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• How can I convert a 2D bitmap (Used for terrain) to a 2D polygon mesh for collision?

  - by Megadanxzero

  So I'm making an artillery type game, sort of similar to Worms with all the usual stuff like destructible terrain etc... and while I could use per-pixel collision that doesn't give me collision normals or anything like that. Converting it all to a mesh would also mean I could use an existing physics library, which would be better than anything I can make by myself. I've seen people mention doing this by using Marching Squares to get contours in the bitmap, but I can't find anything which mentions how to turn these into a mesh (Unless it refers to a 3D mesh with contour lines defining different heights, which is NOT what I want). At the moment I can get a basic Marching Squares contour which looks something like this (Where the grid-like lines in the background would be the Marching Squares 'cells'): That needs to be interpolated to get a smoother, more accurate result but that's the general idea. I had a couple ideas for how to turn this into a mesh, but many of them wouldn't work in certain cases, and the one which I thought would work perfectly has turned out to be very slow and I've not even finished it yet! Ideally I'd like whatever I end up using to be fast enough to do every frame for cases such as rapidly-firing weapons, or digging tools. I'm thinking there must be some kind of existing algorithm/technique for turning something like this into a mesh, but I can't seem to find anything. I've looked at some things like Delaunay Triangulation, but as far as I can tell that won't correctly handle concave shapes like the above example, and also wouldn't account for holes within the terrain. I'll go through the technique I came up with for comparison and I guess I'll see if anyone has a better idea. First of all interpolate the Marching Squares contour lines, creating vertices from the line ends, and getting vertices where lines cross cell edges (Important). Then, for each cell containing vertices create polygons by using 2 vertices, and a cell corner as the 3rd vertex (Probably the closest corner). Do this for each cell and I think you should have a mesh which accurately represents the original bitmap (Though there will only be polygons at the edges of the bitmap, and large filled in areas in between will be empty). The only problem with this is that it involves lopping through every pixel once for the initial Marching Squares, then looping through every cell (image height + 1 x image width + 1) at least twice, which ends up being really slow for any decently sized image...

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• How to resolve concurrent ramp collisions in 2d platformer?

  - by Shaun Inman

  A bit about the physics engine: Bodies are all rectangles. Bodies are sorted at the beginning of every update loop based on the body-in-motion's horizontal and vertical velocity (to avoid sticky walls/floors). Solid bodies are resolved by testing the body-in-motion's new X with the old Y and adjusting if necessary before testing the new X with the new Y, again adjusting if necessary. Works great. Ramps (rectangles with a flag set indicating bottom-left, bottom-right, etc) are resolved by calculating the ratio of penetration along the x-axis and setting a new Y accordingly (with some checks to make sure the body-in-motion isn't attacking from the tall or flat side, in which case the ramp is treated as a normal rectangle). This also works great. Side-by-side ramps, eg. \/ and /\, work fine but things get jittery and unpredictable when a top-down ramp is directly above a bottom-up ramp, eg. < or > or when a bottom-up ramp runs right up to the ceiling/top-down ramp runs right down to the floor. I've been able to lock it down somewhat by detecting whether the body-in-motion hadFloor when also colliding with a top-down ramp or hadCeiling when also colliding with a bottom-up ramp then resolving by calculating the ratio of penetration along the y-axis and setting the new X accordingly (the opposite of the normal behavior). But as soon as the body-in-motion jumps the hasFloor flag becomes false, the first ramp resolution pushes the body into collision with the second ramp and collision resolution becomes jittery again for a few frames. I'm sure I'm making this more complicated than it needs to be. Can anyone recommend a good resource that outlines the best way to address this problem? (Please don't recommend I use something like Box2d or Chipmunk. Also, "redesign your levels" isn't an answer; the body-in-motion may at times be riding another body-in-motion, eg. a platform, that pushes it into a ramp so I'd like to be able to resolve this properly.) Thanks!

