Box2D - Collisions

BlitzMax Forums/Brucey's Modules/Box2D - Collisions

Rick_72 (Posted 2010) [#1]

Hi guys! I am very enthusiastic about Box2D since this is a really cool physics engine (IMHO). But I don't get the collision detect (i.e. the removal of collided objects) to work properly. I've tried to expand the ContactListener in test.bmx (using the destruction code provided in the manual in chap. 8) based on the example verticalstack.bmx: I've tried to get boxes deleted that were hit by a bullet. As soon as a contact occurs I get an exception - crash. Could anyone show me how using the example verticalstack.bmx to do this correctly, plz? I have absolutely no idea. Thx!

Volker (Posted 2010) [#2]

You can't delete bodies bodies directly when the contact occurs. "Box2D does not allow you to alter the physics world inside a callback because you might destroy objects that Box2D is currently processing, leading to orphaned references. " Set a field of your objects to true when a contact occurs and delete them later in your main loop.

Rick_72 (Posted 2010) [#3]

That is exactly what I did. I was aware of waiting for the engine to finish its job before deleting any objects. But anyways it throws an exception and crashes. So if you have a working piece of code I would appreciate you sharing it with us.

Robert Cummings (Posted 2010) [#4]

In the callback, just set a variable for that object, and kill it after an update in your main code.

Volker (Posted 2010) [#5]

