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SetMassData problem

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viqtor

Post subject: SetMassData problem

Posted: Sat Jun 11, 2011 1:45 am

Joined: Mon May 23, 2011 9:41 am
Posts: 18

Hi,
I need to set the mass of a specific BodyShape. I tried to do so using the b2body.SetMassData:

Code:

   
var MassData:b2MassData = new b2MassData(); 
var b2Vec:b2Vec2 = new b2Vec2(0);
b2Vec.v2 = new V2(0, 0);
MassData.center = b2Vec;
MassData.mass = mass; 
MassData.I = 0;
b2body.SetMassData(MassData); 

After I set the mass, the body starts acting like SetFixedRotation=true.
How can I solve this?

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irresistible force

Post subject: Re: SetMassData problem

Posted: Sat Jun 11, 2011 2:26 am

Joined: Tue Jun 24, 2008 8:25 pm
Posts: 1518
Location: Tokyo

Yes, setting the rotational inertia value I to zero will cause that.

What I would do is first set all the fixtures in the body as normal, to let the engine calculate what the mass and inertia should be with say, a density of 1.
Using GetMassData you can get the values that were set. You might find for example that the body has a mass of 2.5.
Calculate the factor that the mass needs to be multiplied by to become the mass you want. So if you want a mass of 10, that would be a factor of 4.
Do that for both the mass and inertia, then set it back in the body with SetMassData.

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viqtor

Post subject: Re: SetMassData problem

Posted: Sat Jun 11, 2011 3:27 am

Joined: Mon May 23, 2011 9:41 am
Posts: 18

Thanks a lot!

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irresistible force

Post subject: Re: SetMassData problem

Posted: Sat Jun 11, 2011 5:16 am

Joined: Tue Jun 24, 2008 8:25 pm
Posts: 1518
Location: Tokyo

irresistible force wrote:

Yes, setting the rotational inertia value I to zero will cause that.

What I would do is first set all the fixtures in the body as normal, to let the engine calculate what the mass and inertia should be with say, a density of 1.
Using GetMassData you can get the values that were set. You might find for example that the body has a mass of 2.5.
Calculate the factor that the mass needs to be multiplied by to become the mass you want. So if you want a mass of 10, that would be a factor of 4.
Multiply both the mass and inertia by that factor, then set them back in the body with SetMassData.

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