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• Best way to implement a simple bullet trajectory

  - by AirieFenix

  I searched and searched and although it's a fair simple question, I don't find the proper answer but general ideas (which I already have). I have a top-down game and I want to implement a gun which shoots bullets that follow a simple path (no physics nor change of trajectory, just go from A to B thing). a: vector of the position of the gun/player. b: vector of the mouse position (cross-hair). w: the vector of the bullet's trajectory. So, w=b-a. And the position of the bullet = [x=x0+speed*time*normalized w.x , y=y0+speed*time * normalized w.y]. I have the constructor: public Shot(int shipX, int shipY, int mouseX, int mouseY) { //I get mouse with Gdx.input.getX()/getY() ... this.shotTime = TimeUtils.millis(); this.posX = shipX; this.posY = shipY; //I used aVector = aVector.nor() here before but for some reason didn't work float tmp = (float) (Math.pow(mouseX-shipX, 2) + Math.pow(mouseY-shipY, 2)); tmp = (float) Math.sqrt(Math.abs(tmp)); this.vecX = (mouseX-shipX)/tmp; this.vecY = (mouseY-shipY)/tmp; } And here I update the position and draw the shot: public void drawShot(SpriteBatch batch) { this.lifeTime = TimeUtils.millis() - this.shotTime; //position = positionBefore + v*t this.posX = this.posX + this.vecX*this.lifeTime*speed*Gdx.graphics.getDeltaTime(); this.posY = this.posY + this.vecY*this.lifeTime*speed*Gdx.graphics.getDeltaTime(); ... } Now, the behavior of the bullet seems very awkward, not going exactly where my mouse is (it's like the mouse is 30px off) and with a random speed. I know I probably need to open the old algebra book from college but I'd like somebody says if I'm in the right direction (or points me to it); if it's a calculation problem, a code problem or both. Also, is it possible that Gdx.input.getX() gives me non-precise position? Because when I draw the cross-hair it also draws off the cursor position. Sorry for the long post and sorry if it's a very basic question. Thanks!

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• Movement prediction for non-shooters

  - by ShadowChaser

  I'm working on an isometric (2D) game with moderate-scale multiplayer - 20-30 players. I've had some difficulty getting a good movement prediction implementation in place. Right now, clients are authoritative for their own position. The server performs validation and broad-scale cheat detection, and I fully realize that the system will never be fully robust against cheating. However, the performance and implementation tradeoffs work well for me right now. Given that I'm dealing with sprite graphics, the game has 8 defined directions rather than free movement. Whenever the player changes their direction or speed (walk, run, stop), a "true" 3D velocity is set on the entity and a packet it sent to the server with the new movement state. In addition, every 250ms additional packets are transmitted with the player's current position for state updates on the server as well as for client prediction. After the server validates the packet, it gets automatically distributed to all of the other "nearby" players. Client-side, all entities with non-zero velocity (ie/ moving entities) are tracked and updated by a rudimentary "physics" system - basically nothing more than changing the position by the velocity according to the elapsed time slice (40ms or so). What I'm struggling with is how to implement clean movement prediction. I have the nagging suspicion that I've made a design mistake somewhere. I've been over the Unreal, Half-life, and all other movement prediction/lag compensation articles I could find, but they all seam geared toward shooters: "Don't send each control change, send updates every 120ms, server is authoritative, client predicts, etc". Unfortunately, that style of design won't work well for me - there's no 3D environment so each individual state change is important. 1) Most of the samples I saw tightly couple movement prediction right into the entities themselves. For example, storing the previous state along with the current state. I'd like to avoid that and keep entities with their "current state" only. Is there a better way to handle this? 2) What should happen when the player stops? I can't interpolate to the correct position, since they might need to walk backwards or another strange direction if their position is too far ahead. 3) What should happen when entities collide? If the current player collides with something, the answer is simple - just stop the player from moving. But what happens if two entities take up the same space on the server? What if the local prediction causes a remote entity to collide with the player or another entity - do I stop them as well? If the prediction had the misfortune of sticking them in front of a wall that the player has gone around, the prediction will never be able to compensate and once the error gets to high the entity will snap to the new position.