Hi Ralph, my own code is to bloated to post it, so I tried to modify the verticalstack.bmx and ran into the same problem you have. Here is a working example; dont' harm me, I don't know what I'm doing. Especially I have no idea why I have to NULLifiy the userdata (just a string) of the bodies AFTER destroying it. Curious. Changes made by me are marked with '***********' verticalstack.bmx SuperStrict Framework BaH.Box2d Import brl.GLMax2D Import "test.bmx" Graphics 800, 600, 0 SetBlend ALPHABLEND Global bodylist:TList = New TList '************************* Run(New VerticalStack.Create(), New TSettings) Type VerticalStack Extends Test Field m_bullet:b2Body Field contactlistener:TMyContactListener Method Create:VerticalStack() Init() Self.contactlistener = New TMyContactListener ' **************************** m_world.SetContactListener(Self.contactlistener) Local ground:b2Body Local sd:b2PolygonDef = New b2PolygonDef sd.SetAsOrientedBox(50.0, 10.0, New b2Vec2.Create(0.0, -10.0), 0) Local bd:b2BodyDef = New b2BodyDef bd.SetPositionXY(0.0, -10.0) ground = m_world.CreateBody(bd) ground.CreateShape(sd) sd.SetAsOrientedBox(0.1, 10.0, New b2Vec2.Create(20.0, 10.0), 0) ground.CreateShape(sd) Local xs:Float[] = [0.0, -10.0, -5.0, 5.0, 10.0] For Local j:Int = 0 Until 5 sd = New b2PolygonDef sd.SetAsBox(0.5, 0.5) sd.SetDensity(1.0) ' sd.SetFriction(0.3) For Local i:Int = 0 Until 16 bd = New b2BodyDef bd.SetPositionXY(xs[j], 0.752 + 1.4 * i) Local body:b2Body = m_world.CreateBody(bd) body.userData = "Cube" ' ************************************ body.CreateShape(sd) body.SetMassFromShapes() Next Next m_bullet = Null Return Self End Method Method Keyboard() Super.Keyboard() If KeyHit(KEY_COMMA) Then If m_bullet Then m_world.DestroyBody(m_bullet) m_bullet = Null End If Local sd:b2CircleDef = New b2CircleDef sd.SetDensity(20.0) sd.SetRadius(0.25) sd.SetRestitution(0.05) Local bd:b2BodyDef = New b2BodyDef bd.SetIsBullet(True) bd.SetPositionXY(-31.0, -5.0) m_bullet = m_world.CreateBody(bd) m_bullet.CreateShape(sd) m_bullet.SetMassFromShapes() m_bullet.userdata = "Bullet" ' ********************************************* m_bullet.SetLinearVelocity(New b2Vec2.Create(400.0, 0.0)) End If End Method Method DoStep(settings:TSettings) Super.DoStep(settings) DrawString "Press: (,) to launch a bullet.", 5, m_textLine m_textLine:+12 DebugLog("bodylist size:" + bodylist.Count()) ' ************************************** For Local body:b2Body = EachIn bodylist ' Remove bodies DebugLog("destroying body:" + String(body.userdata)) If String(body.GetUserData()) = "Cube" ' needed, or crash.. curious m_world.destroybody(body) body.userData = Null ' needed, or crash.. EndIf Next bodylist.Clear() ' End Method End Type Type TMyContactListener Extends b2ContactListener ' ************************************** Method Add(point:b2ContactPoint) Local userdata1:String = String(point.GetShape1().GetBody().userData) Local userdata2:String = String(point.GetShape2().GetBody().userData) If userdata1 = "Bullet" And userdata2 = "Cube" bodylist.AddLast(point.GetShape2().GetBody()) End If If userdata2 = "Bullet" And userdata1 = "Cube" bodylist.AddLast(point.GetShape1().GetBody()) End If End Method End Type test.bmx ' ' Part of the examples framework... imported by the examples. ' SuperStrict Import BRL.FreeTypeFont Import "render.bmx" Global viewZoom:Float = 1 Global tx:Int, ty:Int, tw:Int = 800, th:Int = 600 Global viewCenter:b2Vec2 = New b2Vec2.Create(0.0, 0.0) Global mouseState:Int = False Function Run(tst:Test, settings:TSettings) detailFont = LoadImageFont(CurrentDir() + "/04B_03__.ttf", 8) SetOrigin(400,500) While Not KeyDown(KEY_ESCAPE) Cls Mouse(tst) tst.Keyboard() tst.DoStep(settings) Flip Wend End Function Function Mouse(tst:Test) Local x:Int = MouseX() Local y:Int = MouseY() ' Use the mouse to move things around. If MouseDown(1) Then Local p:b2Vec2 = ConvertScreenToWorld(x, y) ' button pressed If Not mouseState Then tst.MouseDown(p) End If mouseState = True tst.MouseMove(p) Else If mouseState Then Local p:b2Vec2 = ConvertScreenToWorld(x, y) tst.MouseUp(p) End If mouseState = False End If End Function Type Test Const k_maxContactPoints:Int = 2048 Field m_worldAABB:b2AABB = New b2AABB.Create() Field m_world:b2World Field m_debugDraw:b2DebugDraw = New debugDraw Field m_textLine:Int Field m_mouseWorld:b2Vec2 Field m_mouseJoint:b2MouseJoint Field