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Because the body would have zero mass, and any forces or impulses applied to it would need to be divided by the mass giving you an infinite amount of acceleration.
That's a good question actually. What is "works properly" in this case?
If the body really has no mass, then the correct response would be that the body's velocity should end up being infinite. If "works properly" is implying that it's giving you a finite result, then there is definitely a bug somewhere. If you print out the mass of a 0 density body, what does it say it is?
Dynamic bodies that would have no mass (due to either having no fixtures at all, or only fixtures without volume eg. edge/chain) are given a mass of 1. I believe this is done to allow them to remain dynamic without any natural mass of their own. Why rotational inertia is not handled in the same way though, is indeed a good question.
DifferentSparks
Member
By "works properly" i meant that the impulse is applied to the body.
i asked the mass by GetMass and the result is :
1.000000
but my density is 0 in the plist.
I had to use a 0 density body because this body is attached to another one(main body) by a revolute joint. and the main body reacts strangely to impulses and forces if the body attached with the joint has a density>0.
So ApplyLinearImpulse works, ApplyAngularImpulse doesn't but SetAngularVelocity does.
DifferentSparks
Member
Well, i have a boss, made by one big sprite (the boss's body) and one smaller(the tuft). i attached the tuft to the main body by a revolute joint,
revJointDef.Initialize(_tuftBody, _boss._bossBody,
_tuftBody->GetWorldPoint(b2Vec2(-self.contentSize.width/3/PTM_RATIO2, 0/PTM_RATIO2)) );and simply move around the boss and with him the tuft too. all seems to work, except when the density of the tuft's body is >0, also a value of 0.0001 has bad effects on the main body.
What are the bad effects? I'm guessing you want things to behave as if the tuft was weightless and not affect the boss at all? Physics engines are usually not designed to deal with bodies that have zero mass, so the best you could do is make the mass of the boss much larger than the tuft. For example, I would have expected that using an extremely small density for the tuft would be ok... what are the masses of the two bodies when you do that?
I can't tell from your code, but is the revolute joint anchor at the center of the boss body?
DifferentSparks
Member
A bad effect is for example that applying an angular impulse to the boss body , the mass of the tuft just stop the rotation after some second(is not enabled the angular limit).
Tuft's mass: 1.000000
Boss's mass: 20.550596
The boss density is 2.
Don't think i've anchored it
b2RevoluteJointDef revJointDef;
revJointDef.Initialize(_tuftBody, _boss._bossBody,
_tuftBody->GetWorldPoint(b2Vec2(-self.contentSize.width/3/PTM_RATIO2, 0/PTM_RATIO2)) );
revJointDef.lowerAngle = CC_DEGREES_TO_RADIANS(-30.0f);
revJointDef.upperAngle = CC_DEGREES_TO_RADIANS(30.0f);
revJointDef.enableLimit = false;
//revJointDef.enableMotor = true;
//revJointDef.motorSpeed = 0.0f;
// revJointDef.maxMotorTorque = 1000.0f;
_joint = (b2RevoluteJoint*) _world->CreateJoint(&revJointDef);The point that you are passing to Initialize is where the anchor will be, is that the center of the boss body? If so, couldn't it just be:
revJointDef.Initialize(_tuftBody, _boss._bossBody, _boss._bossBody->GetPosition() );Initialize requires that the two bodies be positioned in the correct place already. If they are not, you can set the anchor positions in each body yourself - this might be helpful: http://www.iforce2d.net/b2dtut/joints-revolute
DifferentSparks
Member
They are already in the correct position, the tuft position is based on the boss position
bodyDef.position = _boss._bossBody->GetWorldPoint(b2Vec2(64/PTM_RATIO2, _boss.contentSize.height/3.0/PTM_RATIO2));the anchor point then is not at the center of the boss but where the 2 sprites meet.
-self.contentSize.width/3/PTM_RATIO2, 0/PTM_RATIO2 // with self that is tuft's spriteCould be that i attached the boss to the tuft and not the tuft on the boss? or doesn't make difference?
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