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• Simulate 'Shock absorbtion' with tire rubber in PhysX (2.8.x)

  - by Mungoid

  This is a kinda tricky question and I fear there is no easy enough solution, but I figured I'd hit SE up before giving up on it and just doing what I can. A machine I am working on has no suspension or shocks or springs of any sort in the real machine, so you would think that when it drives over bumps, it would shake like crazy but because its tires (6 of them) are quite large they seem to absorb a lot of shock from the bumps. Part of this is because the machine is around 30k lbs and it just smashes/compresses any bumps in the ground down (This is another issue im still working on) and the other part is that the tires seem to have a lot of flex to them with a lot of air as well. So my current task is to simulate shock absorption in physx without visibly separating the tires from the spindle/axle.. I have been messing with all kinds of NxMaterial, NxSpring, Joints, etc. and have had no luck getting this to work. The main problem is that the spindle attached to the tire is directly in the center and the axle is basically solidly attached to the chassis, so if i give it any spring or suspension travel, that spindle on the tires will move upwards or downwards, looking very odd because now its not any longer in the center of the tire. I tried giving it a higher restitution but that just makes it bouncy without any shock absorption. Another avenue I am messing with is to actively smooth the terrain in front of the tires so that before it hits a bumpy patch, that patch is smoothed and it doesn't bounce. The only issue with this is that it is pretty expensive to do with 6 tires, high tesselation of the terrain and other complex things going on at the same time in this simulation. I am still working on this but I am hoping to mix and match a few different aspects to get the best possible outcome. This is a bit of a complex issue so I'm not expecting anyone to have a definitive answer, just hoping someone may think of something I haven't =-) -Side note: Yes i know PhysX 2.8.x is quite outdated but we have to stick with it for this implementation. We are in the process of going to another physics engine but it is out of scope to apply that engine to this project.

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• What is the best Broadphase Interface for moving spheres?

