Bounce always becomes perpendicular to surface

l0calh05t · **Joined:** Tue Jul 22, 2008 11:36 pm **Posts:** 5

I decided to make a pong-like game with "improved physics", and decided to use Box2D. But for some reason, the ball always bounces perpendicular to the walls after a while. Is there any way to solve this?

If it helps, here's the code adapted to the testbed:

Code:

#ifndef PONG_H
#define PONG_H

class Pong : public Test
{
public:
   Pong()
   {
      m_world->SetGravity(b2Vec2(0.0f, 0.0f));

      b2BodyDef bodyDef;
      b2PolygonDef polyDef;
      bodyDef.position.Set(-51.f,0.f);
      polyDef.SetAsBox(1.f,38.5f);
      polyDef.friction = 0.1f;

      b2Body* framel = m_world->CreateBody(&bodyDef);
      framel->CreateShape(&polyDef);

      bodyDef.position.Set(51.f,0.f);
      b2Body* framer = m_world->CreateBody(&bodyDef);
      framer->CreateShape(&polyDef);

      bodyDef.position.Set(0.f,38.5f);
      polyDef.SetAsBox(51.f,1.f);
      b2Body* framet = m_world->CreateBody(&bodyDef);
      framet->CreateShape(&polyDef);

      bodyDef.position.Set(0.f,-38.5f);
      b2Body* frameb = m_world->CreateBody(&bodyDef);
      frameb->CreateShape(&polyDef);

      b2CircleDef circleDef;
      circleDef.radius = 1.5f;
      circleDef.friction = 0.4f;
      circleDef.density = 1.f;
      circleDef.restitution = 1.25f;

      bodyDef.position.Set(0.f,0.f);
      b2Body* ball = m_world->CreateBody(&bodyDef);
      ball->CreateShape(&circleDef);
      ball->SetMassFromShapes();
      ball->SetBullet(true);
      ball->SetLinearVelocity(b2Vec2(200.f,0.f));

      bodyDef.position.Set(-45.f,0.f);
      polyDef.SetAsBox(1.5f,6.f);
      polyDef.density = 10.f;
      polyDef.friction = 0.4f;
      circleDef.density = 5.f;
      circleDef.localPosition.Set(0.f,6.f);
      circleDef.restitution = 0.5f;

      b2Body* paddle1 = m_world->CreateBody(&bodyDef);
      paddle1->CreateShape(&polyDef);
      paddle1->CreateShape(&circleDef);
      circleDef.localPosition.Set(0.f,-6.f);
      paddle1->CreateShape(&circleDef);
      paddle1->SetMassFromShapes();

      bodyDef.position.Set(45.f,0.f);
      b2Body* paddle2 = m_world->CreateBody(&bodyDef);
      paddle2->CreateShape(&polyDef);
      paddle2->CreateShape(&circleDef);
      circleDef.localPosition.Set(0.f,6.f);
      paddle2->CreateShape(&circleDef);
      paddle2->SetMassFromShapes();

      b2PrismaticJointDef jointDef;
      b2Vec2 worldAxis(0.0f, 1.0f);
      jointDef.motorSpeed = 0.0f;
      jointDef.maxMotorForce = 10e4f;
      jointDef.enableMotor = true;
      jointDef.collideConnected = true;
      jointDef.Initialize(paddle1, frameb, paddle1->GetWorldCenter(), worldAxis);
      paddle1joint = (b2PrismaticJoint*)m_world->CreateJoint(&jointDef);
      jointDef.Initialize(paddle2, frameb, paddle2->GetWorldCenter(), worldAxis);
      paddle2joint = (b2PrismaticJoint*)m_world->CreateJoint(&jointDef);

      ball->AllowSleeping(false);
      paddle1->AllowSleeping(false);
      paddle2->AllowSleeping(false);
   }

   void Keyboard(unsigned char key)
   {
      switch (key)
      {
      case 'e':
         {
            paddle1joint->SetMotorSpeed(-100.f);
         }
         break;

      case 'd':
         {
            paddle1joint->SetMotorSpeed(100.f);
         }
         break;

      case 'i':
         {
            paddle2joint->SetMotorSpeed(-100.f);
         }
         break;

      case 'k':
         {
            paddle2joint->SetMotorSpeed(100.f);
         }
         break;
      }
   }

   static Test* Create()
   {
      return new Pong;
   }

   b2PrismaticJoint* paddle1joint;
   b2PrismaticJoint* paddle2joint;
};

#endif

Erin Catto · **Joined:** Thu Sep 06, 2007 12:34 am **Posts:** 2211

Thanks for writing the test case. This makes it much easier for me to look at the problem.

