Drawing custom polygons in Box2D

Question

I'm making a Flash game, and I've encountered a really weird problem while trying to draw a polygonal shape in Box2D.

Here's the code I use:

var fixtureDefs:Array = new Array();

...

var fDef:b2FixtureDef = new b2FixtureDef();
fDef.density = 0;
fDef.shape = new b2PolygonShape();
b2PolygonShape(fDef.shape).SetAsArray(vertexArray);
fixtureDefs.push(fDef);

//This gets repeated several times, so that at the end you get a body consisting of several convex shapes.

...

var bD:b2BodyDef = new b2BodyDef();
bD.type = b2Body.b2_staticBody;
bD.position.Set(300/Constants.RATIO,200/Constants.RATIO);

var body:b2Body = Constants.world.CreateBody(bD);

...

for each(var fD:b2FixtureDef in fixtureDefs) {
body.CreateFixture(fD);
}

Where vertexArray is a valid array containing 4 b2Vec2 vertices, making up a convex shape.

The problem is, when I test, collisions don't work right for that body. Most other objects -enemies, user-controlled characters - pass straight through, as if the body isn't there at all. Some raycasts pass through as well.

Infuriatingly enough, one kind of bodies I have (a custom enemy) somehow does detect the body and collides with it. The raycasts that particular kind of enemy attempts do work - when your character hides behind the polygon, it's like they can't see him.

The other weird thing: when I try the same code, only go for SetAsBox instead of SetAsArray, it works exactly as it should.

I'm using a custom ContactListener class, but I haven't done any contact filtering (unless it's possible to do that without realising).

I'm using the Flash 9 version of Box2D 2.1a.

Any suggestions? Am I missing something obvious or have I (God forbid!) discovered a bug? Thanks for your help!

Andrey

b2Zero b2Zero · Accepted Answer · 2011-04-15T11:47:38

Heyo Andrey,

2.1a is more on the strict side so you have to declare almost everything for it to work proper.

AS3 OOP-

private var body:b2Body;
private var bodyDef:b2BodyDef = new b2BodyDef();
private var bodyPoly:b2PolygonShape = new b2PolygonShape();
private var bodyFix:b2FixtureDef = new b2FixtureDef();

private var vertexArray:Array = new Array();

public function Example() 
{

//Setting up Vertices in an Array   
var ver1:b2Vec2 = new b2Vec2( -1, -1);
var ver2:b2Vec2 = new b2Vec2( 1, -1);
var ver3:b2Vec2 = new b2Vec2( 1, 1);
var ver4:b2Vec2 = new b2Vec2( -1, 1);

//Push in order
vertexArray.push(ver1, ver2, ver3, ver4);

bodyDef.type = b2Body.b2_dynamicBody;
//b2Vec2 Array then Vertex Count
bodyPoly.SetAsArray(vertexArray, vertexArray.length);
bodyFix.shape = bodyPoly;
bodyFix.density = 0.5;
bodyFix.friction = 0.5;
bodyFix.restitution = 0.5;
bodyDef.position.Set(0, 0);
body = m_world.CreateBody(bodyDef);
body.CreateFixture(bodyFix);

}

Remember that each point must be to the right of the one before.

var ver1:b2Vec2 = new b2Vec2( -1, -1);//Top Left
var ver2:b2Vec2 = new b2Vec2( 1, -1);//Top Right
var ver3:b2Vec2 = new b2Vec2( 1, 1);//Bottom Right
var ver4:b2Vec2 = new b2Vec2( -1, 1);//Bottom Left

Placement from left to right

vertexArray.push(ver1, ver2, ver3, ver4);

I'm not very good at explaining thing's but i hope this help's! -Zero

Drawing custom polygons in Box2D

2 Answers