How to tell which is body A and which is body B upon collision

Tom2012 · April 2013

Hi,

My Box2D collision code is full of code like this:

		local fixtureA = event.fixtureA
		local fixtureB = event.fixtureB
		local bodyA = fixtureA:getBody()
		local bodyB = fixtureB:getBody()
 
		if (bodyA.name == "Drag block button" and bodyB.name == "Drag block") then
 
			-- do something
 
		elseif(bodyB.name == "Drag block button" and bodyA.name == "Drag block") then
 
			-- do something
		end

Which doesn't seem like an efficient way to do it.

How do you determine which body is body A and which is body B in a collision?

Thanks

tkhnoman · April 2013

For me, i do some swapping.

For example, i make sure that "Drag block" is at B, so if i found "Drag block" at A, then i do something like.
local temp = bodyA
bodyA = bodyB
bodyB = temp

hgvyas123 · April 2013

i usually put if else condition but i like the idea of @tkhnoman

Tom2012 · April 2013

Thanks for the replies.

@hgvyas123 - glad to know I'm not the only one doing that.

It hasn't caused any performance issues I can see in my game so probably keep doing it.

I'm still curious about Box2D though... If you have two bodies moving towards each other, how does the engine determine which is A and B?

ar2rsawseen · April 2013

@Tom2012 as far as I've noticed, the body that was created first is always A, and the body that was created after is B, but I have not found any confirmation for this

john26 · April 2013

I normally add a "parent" entry to the body table which is the sprite that "owns" the body. Then I can do, for example,

if body.parent==ego

so ego.body.parent==ego. That way I can test directly against the name of the sprite in the source code rather than having to introduce a "name" string. Saves typing!

WauloK · April 2013

@tkhnoman I'm pretty sure in lua it's valid to do:

bodyA,bodyB = bodyB,bodyA

Someone can correct me if not

tkhnoman · April 2013

@WauloK Yep, you're correct. Thanks.

hgvyas123 · May 2013

i think this thread is turning into tips & tricks

Tom2012 · May 2013

@tkhnoman I'm pretty sure in lua it's valid to do:

bodyA,bodyB = bodyB,bodyA

Someone can correct me if not

Thanks for that. Can't wait to get back from work and try this out.

Tom2012 · May 2013

Hmm...

The code below doesn't work -

		if (bodyA.name,bodyB.name == "Ground","feet") then
 
			self:heroOnGround()
 
		end

How do you use bodyA,bodyB = bodyB,bodyA?

hgvyas123 · May 2013

bodyA,bodyB = bodyB,bodyA is useful for the swapping just alternate way of

local temp = bodyA
bodyA = bodyB
bodyB = temp

and it can not be use for compare

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