Introducing Box2D filtering

In Box2D, normally every object collides with other objects in the stage.

But sometimes we may need a way to make an object collide with only certain objects, ignoring the rest. I am not talking about sensors, but real objects that won’t collide with certain other objects.

This feature can be done with filtering.

Collision filtering is a system for preventing collision between shapes. For example, say you make a character that rides a bicycle. You want the bicycle to collide with the terrain and the character to collide with the terrain, but you don’t want the character to collide with the bicycle (because they must overlap). Box2D supports such collision filtering using categories and groups.

Box2D supports 16 collision categories. For each shape you can specify which category it belongs to. You also specify what other categories this shape can collide with. For example, you could specify in a multiplayer game that all players don’t collide with each other and monsters don’t collide with each other, but players and monsters should collide. This is done with masking bits.

The following script is the same we saw a Understanding how Box2D manages boundaries but I am using a random filtering to make the ball ignore the ramp, the wall, or none of them.

package　{
　import　flash.display.Sprite;
　import　flash.events.Event;
　import　Box2D.Dynamics.*;
　import　Box2D.Collision.*;
　import　Box2D.Collision.Shapes.*;
　import　Box2D.Common.Math.*;
　public　class　bound　extends　Sprite　{
　　var　body:b2Body;
　　public　var　m_world:b2World;
　　public　var　m_iterations:int=10;
　　public　var　m_timeStep:Number=1.0/30.0;
　　var　m_boundaryListener=new　b2BoundaryListener();
　　var　bodyDef:b2BodyDef;
　　var　boxDef:b2PolygonDef;
　　var　circleDef:b2CircleDef;
　　public　function　bound()　{
　　　addEventListener(Event.ENTER_FRAME,　Update,　false,　0,　true);
　　　var　worldAABB:b2AABB　=　new　b2AABB();
　　　worldAABB.lowerBound.Set(-100.0,　-100.0);
　　　worldAABB.upperBound.Set(100.0,　100.0);
　　　var　gravity:b2Vec2=new　b2Vec2(0.0,10.0);
　　　var　doSleep:Boolean=true;
　　　m_world=new　b2World(worldAABB,gravity,doSleep);
　　　m_world.SetBoundaryListener(m_boundaryListener);
　　　//　debug　draw　start
　　　var　m_sprite:Sprite;
　　　m_sprite　=　new　Sprite();
　　　addChild(m_sprite);
　　　var　dbgDraw:b2DebugDraw　=　new　b2DebugDraw();
　　　var　dbgSprite:Sprite　=　new　Sprite();
　　　m_sprite.addChild(dbgSprite);
　　　dbgDraw.m_sprite=m_sprite;
　　　dbgDraw.m_drawScale=30;
　　　dbgDraw.m_alpha=1;
　　　dbgDraw.m_fillAlpha=0.5;
　　　dbgDraw.m_lineThickness=1;
　　　dbgDraw.m_drawFlags=b2DebugDraw.e_shapeBit;
　　　m_world.SetDebugDraw(dbgDraw);
　　　//　ground
　　　bodyDef　=　new　b2BodyDef();
　　　bodyDef.position.Set(4,　12);
　　　boxDef　=　new　b2PolygonDef();
　　　boxDef.filter.categoryBits=2;
　　　boxDef.SetAsOrientedBox(10,　1,new　b2Vec2(5,　1.5),　Math.PI/32);
　　　boxDef.friction=0.3;
　　　boxDef.density=0;
　　　body=m_world.CreateBody(bodyDef);
　　　body.CreateShape(boxDef);
　　　body.SetMassFromShapes();
　　　//　another　object
　　　bodyDef　=　new　b2BodyDef();
　　　bodyDef.position.Set(8,　12);
　　　boxDef　=　new　b2PolygonDef();
　　　boxDef.filter.categoryBits=4;
　　　boxDef.SetAsBox(1,　3);
　　　boxDef.friction=0.3;
　　　boxDef.density=0;
　　　body=m_world.CreateBody(bodyDef);
　　　body.CreateShape(boxDef);
　　　body.SetMassFromShapes();
　　　//　circle
　　　create_circle();
　　}
　　public　function　create_circle()　{
　　　bodyDef　=　new　b2BodyDef();
　　　bodyDef.position.x=6;
　　　bodyDef.position.y=2;
　　　circleDef　=　new　b2CircleDef();
　　　circleDef.filter.maskBits=Math.ceil(Math.random()*3)*2;
　　　circleDef.radius=2;
　　　circleDef.density=1.0;
　　　circleDef.friction=0.5;
　　　circleDef.restitution=0.2;
　　　body=m_world.CreateBody(bodyDef);
　　　body.CreateShape(circleDef);
　　　body.SetMassFromShapes();
　　}
　　public　function　Update(e:Event):void　{
　　　m_world.Step(m_timeStep,　m_iterations);
　　　if　(m_boundaryListener.get_contact())　{
　　　　m_boundaryListener.no_contact();
　　　　m_world.DestroyBody(body);
　　　　bodyDef　=　new　b2BodyDef();
　　　　create_circle();
　　　}
　　}
　}
}