v;
    b2WheelJointDef jd;

    v.x = 0.0f;
    v.y = 1.0f;

    jd.Initialize(
      box_fixture->GetBody(),
      wheel_fixture->GetBody(),
      wheel_fixture->GetBody()->GetWorldCenter(),
      v);

    jd.motorSpeed = 100.0f;
    jd.maxMotorTorque = 1000.0f;
    jd.enableMotor = true;

    joint = (b2WheelJoint *) world->CreateJoint(&jd);
  }
}

static void drawCircle(float radius)
{
  glBegin(GL_LINES);
  {
    for (int index = 0; index < 180; ++index) {
      double x0 = radius * cos(index);
      double y0 = radius * sin(index);
      glVertex3d(x0, y0, 0.0f);
      double x1 = radius * cos(index + 0.1);
      double y1 = radius * sin(index + 0.1);
      glVertex3d(x1, y1, 0.0f);
    }
  }
  glEnd();
}

static void drawFixture(b2Fixture *fix)
{
  b2Body *b = fix->GetBody();
  b2Shape *s = fix->GetShape();

  glPushMatrix();
  {
    b2Vec2 p = b->GetWorldCenter();
    glColor3d(1.0f, 0.0f, 0.0f);
    glTranslated(p.x, p.y, 0.0);
    drawCircle(0.125f);
  }
  glPopMatrix();

  glPushMatrix();
  {
    glColor3d(0.0f, 1.0f, 0.0f);

    b2Shape::Type type = s->GetType();

    switch (s->GetType()) {
      case b2Shape::e_circle:
      {
        b2CircleShape *c = (b2CircleShape *) s;
        b2Vec2 p = b->GetWorldPoint(c->m_p);
 
        glTranslated(p.x, p.y, 0.0);
        glRotated(b->GetAngle() * (180 / 3.14), 0, 0, 1);
 
        drawCircle(c->m_radius);
        glBegin(GL_LINES);
        {
          glVertex3d(0.0, 0.0, 0.0);
          glVertex3d(0.0, 1.0, 0.0);
        }
        glEnd();
      }
      case b2Shape::e_polygon:
      {
        b2PolygonShape *ps = (b2PolygonShape *) s;
 
        glBegin(GL_LINE_LOOP);
        {
          b2Vec2 *vs = ps->m_vertices;
          for (int index = 0; index < ps->GetVertexCount(); ++index) {
            b2Vec2 v = b->GetWorldPoint(vs[index]);
            glVertex3d(v.x, v.y, 0.0);
          }
        }
        glEnd();
      }
    }
  }
  glPopMatrix();
}

static void
display (void)
{
  world->Step(1.0 / 60.0, 8, 4);

  glClearColor(0.2, 0.2, 0.2, 1.0);
  glClear(GL_COLOR_BUFFER_BIT);

  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  glOrtho(0, 40, 0, 20, 0, 100);

  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();

  drawFixture(floor_fixture);
  drawFixture(roof_fixture);
  drawFixture(left_fixture);
  drawFixture(right_fixture);
  drawFixture(wheel_fixture);
  drawFixture(box_fixture);
  drawJoint(joint);

  glutSwapBuffers();
  glutPostRedisplay();
}

int
main (int argc, char *argv[])
{
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
  glutInitWindowSize(640, 480);
  glutCreateWindow("B2D");
  glutDisplayFunc(display);
  init();
  glutMainLoop();

  return 0;
}