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Hi,
How would you use ApplyForce for instance to move a player left, a player up and a player right.
I've tried:
_playerBody->ApplyForce(b2Vec2(14.0f,0.0f),_playerBody->GetWorldCenter());But all this does is move the player right and how would I limit the max force that can be applied?
Regards,
Steve
robodude666
Member
From what I remember about working with Box2D, ApplyLinearImpulse is the preferred way of moving something in a particular direction. Simply swap out ApplyForce for ApplyLinearImpulse.
In physics, an Impulse (P) is defined as P = Fs, where F is the Force in Newtons, and s is the length of application in seconds. If you set the impulse to be 10 Ns, then that is the same thing as applying a force of 1 N every 10 seconds (i.e every 600 frames).
Hi,
Thanks for that. This is what I've done:
b2Vec2 velocityVec = _playerBody->GetLinearVelocity(); // Get current player velocity
if(Moving)
{
if(_playerAction == kWalkingLeft && velocityVec.x <= 0)
{
velocityVec.x = -4.5;
}
else
{
if(_playerAction == kWalkingRight && velocityVec.x >=0)
{
velocityVec.x = 4.5;
}
else {
if(_playerAction == kJumping) // jumping
{
velocityVec.y = 12;
Moving = NO;
}
}
}
_playerBody->SetLinearVelocity(velocityVec);
}
[self UpdateCamera];Means the player has to come to a stop before being able to turn around, almost like sliding on ice, lol.
Is ApplyLinearImpulse a better method than using SetLinearVelocity?
Thanks again,
Steve
robodude666
Member
They are both actually very similar, but very different at the same time.
SetLinearVelocity hardcodes the number in. That is the velocity vector that will represent its movement for the next step. If you set it to go at 5 meters per second, it'll go at 5 meters per second regardless of the object's mass.
ApplyLinearImpulse adds the impulse to the existing velocity, but is modified by the object's mass (inverse of it, technically). This means that an impulse of 10 Ns applied on an object of mass 100 kg will have 10x less affect than on an object of mass 10. The other difference between the to is that ApplyLinearImpulse also allows you to set the point the force will be applied to. Applying it offset from the center of mass will add angular velocity (spin) to the body. This would be helpful in situations like a skateboard game where the character does a flip, etc.
So when choosing which one to use, keep the following in mind:
- If you want to add spin while making the object move in a particular direction ApplyLinearImpulse is your friend.
- If you use SetLinearVelocity and later change the mass of the object, and notice collisions are more extreme (objects move away faster) it's because SetLinearVelocity ignores mass and the object can apply more Force to something during a collision.
-robodude666
robodude666
Member
Because gravity and other forces are always affecting the body, there is no way to actually stop it proper but here are two possible solutions.
1) You can apply a negative velocity which will make it look like the object is standing still:
b2Vec2 vel = myBody->GetLinearVelocity() * -1;
myBody->SetLinearVelocity(vel);However, this won't work too well on free falling objects as gravity will still pull it down (it will appear as if it's falling reeeeeeeeeeeeeeeeaalllly slowly). If the body is moving only left/right you might be able to get away with it.
2) Alternatively, you can do something hacky like this and force Box2D to not update the body's position:
if(myBody->GetType() == b2_dynamicBody)
myBody->SetType(b2_staticBody);
else
myBody->SetType(b2_dynamicBody);This will toggle the body's type between a static (stationary) and dynamic (moving) body type. This will prevent the body from moving altogether, but it will also mean the body will no longer respond to being collided with.
Or, like Techy mentioned you could set the velocity to 0. Though, this will have the same result as method #1. Object will move slowly in the direction of gravity, but otherwise will be "standing still".
-robodude666
you can always do something like:
static float MAX_SPEED = 10.0f;
b2Body *player = ...;
b2Vec2 initVel = player->GetLinearVelocity();
if( initVel.x < MAX_SPEED ){
b2Vec2 f = player->GetWorldVector(b2Vec2(MAX_SPEED -initVel.x, 0.0f));
b2Vec2 p = player->GetWorldPoint(b2Vec2(0.0f, 0.0f));
player->ApplyForce(f, p);
}same with ApplyLinearImpulse
robodude666
Member
Off the top of my head, you could try adding a timed event. X seconds after the player jumps apply a downword impulse on them, or it could be triggered by the distance between them and the ground. If they jump high enough, they'll fall down significantly faster due to the applied impulse.
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