  - by Molmasepic

  As of now I am working on optimizing the performance of the physics and collision, and as of now I am having some slowdowns on my other computers from my main. I have well over 3000 btSphereShape Rigidbodies and 2/3 of them do not move at all, but I am noticing(by the profile below) that collision is taking a bit of time to maneuver. Each sample counts as 0.01 seconds. % cumulative self self total time seconds seconds calls ms/call ms/call name 10.09 0.65 0.65 SphereTriangleDetector::collide(btVector3 const&, btVector3&, btVector3&, float&, float&, float) 7.61 1.14 0.49 btSphereTriangleCollisionAlgorithm::processCollision(btCollisionObject*, btCollisionObject*, btDispatcherInfo const&, btManifoldResult*) 5.59 1.50 0.36 btConvexTriangleCallback::processTriangle(btVector3*, int, int) 5.43 1.85 0.35 btQuantizedBvh::reportAabbOverlappingNodex(btNodeOverlapCallback*, btVector3 const&, btVector3 const&) const 4.97 2.17 0.32 btBvhTriangleMeshShape::processAllTriangles(btTriangleCallback*, btVector3 const&, btVector3 const&) const::MyNodeOverlapCallback::processNode(int, int) 4.19 2.44 0.27 btSequentialImpulseConstraintSolver::resolveSingleConstraintRowGeneric(btRigidBody&, btRigidBody&, btSolverConstraint const&) 4.04 2.70 0.26 btSequentialImpulseConstraintSolver::resolveSingleConstraintRowLowerLimit(btRigidBody&, btRigidBody&, btSolverConstraint const&) 3.73 2.94 0.24 Ogre::OctreeSceneManager::walkOctree(Ogre::OctreeCamera*, Ogre::RenderQueue*, Ogre::Octree*, Ogre::VisibleObjectsBoundsInfo*, bool, bool) 3.42 3.16 0.22 btTriangleShape::getVertex(int, btVector3&) const 2.48 3.32 0.16 Ogre::Frustum::isVisible(Ogre::AxisAlignedBox const&, Ogre::FrustumPlane*) const 2.33 3.47 0.15 1246357 0.00 0.00 Gorilla::Layer::setVisible(bool) 2.33 3.62 0.15 SphereTriangleDetector::getClosestPoints(btDiscreteCollisionDetectorInterface::ClosestPointInput const&, btDiscreteCollisionDetectorInterface::Result&, btIDebugDraw*, bool) 1.86 3.74 0.12 btCollisionDispatcher::findAlgorithm(btCollisionObject*, btCollisionObject*, btPersistentManifold*) 1.86 3.86 0.12 btSequentialImpulseConstraintSolver::setupContactConstraint(btSolverConstraint&, btCollisionObject*, btCollisionObject*, btManifoldPoint&, btContactSolverInfo const&, btVector3&, float&, float&, btVector3&, btVector3&) 1.71 3.97 0.11 btTriangleShape::getEdge(int, btVector3&, btVector3&) const 1.55 4.07 0.10 _Unwind_SjLj_Register 1.55 4.17 0.10 _Unwind_SjLj_Unregister 1.55 4.27 0.10 Ogre::D3D9HardwareVertexBuffer::updateBufferResources(char const*, Ogre::D3D9HardwareVertexBuffer::BufferResources*) 1.40 4.36 0.09 btManifoldResult::addContactPoint(btVector3 const&, btVector3 const&, float) 1.40 4.45 0.09 btSequentialImpulseConstraintSolver::setupFrictionConstraint(btSolverConstraint&, btVector3 const&, btRigidBody*, btRigidBody*, btManifoldPoint&, btVector3 const&, btVector3 const&, btCollisionObject*, btCollisionObject*, float, float, float) 1.24 4.53 0.08 btSequentialImpulseConstraintSolver::convertContact(btPersistentManifold*, btContactSolverInfo const&) 1.09 4.60 0.07 408760 0.00 0.00 Living::MapHide() 1.09 4.67 0.07 btSphereTriangleCollisionAlgorithm::~btSphereTriangleCollisionAlgorithm() 