You are using a restitution greater than 1, so the velocity of the ball is hitting the maximum linear velocity limit of 200 meters/second (around 600mph). As the velocity is scaled down the perpendicular velocity is reduced since the parallel velocity wants to increase (restitution greater than 1).

l0calh05t · **Joined:** Tue Jul 22, 2008 11:36 pm **Posts:** 5

Erin Catto wrote:

Thanks for writing the test case. This makes it much easier for me to look at the problem.

np. That's exactly why I wrote it :-)

Quote:

You are using a restitution greater than 1, so the velocity of the ball is hitting the maximum linear velocity limit of 200 meters/second (around 600mph). As the velocity is scaled down the perpendicular velocity is reduced since the parallel velocity wants to increase (restitution greater than 1).

hm, makes sense, but i also tried restitution = 1, and the same problem occurred. ideally, only the restitution when colliding with one of the paddles should be > 1, but at the same time the paddles restitution when colliding with a wall should be low (a little bounce, but not too much)

Erin Catto · **Joined:** Thu Sep 06, 2007 12:34 am **Posts:** 2211

You can use joint limits to prevent the paddles from hitting the wall.

l0calh05t · **Joined:** Tue Jul 22, 2008 11:36 pm **Posts:** 5

I don't want to prevent it, i think it looks cool how they bounce of the walls if you move them too far :-)

Any ideas on what's causing the strange bouncing behavior? As long as it persists, I can't really do anything...

Erin Catto · **Joined:** Thu Sep 06, 2007 12:34 am **Posts:** 2211

Once you keep the velocity within the limit, you next need to set all friction values to zero. I would also setup the ball to have fixed rotation.

l0calh05t · **Joined:** Tue Jul 22, 2008 11:36 pm **Posts:** 5

But then i can't add spin to the ball to make it bounce in a different direction... I guess what I need is some fake physics...

shaktool · **Joined:** Sun Jan 20, 2008 7:52 pm **Posts:** 434

Yes, you definitely need some fake physics. In your case, probably a minimum horizontal velocity.

l0calh05t · **Joined:** Tue Jul 22, 2008 11:36 pm **Posts:** 5

I decided to go for a minimum angle (and a minimum velocity) instead. It seems to work.

Code:

void Step(Settings* settings)
   {
      Test::Step(settings);

      b2Vec2 ballVelocity = ball->GetLinearVelocity();
      float32 absX = fabs(ballVelocity.x);
      float32 length = ballVelocity.Length();

      const float32 xmul = 0.2f;
      const float32 ymul = (2.f*sqrt(6.f))/5.f;

      if(absX < 0.2f*length)
      {
         if(absX > 1e-6f)
         {
            float32 sgnX = (ballVelocity.x < 0.f) ? -1.f : 1.f;
            float32 sgnY = (ballVelocity.y < 0.f) ? -1.f : 1.f;
            ballVelocity.x = sgnX * xmul * length;
            ballVelocity.y = sgnY * ymul * length;
         }
         else
         {
            float32 sgnPos = (ball->GetPosition().x < 0.f) ? 1.f : -1.f;
            float32 sgnY = (ballVelocity.y < 0.f) ? -1.f : 1.f;
            ballVelocity.x = sgnPos * xmul * length;
            ballVelocity.y = sgnY * ymul * length;
         }
      }

      if(length < 10.f)
      {
         if(length > 1e-6f)
         {
            float32 mul = 10.f/length;
            ballVelocity *= mul;
         }
         else
         {
            float32 sgnPos = (ball->GetPosition().x < 0.f) ? 1.f : -1.f;
            ballVelocity.x = sgnPos * 1.f;
            ballVelocity.y = 0.f;
         }
      }
      else if(length > 30.f)
      {
         float32 mul = 30.f/length;
         ballVelocity *= mul;
      }

      ball->SetLinearVelocity(ballVelocity);
   }

Any suggestions on how to increase ball velocity on contact with one of the paddles? (probably something like increasing the minimum velocity after each ball-paddle contact)

Box2D Forums

Bounce always becomes perpendicular to surface

Who is online