Farseer Physics collision detection vs intersection detection with sensors

Question

I'm trying to make a game that uses Farseer physics engine as its main collision detection engine. I have a 2d circle body object that has a matching fixture (I used the CreatCircle method. I did not create the fixtures and bodies separate). When IsSensor is set to false, objects collide with this circle and bounce off. My goal was to have events triggered when other physics enabled bodies move through this field and just use the engine to detect the collisions (or rather intersections) so I set it to IsSensor = true, and now all the other bodies pass right through but I do not get collision events. The be specific, sometimes I do get events but not continuously as I would expect. My intention was to have this collision code fired each time the world re evaluated collisions.

Does anyone know a way to do this. My googling has led me nowhere and I'll be fudging with the limited farseer documentation I can find this weekend to try and get this right.

Thanks ahead of time.

Answer 1

Get the Body.ContactList which is populated by all contact resulting from BB collisions. If you want contacts that actually have fixtures touching check to see if Contact.IsTouching() is true.

Edit: There are performance issues with this approach so I suggest setting a flag when the sensor collision event fires and setting the flag to false when none of the items in the ContactList are touching. Only iterate though the contact while this flag it set to true. This would prevent you from iterating every every time a BB intersects with your sensor.

Answer 2

//declare variables 
private List<Dynamics.Contacts.Contact> contacts;

public int SensorData;
//you need your fixture
//my fixture create by Bayazit
TactSensorTexture = Content.Load<Texture2D>("test_1");
uint[] data1 = new uint[TactSensorTexture.Width * TactSensorTexture.Height];
TactSensorTexture.GetData(data1);
Vertices verts1 = PolygonTools.CreatePolygon(data1, TactSensorTexture.Width);
Vector2 scale = new Vector2(0.07f, 0.07f);
List<Vertices> _list = FarseerPhysics.Common.Decomposition.Triangulate.ConvexPartition(verts1, TriangulationAlgorithm.Bayazit);
foreach (Vertices vertices in _list)
        {
            vertices.Scale(ref scale);
        }
//attach to create body (already have a circle shape)
compaund = FixtureFactory.AttachCompoundPolygon(_list, 0.000f, _circleBody);
//FixtureFactory create two fixture
compaund[1].IsSensor = true;
compaund[0].IsSensor = true;

//in function update i white next construction (insert on right place)
SensorData = 0; //always set zero because you in update funciton
contacts = _world.ContactList; //get list of contacts
foreach(Dynamics.Contacts.Contact contact in contacts)
            {   

                if ((contact.FixtureA == compaund[0] && contact.IsTouching) || (contact.FixtureB == compaund[0] && contact.IsTouching)) //check only for one fixture because it's just example
                {
                    SensorData = 1;
                }
                else
                {
                   //dont need do anything
                }

            }
//also you can drow on screen (insert on right place)
_batch.DrawString(_font, System.Convert.ToString(SensorData), new Vector2(14f, 20f), Color.Red);