1.09 4.74 0.07 inflate_fast EDIT: Updated to show current Profile. I have only listed the functions using over 1% time from the many functions that are being used. Another thing is that each monster has a certain area that they stay in and are only active when a player is in said area. I was wondering if maybe there is a way to deactivate the non-active monsters from bullet(reactivating once in the area again) or maybe theres a different broadphase interface that I should use. The current BPI is btDbvtBroadphase. EDIT: Here is the Profile on the other computer(the top one is my main) Each sample counts as 0.01 seconds. % cumulative self self total time seconds seconds calls ms/call ms/call name 12.18 1.19 1.19 SphereTriangleDetector::collide(btVector3 const&, btVector3&, btVector3&, float&, float&, float) 6.76 1.85 0.66 btSphereTriangleCollisionAlgorithm::processCollision(btCollisionObject*, btCollisionObject*, btDispatcherInfo const&, btManifoldResult*) 5.83 2.42 0.57 btQuantizedBvh::reportAabbOverlappingNodex(btNodeOverlapCallback*, btVector3 const&, btVector3 const&) const 5.12 2.92 0.50 btConvexTriangleCallback::processTriangle(btVector3*, int, int) 4.61 3.37 0.45 btTriangleShape::getVertex(int, btVector3&) const 4.09 3.77 0.40 _Unwind_SjLj_Register 3.48 4.11 0.34 btBvhTriangleMeshShape::processAllTriangles(btTriangleCallback*, btVector3 const&, btVector3 const&) const::MyNodeOverlapCallback::processNode(int, int) 2.46 4.35 0.24 btSequentialImpulseConstraintSolver::resolveSingleConstraintRowLowerLimit(btRigidBody&, btRigidBody&, btSolverConstraint const&) 2.15 4.56 0.21 _Unwind_SjLj_Unregister 2.15 4.77 0.21 SphereTriangleDetector::getClosestPoints(btDiscreteCollisionDetectorInterface::ClosestPointInput const&, btDiscreteCollisionDetectorInterface::Result&, btIDebugDraw*, bool) 1.84 4.95 0.18 btTriangleShape::getEdge(int, btVector3&, btVector3&) const 1.64 5.11 0.16 btSequentialImpulseConstraintSolver::resolveSingleConstraintRowGeneric(btRigidBody&, btRigidBody&, btSolverConstraint const&) 1.54 5.26 0.15 btSequentialImpulseConstraintSolver::setupContactConstraint(btSolverConstraint&, btCollisionObject*, btCollisionObject*, btManifoldPoint&, btContactSolverInfo const&, btVector3&, float&, float&, btVector3&, btVector3&) 1.43 5.40 0.14 Ogre::D3D9HardwareVertexBuffer::updateBufferResources(char const*, Ogre::D3D9HardwareVertexBuffer::BufferResources*) 1.33 5.53 0.13 btManifoldResult::addContactPoint(btVector3 const&, btVector3 const&, float) 1.13 5.64 0.11 btRigidBody::predictIntegratedTransform(float, btTransform&) 1.13 5.75 0.11 btTriangleIndexVertexArray::getLockedReadOnlyVertexIndexBase(unsigned char const**, int&, PHY_ScalarType&, int&, unsigned char const**, int&, int&, PHY_ScalarType&, int) const 1.02 5.85 0.10 btSphereTriangleCollisionAlgorithm::CreateFunc::CreateCollisionAlgorithm(btCollisionAlgorithmConstructionInfo&, btCollisionObject*, btCollisionObject*) 1.02 5.95 0.10 btSphereTriangleCollisionAlgorithm::btSphereTriangleCollisionAlgorithm(btPersistentManifold*, btCollisionAlgorithmConstructionInfo const&, btCollisionObject*, btCollisionObject*, bool) Edited same as other Profile.

