elliptical fixture in box2d and cocos2d - objective-c

Question

I am trying to develop an iOS game in Cocos2d + Box2d. I want to use elliptical fixtures in Box2D. I tried using b2Capsule shape, but its not exactly what I want as the collision is not proper. Anyone has done this before?

Answer 1

For specific shapes in Box2D you will have to triangulate you original polygon (in your case an ellipse in which you keep a certain number of vertices).
For this, you can use the poly2tri excellent constrained Delaunay triangulation at http://code.google.com/p/poly2tri/
It is very simple. Here is the way I get my triangles :
- (NSArray*) triangulate:(NSArray*)verticesArray
{
NSMutableArray* outputTriangles = [[[NSMutableArray alloc] init] autorelease];
p2t::CDT* triangulationContainer;
vector<p2t::Triangle*> p2tTriangles;
vector< vector<p2t::Point*> > polylines;
vector<p2t::Point*> polyline;
for (hOzPoint2D *point in verticesArray) {
polyline.push_back(new p2t::Point([point x], [point y]));
}
polylines.push_back(polyline);
triangulationContainer = new p2t::CDT(polyline);
triangulationContainer->Triangulate();
p2tTriangles = triangulationContainer->GetTriangles();
for (int i = 0; i < p2tTriangles.size(); i++) {
p2t::Triangle& t = *p2tTriangles[i];
p2t::Point& a = *t.GetPoint(0);
p2t::Point& b = *t.GetPoint(1);
p2t::Point& c = *t.GetPoint(2);
[outputTriangles addObject:[NSArray arrayWithObjects:
[hOzPoint2D point2DWithDoubleX:a.x doubleY:a.y],
[hOzPoint2D point2DWithDoubleX:b.x doubleY:b.y],
[hOzPoint2D point2DWithDoubleX:c.x doubleY:c.y], nil]];
}
delete triangulationContainer;
for(int i = 0; i < polylines.size(); i++) {
vector<p2t::Point*> poly = polylines[i];
FreeClear(poly);
}
return [outputTriangles copy];
}
hOzPoint2D here is my custom point class, but you can pass any couple of coordinates. You don't even have to output a NSArray : you can insert this method in your body creation one.
Be careful that poly2tri has some restrictions :
you can't have twice the same point in your polygon ;
the polygon must not be self-intersecting ;
...
Read the poly2tri page to know more.
The resulting array contains triangles that you attach as fixtures to the same body.

Answer 2

I have used approximation as well. This has some performance drawbacks, but nothing major I guess. Code (Flash ActionScript 3, but you should be able to port that easily):
var vertices:Vector.<b2Vec2> = new Vector.<b2Vec2>();
var a:Number = _image.width / 2 / PhysicsVals.RATIO;
var b:Number = _image.height / 2 / PhysicsVals.RATIO;
var segments:int = ellipse_approximation_vertices_count; (the more the more precise shape is, but the more time it takes to do collision detection)
var segment:Number = 2 * Math.PI / segments;
for (var i:int = 0; i < segments; i++)
{
vertices.push(new b2Vec2(a * Math.cos(segment * i), b * Math.sin(segment * i)));
}
var shape:b2PolygonShape = new b2PolygonShape();
shape.SetAsVector(vertices, vertices.length);
var fixtureDef:b2FixtureDef = new b2FixtureDef();
fixtureDef.shape = shape;