m_points:ContactPoint[] = New ContactPoint[k_maxContactPoints] ' Field m_contactListener:ContactListener = New ContactListener Field m_destructionListener:DestructionListener = New DestructionListener Field m_pointCount:Int Field _settings:TSettings Method Init(xs:Float = 0, ys:Float = 0) If xs Then xScale = xs End If If ys Then yScale = ys End If m_worldAABB.SetLowerBound(New b2Vec2.Create(-200.0, -100.0)) m_worldAABB.SetUpperBound(New b2Vec2.Create(200.0, 200.0)) Local gravity:b2Vec2 = New b2Vec2.Create(0.0, -10.0) m_world = New b2World.Create(m_worldAABB, gravity, True) ' m_contactListener._test = Self m_destructionListener._test = Self m_world.SetDebugDraw(m_debugDraw) ' m_world.SetContactListener(m_contactListener) m_world.SetDestructionListener(m_destructionListener) End Method Method DoStep(settings:TSettings) _settings = settings m_textLine = 10 Local timeStep:Float If settings.hz > 0.0 Then timeStep = 1.0 / settings.hz Else timeStep = 0.0 End If If settings.pause Then If settings.singleStep Then settings.singleStep = 0 Else timeStep = 0.0 End If 'DrawString(5, m_textLine, "****PAUSED****") 'm_textLine :+ 15 End If Local flags:Int = 0 flags :+ settings.drawShapes * b2DebugDraw.e_shapeBit flags :+ settings.drawJoints * b2DebugDraw.e_jointBit flags :+ settings.drawCoreShapes * b2DebugDraw.e_coreShapeBit flags :+ settings.drawControllers * b2DebugDraw.e_controllerBit flags :+ settings.drawAABBs * b2DebugDraw.e_aabbBit flags :+ settings.drawOBBs * b2DebugDraw.e_obbBit flags :+ settings.drawPairs * b2DebugDraw.e_pairBit flags :+ settings.drawCOMs * b2DebugDraw.e_centerOfMassBit m_debugDraw.SetFlags(flags) m_world.SetWarmStarting(settings.enableWarmStarting) ' m_world.SetPositionCorrection(settings.enablePositionCorrection) m_world.SetContinuousPhysics(settings.enableTOI) m_pointCount = 0 m_world.DoStep(timeStep, settings.velocityIterations, settings.positionIterations) m_world.Validate() If m_mouseJoint Then Local body:b2Body = m_mouseJoint.GetBody2() Local p1:b2Vec2 = body.GetWorldPoint(m_mouseJoint.GetLocalAnchor()) Local p2:b2Vec2 = m_mouseJoint.GetTarget() debugDraw(m_debugDraw).DrawPoint(p1, 4.0, b2Color.Set(0, 255, 0)) debugDraw(m_debugDraw).DrawPoint(p2, 4.0, b2Color.Set(0, 255, 0)) m_debugDraw.DrawSegment(p1, p2, b2Color.Set(204, 204, 204)) End If If settings.drawContactPoints Then Const k_axisScale:Int = 0.3 For Local i:Int = 0 Until m_pointCount Local point:ContactPoint = m_points[i] If point.state = 0 Then ' Add debugDraw(m_debugDraw).DrawPoint(point.position, 5.0, b2Color.Set(76, 242, 76)) Else If point.state = 1 Then ' Persist debugDraw(m_debugDraw).DrawPoint(point.position, 3.0, b2Color.Set(76, 76, 242)) Else ' Remove debugDraw(m_debugDraw).DrawPoint(point.position, 5.0, b2Color.Set(242, 76, 76)) End If Rem If (settings->drawContactNormals == 1) { b2Vec2 p1 = point->position; b2Vec2 p2 = p1 + k_axisScale * point->normal; m_debugDraw.DrawSegment(p1, p2, b2Color(0.4f, 0.9f, 0.4f)); } Else If (settings->drawContactForces == 1) { //b2Vec2 p1 = point->position; //b2Vec2 p2 = p1 + k_forceScale * point->normalForce * point->normal; //DrawSegment(p1, p2, b2Color(0.9f, 0.9f, 0.3f)); } If (settings->drawFrictionForces == 1) { //b2Vec2 tangent = b2Cross(point->normal, 1.0f); //b2Vec2 p1 = point->position; //b2Vec2 p2 = p1 + k_forceScale * point->tangentForce * tangent; //DrawSegment(p1, p2, b2Color(0.9f, 0.9f, 0.3f)); } End Rem Next End If End Method Method Keyboard() If KeyHit(KEY_F) Then debugDraw(m_debugDraw).showDetails = Not debugDraw(m_debugDraw).showDetails End If If KeyHit(KEY_C) Then _settings.drawContactPoints = Not _settings.drawContactPoints End If End Method Method DrawString(text:String, x:Int, y:Int) SetColor(255, 255, 255) Local ox:Float, oy:Float GetOrigin(ox, oy) SetOrigin 0,0 DrawText text, x, y SetOrigin ox, oy End Method Method ShiftMouseDown(p:b2Vec2) End Method Method MouseDown(p:b2Vec2) m_mouseWorld = p If m_mouseJoint Then Return End If ' Make a small box. Local aabb:b2AABB = New b2AABB.Create() Local d:b2Vec2 = New b2Vec2.Create(0.001, 0.001) aabb.SetLowerBound(p.Subtract(d)) aabb.SetUpperBound(p.Plus(d)) ' Query the world for overlapping shapes. Local k_maxCount:Int = 10 Local shapes:b2Shape[] = New b2Shape[k_maxCount] Local count:Int = m_world.Query(aabb, shapes) Local body:b2Body For Local i:Int = 0 Until count Local shapeBody:b2Body = shapes[i].GetBody() If Not shapeBody.IsStatic() And