How to write a plugin to get contact info of Box2D

1dot44mb

I need to get contact info of colliding bodies however it seems that it is not implemented in Gideros yet. I would like to write a C++plugin however i have not written a plugin for Gideros before and i wonder if it is possible to get the Box2D world state in the plugin and get the contact info (colliding points) out of it?

Thanks.

1dot44mb

Anyone? I really need help about this as i am stuck with my project at the moment.

ar2rsawseen

Well not exactly a plugin for existing box2d, but probably you'll need to create your own version of box2d as a plugin.
You'll need to take box2d source and than create a lua wrapper and compile as a plugin, which would be very tiresome, manageable, but I would not go that way without a deeper experience in C and Lua bridge.

Other options would be to do a pretty squeaky please to @atilim to wrap it and add in next Gideros version (which could take some time, because it's not a simple method of existing object, as most of the previous requests are, but it's a completely new object that needs to be implemented and wrapped).

Or @atilim could open source the box2d plugin, this way you'd only have to wrap contact list and recompile player with new plugin.

1dot44mb

@arturs, thank you very much for your answer and directions. I think best option is the third one as i am stuck and the project will not go further without contact info at this point. So, i hope @atilim shows up and gives us sources of box2d plugin.

In the meantime, i am using raycast function continuously around my object (rectangle) to get contact info, not the best but i am able to get contact points this way. It is not pixel perfect but contact info is not pixel perfect either as long as your object is not bullet.

Thanks again.

ar2rsawseen

@1dot44mb what shapes do you use for bodies, actually maybe this issue could be solved with geometry?
For example if two bodies are circles, you can simply take both centers as vector and calculate the coordinates of vector which length equals radius of one circle

1dot44mb

Thanks, good point, however the object is rectangle in my case. I only need contact info for that rectangle around the level. If a solution comes up please share.

ar2rsawseen

@1dot44mb can you provide a pic of example collision and what information do you exactly, need, maybe I'll come up with something

1dot44mb

http://gamedev.stackexchange.com/questions/558/implementing-a-wrapping-wire-like-the-worms-ninja-rope-in-a-2d-physics-engine

This is exactly my problem. I have a rope (a rectangle body) and it needs to wrap around the terrain. To wrap it (split it into to seperate rectangles) i need to get contact info. And when the collision ends, i need to join two splitted rectangles so it becomes one.

ar2rsawseen

Is there any particular reason you want to split a rectangle instead of using rope emulation (distance joint + rope joint)? that would (at least I think) act like a real rope itself.

Here's some discussion on rope topic
http://www.giderosmobile.com/forum/discussion/2171/is-there-a-rope-joint-alternative/p1

1dot44mb

I had implemented the chain solution (which looks like a rope) but it changes the gameplay hugely. The gameplay i want must be simple and the rope should only go left or right, but if i implemented (which i tried) the chain solution, there is too much realism I set the fixedRotation of each chain to true, and i achieved the same rectangle solution. However, this time i could not do the 'wrapping' feature because setFixedRotation is not implemented (I would change the fixedRotation to false whichever chain contacts the terrain, this way i would have the wrapping effect).

In short, the particular reason for rectangle is the desired gameplay. Rope swinging left and right as a whole.

ar2rsawseen

Oh yeah, I see what you mean.
And if you implement your own setFixedRotation method while @atilim releases new version? Would it be too inefficient?

For each link in the chain (each rectangle) you can create two bodies, one with fixed rotation other without. Set without fixed rotation body as inactive, so it does not consume any processing.

And then when you need to change fixed rotation, simply switch the bodies by setting on as active and other as inactive and copying their positions.

Just an idea

1dot44mb

That's a cool solution, i will try right away and let you know the result.

1dot44mb

I was not able to produce a tight, firm rope. It is saggy even though i set distance joints between links and between first and last link. I will work more on it and tell you if i achieve something. Thanks for your great help.

ar2rsawseen

It would be awesome to see a video or at least screenshot.

BTW distance joint is used to limit minimal distance and rope joint to limit maximal distance.

atilim

btw, I've implemented contact info. It'll be available with the next version (hopefully soon).

atilim

And currently I cannot publish box2d wrapper as a plugin only because of b2.DebugDraw.

1dot44mb

@atilim Thanks.

@arturs i will try to make a video of my problem and send it to you today.