Complex polygons with Box2D

[plandry]

Hi all!

I'm trying to fix different shapes(triangle) in the same body so to create a complex polygon. Does anybody know how to do it?

Thanks.

[julapy]

Sounds like you need to work out the delaunay triangulation within the complex polygon.

Vanderlin has done some work with this,

http://toddvanderlin.com/2009/04/delaunay-contour/

L.

[kikko]

Hi!

Here is a small addon (with example) that wraps triangle c++ wrapper from Piyush Kumar for openframeworks according to the method described by Todd in his blog. It also uses the great PointInsidePolygon function of Theo! You can use these triangles to reproduce your shape in box2d.

ofxTriangle

hope it helps!

update: memory leak fixed! ^^

[julapy]

very cool kikko, thanks for sharing that.

can you think of any way of stabilising the triangles?
looks like on every frame it works out a new contour and re-calculates all the triangles.

just trying to think of ways of smoothing out the triangle transitions...

L.

[kikko]

I think you find the triangulation messy because the debug drawing gives a random color for each triangle on every update. It looks way more stable when you only draw the strokes of the triangles!

I'm not sure about the best way to interpolate the triangles instead of replacing on each update. But if your goal is to use the triangles as rigid bodies for human interactions, it's likely that you want them to be accurate as often as possible!

[plandry]

That's good stuff!

Thank you both!

[plandry]

There's still a problem when I want to assemble all these triangles in the same body. Most of the time the program fail.

Here the code, maybe you can see what could be the problem :?

Code:

void init(b2World * b2dworld, float x, float y, float fMass, float fFriction, vector<ofPoint> pTriangleA, vector<ofPoint>  pTriangleB, vector<ofPoint>  pTriangleC)
{
	if(b2dworld == NULL) {
		ofLog(OF_LOG_NOTICE, "- must have a valid world -");
		return;
	}

	world = b2dworld;

	b2BodyDef bodyDef;
	bodyDef.position.Set(x/OFX_BOX2D_SCALE, y/OFX_BOX2D_SCALE);
	body = world->CreateBody(&bodyDef);


        int iCptPoly=0;
	for(int i=0;i<pTriangleA.size();i++)
	{
            b2PolygonDef poly;
            poly.vertexCount = 3;

            poly.density	     = fMass;
            poly.restitution     = 0.0;
            poly.friction	     = fFriction;

            poly.vertices[0].Set(pTriangleA[i].x, pTriangleA[i].y);
            poly.vertices[1].Set(pTriangleB[i].x, pTriangleB[i].y);
            poly.vertices[2].Set(pTriangleC[i].x, pTriangleC[i].y);

            for(int j=0;j<3;j++)
            {
                poly.vertices[j].x /= OFX_BOX2D_SCALE;
                poly.vertices[j].y /= OFX_BOX2D_SCALE;
            }
          
            body->CreateShape(&poly);  ///--------->IT FAIL THERE<-------
	}

       body->SetMassFromShapes();

	//set the filter data
	b2FilterData data;
	data.categoryBits = 0x0003;
	data.maskBits = 0x1;
	data.groupIndex = -3;
}

Thanks

[kikko]

If it's a box2d assertion that make your app crash, it's probably because the points have to be ordered clockwise and the triangles have to fit some requirements :

- Edges can't be too close to parallel
- Area must be greater than a minimum value
- Each edge must be more than b2_toiSlop away from the centroid

I haven't figured yet how to perform these checks but I'll give it a try in the next couple of days!

[kikko]

well after doing some researches i found out 2 things :

- first is that plong0 made a similar addon (ofxDelaunay) a couple of months ago that may be faster than the one i posted!
- second is that Todd Vanderlin already did all the maths of the polygon requierements in the ofxBox2dPolygon.validateShape() method! so for us, validating a triangle is as easy as :

Code:

ofxBox2dPolygon poly;
poly.addVertex(tData->a.x, tData->a.y);
poly.addVertex(tData->b.x, tData->b.y);
poly.addVertex(tData->c.x, tData->c.y);
			
if(poly.validateShape()) {
			
	poly.init(physics.world, 0.0f, 0.0f);
}

awesome! thanks for that Todd!^^

sources of the example below

[Akira Hayasaka]

Hi kikko and all.
Thank you all for sharing code and info :)

i did some experiments using ofxTriangle.



[attachment=99:1g2pno6b]BlobToB2D.zip[/attachment:1g2pno6b]
**edit 20100506**
this is depreciated.
new version is http://forum.openframeworks.cc/index.php/topic,3386.msg20078.html#msg20078


In this experiment, i did some awkward hack to stop box2d assertion.
At b2Settings.h, before #include <assert.h>, place #define NDEBUG.

And how to create custome polygon:
http://www.psyked.co.uk/box2d/simple-box2d-custom-polygon-creation.htm

Code:

void testApp::makeComplexBody(b2Body* body, vector<ofxTriangleData*> triangles) {
	int shapeCnt = 0;
	
	for (int i = 0; i < triangles.size(); i++) {
	
		b2PolygonDef* shapeDef = new b2PolygonDef();
		shapeDef->density = 1;
		shapeDef->friction = 5;
		shapeDef->vertexCount = 3;

		shapeDef->vertices[0].Set(triangles[i]->a.x/OFX_BOX2D_SCALE, triangles[i]->a.y/OFX_BOX2D_SCALE);
		shapeDef->vertices[1].Set(triangles[i]->b.x/OFX_BOX2D_SCALE, triangles[i]->b.y/OFX_BOX2D_SCALE);
		shapeDef->vertices[2].Set(triangles[i]->c.x/OFX_BOX2D_SCALE, triangles[i]->c.y/OFX_BOX2D_SCALE);
		
		body->CreateShape(shapeDef);
		shapeCnt++;
	}
	
	shapeCnts.push_back(shapeCnt);
	body->SetMassFromShapes();
}

cheers

[kikko]

sweet idea and video Akira! love it!

[plandry]

Nice sharing!

Thanks you both!

[julapy]

hi kikko,

im trying to run your example but running into problems, think its do with my version of ofxBox2d.

i got the latest from vanderlin's trunk (rev 73) http://code.google.com/p/vanderlin/ and ofxBox2dPolygon is missing the validateShape() method you call in your code.
not sure why it has been removed.

so i went back a revision to rev 72 and i was able to compile your code but running into this error,

Assertion failed: (m_I > 0.0f), function SetMassFromShapes, file /Volumes/STORAGE/OF_0061/apps/forum/3386_ofxTriangle_box2d/../../../addons/ofxBox2d_rev72/scr/lib/Box2D/Source/Dynamics/b2Body.cpp, line 334.
Program received signal:  “SIGABRT”.

would you mind posting your version of ofxBox2d you are using for your example?

really keen to get it working.

thx, L.

[kikko]

hi julapy,

my knowledge in box2d is quite basic so I can't tell you why your shape is unable to get a mass even if it has been checked by Todd's function..

here is the version I'm working with hope it helps!

[julapy]

hey kikko, works great now!
thx for posting.