shapeBody.GetMass() > 0.0 Then Local inside:Int = shapes[i].TestPoint(shapeBody.GetXForm(), p) If inside Then body = shapes[i].GetBody() Exit End If End If Next If body Then Local md:b2MouseJointDef = New b2MouseJointDef md.SetBody1(m_world.GetGroundBody()) md.SetBody2(body) md.SetTarget(p) md.SetMaxForce(1000.0 * body.GetMass()) m_mouseJoint = b2MouseJoint(m_world.CreateJoint(md)) body.WakeUp() End If End Method Method MouseUp(p:b2Vec2) If m_mouseJoint Then m_world.DestroyJoint(m_mouseJoint) m_mouseJoint = Null End If End Method Method MouseMove(p:b2Vec2) m_mouseWorld = p If debugDraw(m_debugDraw).showDetails Then SetImageFont(detailFont) SetColor(255, 255, 255) DrawText StripPlaces(p.X()) + "," + StripPlaces(p.Y()), MouseX() - 400, MouseY() - 500 - 10 SetImageFont(Null) End If If m_mouseJoint Then m_mouseJoint.SetTarget(p) End If End Method Method JointDestroyed(joint:b2Joint) End Method End Type Type TSettings Field hz:Float = 60 Field velocityIterations:Int = 10 Field positionIterations:Int = 8 Field drawShapes:Int = 1 Field drawJoints:Int = 1 Field drawControllers:Int = 1 Field drawCoreShapes:Int Field drawAABBs:Int Field drawOBBs:Int Field drawPairs:Int Field drawContactPoints:Int Field drawContactNormals:Int Field drawContactForces:Int Field drawFrictionForces:Int Field drawCOMs:Int Field drawStats:Int Field enableWarmStarting:Int = 1 Field enableTOI:Int = 1 Field pause:Int Field singleStep:Int End Type Type ContactPoint Const e_contactAdded:Int = 0 Const e_contactPersisted:Int = 1 Const e_contactRemoved:Int = 2 Field shape1:b2Shape Field shape2:b2Shape Field normal:b2Vec2 Field position:b2Vec2 Field velocity:b2Vec2 Field id:Int Field state:Int End Type 'Type ContactListener Extends b2ContactListener ' ' Field _test:Test ' ' Method Add(point:b2ContactPoint) ' If _test.m_pointCount = Test.k_maxContactPoints Then ' Return ' End If ' ' Local cp:ContactPoint = _test.m_points[_test.m_pointCount] ' If Not cp Then ' cp = New ContactPoint ' _test.m_points[_test.m_pointCount] = cp ' End If ' ' cp.shape1 = point.GetShape1() ' cp.shape2 = point.GetShape2() ' cp.position = point.GetPosition() ' cp.normal = point.GetNormal() '' cp.id = point->id ' cp.state = ContactPoint.e_contactAdded ' ' _test.m_pointCount :+ 1 ' End Method ' ' Method Persist(point:b2ContactPoint) ' If _test.m_pointCount = Test.k_maxContactPoints Then ' Return ' End If ' ' Local cp:ContactPoint = _test.m_points[_test.m_pointCount] ' If Not cp Then ' cp = New ContactPoint ' _test.m_points[_test.m_pointCount] = cp ' End If ' cp.shape1 = point.GetShape1() ' cp.shape2 = point.GetShape2() ' cp.position = point.GetPosition() ' cp.normal = point.GetNormal() '' cp.id = point->id ' cp.state = ContactPoint.e_contactPersisted ' ' _test.m_pointCount :+ 1 ' End Method ' ' Method Remove(point:b2ContactPoint) ' If _test.m_pointCount = Test.k_maxContactPoints Then ' Return ' End If ' ' Local cp:ContactPoint = _test.m_points[_test.m_pointCount] ' If Not cp Then ' cp = New ContactPoint ' _test.m_points[_test.m_pointCount] = cp ' End If ' cp.shape1 = point.GetShape1() ' cp.shape2 = point.GetShape2() ' cp.position = point.GetPosition() ' cp.normal = point.GetNormal() '' cp.id = point->id ' cp.state = ContactPoint.e_contactRemoved ' ' _test.m_pointCount :+ 1 ' End Method ' 'End Type Type DestructionListener Extends b2DestructionListener Field _test:Test Method SayGoodbyeJoint(joint:b2Joint) If _test.m_mouseJoint = joint _test.m_mouseJoint = Null Else _test.JointDestroyed(joint) End If End Method End Type Function ConvertScreenToWorld:b2Vec2(x:Int, y:Int) Local ox:Float, oy:Float GetOrigin(ox, oy) x :- ox y :- oy Local wx:Float = x / xScale Local wy:Float = y / yScale Return Vec2(wx, -wy) 'Local u:Float = x / Float(tw) 'Local v:Float = (th - y) / Float(th) 'Local ratio:Float = tw / Float(th) 'Local extents:b2Vec2 = New b2Vec2.Create(ratio * 38, 50.0) 'extents.Multiply(viewZoom) 'Local _lower:b2Vec2 = viewCenter.Subtract(extents) 'Local _upper:b2Vec2 = viewCenter.Plus(extents) 'Return New b2Vec2.Create((1.0 - u) * _lower.X() + u * _upper.X(), (1.0 - v) * _lower.Y() + v * _upper.Y()) End Function

Rick_72 (Posted 2010) [#6]

OK, this one works! Thanks Volker for helping on this one and posting some working code! I had some unresolved reference bug in my code that crashed the whole stuff before collisions even took place. I should better go to sleep now .. ;-)