[Box2D] Change density of a b2Body (and another questions ;))

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[Box2D] Change density of a b2Body (and another questions ;))

(8 posts) (4 voices)

Started 2 years ago by badawe
Latest reply from badawe

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badawe
Member

Someone know how? The version of box2d coming with the cocos2d 8.2, seens not have this, the only way i can do that, is destroying the body, and create it again, but i lost all joints attached to the body!

I seens in some places in the old versions of box2d you can do something like get the b2Shape and just change the density value, but is not work anymore =/

2 - How i know what is the version of box2D at my project?

3 - How we can get the last version of box2d working with cocos2d?

4 - is the rik doing all this stuff?

Posted 2 years ago #
eshirt
Member

badawe - in b2Fixture.h, there's void b2Fixture::SetDensity(float32 density) Is that not what you need? I don't see a way to SetDensity on a body though, just a Fixture.

2 - in b2Settings.ccp you'll find a string: b2Version b2_version = {2, 1, 0}; So, it's running 2.1.0.

3 & 4 - I've been having trouble with box2d as well - made even harder by the lack of documentation for this version. I thought about going back to a previous (better documented) version, but I never actually did it.

I would imagine that you could just replace the Box2d files with the older version and it would work just fine. There doesn't seem to be any direct link between cocos2d and box2d.

Hope that helps!

Posted 2 years ago #
badawe
Member

@eshirt THANKS for the fast reply man!

1 - my b2Fixture don't have this method, only in the newer versions of box2d, can you see the version you are using?

2 - yeah, my version is the: b2Version b2_version = {2, 1, 0};

3 - I try this, getting the last box2d from the repository, to use the new method b2Body->Freeze() but i get so many erros, i think box2d is change a lot from 2.1.0 for the svn repository =/

Code:
error: cannot convert 'b2PolygonShape*' to 'b2ShapeType' in assignment

Code:
error: 'struct b2FixtureDef' has no member named 'shape'

Code:
error: 'class b2Body' has no member named 'SetTransform'

Seens now you have 2 methods ->SetPosition and ->SetAngle

Code:
error: no matching function for call to 'b2World::b2World(b2Vec2&, bool)'

The worlds now is created by another way oO

Code:
error: no matching function for call to 'GLESDebugDraw::GLESDebugDraw(int)'

The Debug Draw not working on the same way to.

Code:
error: cannot declare variable 'callback' to be of abstract type 'QueryCallback'

Code:
error: 'class b2World' has no member named 'QueryAABB'

Code:
Box2D/Dynamics/b2World.h:222: error: 'void b2World::DrawDebugData()' is private

Code:
error: no matching function for call to 'b2Body::GetMassData(b2MassData*)'

Posted 2 years ago #
eshirt
Member

Wow, yeah - it looks like they've changed a lot in the latest SVN build. It's weird though, I'm also running 2.1.0 and I have all those methods SetDesnity, SetTransform, etc. It makes it very confusing to make those kind of major changes without incrementing the version number...

I am using the build that came with the cocos2d 0.8.1 download. I don't know if it would break anything that you've already done, but you may try using that version since it seems like it's a little more stable.

http://code.google.com/p/cocos2d-iphone/downloads/list for older versions.

Although, if you're trying to use the new freeze method, I don't know if you'd want to do that. I'm assuming you've tried posting on the box2d forums also? They don't seem as active/helpful as these forums though :)

Posted 2 years ago #

badawe
Member

@eshirt all this think seens quite confuse!

By now the only way i can change the density of one body is like that:

if (!bSolid)
	{

		for (b2Fixture *fixture = boxBody->GetFixtureList(); fixture != NULL; fixture = fixture->GetNext())
		{
			boxBody->DestroyFixture(fixture);
		}

		b2FixtureDef fixtureDef;
		fixtureDef.density		= 0.0;
		fixtureDef.userData		= @"Box";
		fixtureDef.filter.categoryBits = box_type;
		fixtureDef.filter.maskBits = box_type | body_type;

		b2PolygonShape dynamicBox;
		dynamicBox.SetAsBox((bWidth/2)/PTM_RATIO, (bHeight/2)/PTM_RATIO);
		fixtureDef.shape = &dynamicBox;	

		boxBody->CreateFixture(&fixtureDef);
		bSolid = YES;

		NSLog(@"Make this box Static");
	}

Posted 2 years ago #

blackmouth
Member
Badawe,

I saw your post here. http://code.google.com/p/box2d/issues/detail?id=20
have you tried the SetMassData? I'm curious if it really is that difficult to compute the mass from the density, or if it will even have the same effect.

density = mass/volume, so mass = density * volume, but in reality you just want your density to be Zero from your code above.

I'm not sure if these would be relevant to the version of Box2D you are using.
```
void b2Body::SetMassData ( const b2MassData * data )   	

Set the mass properties. Note that this changes the center of mass position. If you are not sure how to compute mass properties, use SetMassFromShapes. The inertia tensor is assumed to be relative to the center of mass. You can make the body static by using a zero mass.

Parameters:
    	massData  the mass properties.

void b2Body::SetMassFromShapes 	( 		 )  	

Compute the mass properties from the attached fixture. You typically call this after adding all the fixtures. If you add or remove fixtures later, you may want to call this again. Note that this changes the center of mass position.
```
Maybe you can just create a new shape and add the new shape to your body with a new density using SetMassFromShapes, but I believe that is no longer the case with the box2d version shipped with cocos2d 8.2, as I don't see the method in b2body.cpp.

Here is another Post that links to the one above with Erin Catto mentioning ResetMass();

http://www.box2d.org/forum/viewtopic.php?f=4&t=3719

Anyways, I'm curious as to what your solution is. :)
Posted 2 years ago #

onedayitwillmake
Member

Hi badawe, I do that a lot in littlephysics - here's my procedure.
Basically I create fake mass data, by copying the current mass data.

Then I set the new mass data for the body.
Where it gets weird, and it doesn't look like a concious design change on their part but more maybe an oversight is that GetMassData sends back a struct, but SetMassData requires a pointer.

// First i store it for all bodies, not required but that's why i did
b2MassData massData = body->GetFixtureList()->GetMassData();
LittlePiece._massData = massData; //LittlePiece is an atlassprite subclass

// Later on i change it like this and reset it
// This happens here when you grab a piece

LittlePiece* shape = (LittlePiece*) closestBody->GetUserData();
	// Overwrite the mass data
		b2MassData* fakeMass = new b2MassData(); // Very strange, you can't pass a struct so I have to create this using new

// Change or keep whatever massdata variables i want
		fakeMass->mass = 0.1f;
		fakeMass->I = shape._massData.I;
		fakeMass->center = shape._massData.center
// Set it
		closestBody->SetMassData(fakeMass);

Hope that helps!

oneday,
http://littlephysics.com

Posted 2 years ago #

badawe
Member
Thanks for all the help guys! But i can´t get things work yet, i need make the box solid again, like if i set the density to 0 when i create it, if i change the mass to 0.0 thinks like get heavier and i get some errors, and the box not get solid state!

I get this error after change the density to 0.0:
```
Assertion Failed: (kEqualized > 1.192...), function b2ContactSolver, file:...
```
But if i change the density to 60.0 or 0.000001 things works =/
I don´t know how do that yet. If someone know another way to do that.
Posted 2 years ago #

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