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• RK4 Bouncing a Ball

  - by Jonathan Dickinson

  I am trying to wrap my head around RK4. I decided to do the most basic 'ball with gravity that bounces' simulation. I have implemented the following integrator given Glenn Fiedler's tutorial: /// <summary> /// Represents physics state. /// </summary> public struct State { // Also used internally as derivative. // S: Position // D: Velocity. /// <summary> /// Gets or sets the Position. /// </summary> public Vector2 X; // S: Position // D: Acceleration. /// <summary> /// Gets or sets the Velocity. /// </summary> public Vector2 V; } /// <summary> /// Calculates the force given the specified state. /// </summary> /// <param name="state">The state.</param> /// <param name="t">The time.</param> /// <param name="acceleration">The value that should be updated with the acceleration.</param> public delegate void EulerIntegrator(ref State state, float t, ref Vector2 acceleration); /// <summary> /// Represents the RK4 Integrator. /// </summary> public static class RK4 { private const float OneSixth = 1.0f / 6.0f; private static void Evaluate(EulerIntegrator integrator, ref State initial, float t, float dt, ref State derivative, ref State output) { var state = new State(); // These are a premature optimization. I like premature optimization. // So let's not concentrate on that. state.X.X = initial.X.X + derivative.X.X * dt; state.X.Y = initial.X.Y + derivative.X.Y * dt; state.V.X = initial.V.X + derivative.V.X * dt; state.V.Y = initial.V.Y + derivative.V.Y * dt; output = new State(); output.X.X = state.V.X; output.X.Y = state.V.Y; integrator(ref state, t + dt, ref output.V); } /// <summary> /// Performs RK4 integration over the specified state. /// </summary> /// <param name="eulerIntegrator">The euler integrator.</param> /// <param name="state">The state.</param> /// <param name="t">The t.</param> /// <param name="dt">The dt.</param> public static void Integrate(EulerIntegrator eulerIntegrator, ref State state, float t, float dt) { var a = new State(); var b = new State(); var c = new State(); var d = new State(); Evaluate(eulerIntegrator, ref state, t, 0.0f, ref a, ref a); Evaluate(eulerIntegrator, ref state, t + dt * 0.5f, dt * 0.5f, ref a, ref b); Evaluate(eulerIntegrator, ref state, t + dt * 0.5f, dt * 0.5f, ref b, ref c); Evaluate(eulerIntegrator, ref state, t + dt, dt, ref c, ref d); a.X.X = OneSixth * (a.X.X + 2.0f * (b.X.X + c.X.X) + d.X.X); a.X.Y = OneSixth * (a.X.Y + 2.0f * (b.X.Y + c.X.Y) + d.X.Y); a.V.X = OneSixth * (a.V.X + 2.0f * (b.V.X + c.V.X) + d.V.X); a.V.Y = OneSixth * (a.V.Y + 2.0f * (b.V.Y + c.V.Y) + d.V.Y); state.X.X = state.X.X + a.X.X * dt; state.X.Y = state.X.Y + a.X.Y * dt; state.V.X = state.V.X + a.V.X * dt; state.V.Y = state.V.Y + a.V.Y * dt; } } After reading over the tutorial I noticed a few things that just seemed 'out' to me. Notably how the entire simulation revolves around t at 0 and state at 0 - considering that we are working out a curve over the duration it seems logical that RK4 wouldn't be able to handle this simple scenario. Never-the-less I forged on and wrote a very simple Euler integrator: static void Integrator(ref State state, float t, ref Vector2 acceleration) { if (state.X.Y > 100 && state.V.Y > 0) { // Bounce vertically. acceleration.Y = -state.V.Y * t; } else { acceleration.Y = 9.8f; } } I then ran the code against a simple fixed-time step loop and this is what I got: 0.05 0.20 0.44 0.78 1.23 1.76 ... 74.53 78.40 82.37 86.44 90.60 94.86 99.23 103.05 105.45 106.94 107.86 108.42 108.76 108.96 109.08 109.15 109.19 109.21 109.23 109.23 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 109.24 ... As I said, I was expecting it to break - however I am unsure of how to fix it. I am currently looking into keeping the previous state and time, and working from that - although at the same time I assume that will defeat the purpose of RK4. How would I get this simulation to print the expected results?

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• 2D OBB collision detection, resolving collisions?

