Adding a shape to a body

[Bliddo]

Hi.
I'm trying to figure out how to add at runtime a shape to a body, keeping the distance from the two objects... I'll try to explain
I have the player floating on the screen, when it collides with an object, the player can pick up the object by pressing one button.
Right now I'm using this:

Code: Select all

int grabObject(cpShape *a, cpShape *b, cpContact *contacts, int numContacts, cpFloat normal_coef, void *data)
{
	if(_objCaught == false && _beamOn)
	{
		b->body = a->body;

		_objCaught = true;
	}

	return _objCaught;
}

The player and the object are defined as a box with coords in [1, -1]. The player has a cone shape attached to it, that should work as a winch, when an object collides with this cone, the above code is called.
Right now the object is picked up, but it translates right under the player's position, I want it to stay at the relative distance from the player that it is when the player picks it up... so, in other words, I want the object to stay attached to the winch, not under the player.
Any ideas?


Thanks

[slembcke]

You either need to make a new shape with an offset, or set the offset on the original shape. You might also want to adjust the mass and moment of inertia of the player character.

[Bliddo]

Thanks.

Though I'm considering using a joint, it would make things easier, but I can't quite figure out where to put the two anchors.
I tried a pinjoint, with the two anchors set to cpvzero, the object is grabbed correctly, but while moving, the object is free to rotate around the player, like it was connected by a rod. How can I make the joint stiff?

[slembcke]

You'll need at least two joints to make it stiff, and that means that it won't really be that stiff. Not using joints would be simpler, faster, and more numerically stable.

[Bliddo]

I see.
Which is the best way to set an offset of a shape?
I tried this, but I doesn't work fine

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cpPolyShape *poly = (cpPolyShape *)b;
		
		int num = poly->numVerts;
		cpVect *verts = poly->verts;
		
		cpVect offset;
		offset.x = b->body->p.x - a->body->p.x;
		offset.y = b->body->p.y - a->body->p.y;
		
		for(int i=0; i<num; i++)
			verts[i] = cpvadd(verts[i], offset);

[slembcke]

You need to take into account the current rotation of the shapes as well.

Also, why this:

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      cpVect offset;
      offset.x = b->body->p.x - a->body->p.x;
      offset.y = b->body->p.y - a->body->p.y;

Instead of just this:

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cpVect offset = cpvsub(b->body->p, a->body->p);