  - by Milo

  I currently use OBBs and I have a vehicle that is a rigid body and some buildings. Here is my update() private void update() { camera.setPosition((vehicle.getPosition().x * camera.getScale()) - ((getWidth() ) / 2.0f), (vehicle.getPosition().y * camera.getScale()) - ((getHeight() ) / 2.0f)); //camera.move(input.getAnalogStick().getStickValueX() * 15.0f, input.getAnalogStick().getStickValueY() * 15.0f); if(input.isPressed(ControlButton.BUTTON_GAS)) { vehicle.setThrottle(1.0f, false); } if(input.isPressed(ControlButton.BUTTON_BRAKE)) { vehicle.setBrakes(1.0f); } vehicle.setSteering(input.getAnalogStick().getStickValueX()); vehicle.update(16.6666f / 1000.0f); ArrayList<Building> buildings = city.getBuildings(); for(Building b : buildings) { if(vehicle.getRect().overlaps(b.getRect())) { vehicle.update(-17.0f / 1000.0f); break; } } } The collision detection works well. What doesn't is how they are dealt with. My goal is simple. If the vehicle hits a building, it should stop, and never go into the building. When I apply negative torque to reverse the car should not feel buggy and move away from the building. I don't want this to look buggy. This is my rigid body class: class RigidBody extends Entity { //linear private Vector2D velocity = new Vector2D(); private Vector2D forces = new Vector2D(); private float mass; //angular private float angularVelocity; private float torque; private float inertia; //graphical private Vector2D halfSize = new Vector2D(); private Bitmap image; public RigidBody() { //set these defaults so we don't get divide by zeros mass = 1.0f; inertia = 1.0f; } //intialize out parameters public void initialize(Vector2D halfSize, float mass, Bitmap bitmap) { //store physical parameters this.halfSize = halfSize; this.mass = mass; image = bitmap; inertia = (1.0f / 20.0f) * (halfSize.x * halfSize.x) * (halfSize.y * halfSize.y) * mass; RectF rect = new RectF(); float scalar = 10.0f; rect.left = (int)-halfSize.x * scalar; rect.top = (int)-halfSize.y * scalar; rect.right = rect.left + (int)(halfSize.x * 2.0f * scalar); rect.bottom = rect.top + (int)(halfSize.y * 2.0f * scalar); setRect(rect); } public void setLocation(Vector2D position, float angle) { getRect().set(position, getWidth(), getHeight(), angle); } public Vector2D getPosition() { return getRect().getCenter(); } @Override public void update(float timeStep) { //integrate physics //linear Vector2D acceleration = Vector2D.scalarDivide(forces, mass); velocity = Vector2D.add(velocity, Vector2D.scalarMultiply(acceleration, timeStep)); Vector2D c = getRect().getCenter(); c = Vector2D.add(getRect().getCenter(), Vector2D.scalarMultiply(velocity , timeStep)); setCenter(c.x, c.y); forces = new Vector2D(0,0); //clear forces //angular float angAcc = torque / inertia; angularVelocity += angAcc * timeStep; setAngle(getAngle() + angularVelocity * timeStep); torque = 0; //clear torque } //take a relative Vector2D and make it a world Vector2D public Vector2D relativeToWorld(Vector2D relative) { Matrix mat = new Matrix(); float[] Vector2Ds = new float[2]; Vector2Ds[0] = relative.x; Vector2Ds[1] = relative.y; mat.postRotate(JMath.radToDeg(getAngle())); mat.mapVectors(Vector2Ds); return new Vector2D(Vector2Ds[0], Vector2Ds[1]); } //take a world Vector2D and make it a relative Vector2D public Vector2D worldToRelative(Vector2D world) { Matrix mat = new Matrix(); float[] Vectors = new float[2]; Vectors[0] = world.x; Vectors[1] = world.y; mat.postRotate(JMath.radToDeg(-getAngle())); mat.mapVectors(Vectors); return new Vector2D(Vectors[0], Vectors[1]); } //velocity of a point on body public Vector2D pointVelocity(Vector2D worldOffset) { Vector2D tangent = new Vector2D(-worldOffset.y, worldOffset.x); return Vector2D.add( Vector2D.scalarMultiply(tangent, angularVelocity) , velocity); } public void applyForce(Vector2D worldForce, Vector2D worldOffset) { //add linear force forces = Vector2D.add(forces ,worldForce); //add associated torque torque += Vector2D.cross(worldOffset, worldForce); } @Override public void draw( GraphicsContext c) { c.drawRotatedScaledBitmap(image, getPosition().x, getPosition().y, getWidth(), getHeight(), getAngle()); } } Essentially, when any rigid body hits a building it should exhibit the same behavior. How is collision solving usually done? Thanks

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• What kind of physics to choose for our arcade 3D MMO?

  - by Nick

  We're creating an action MMO using Three.js (WebGL) with an arcadish feel, and implementing physics for it has been a pain in the butt. Our game has a terrain where the character will walk on, and in the future 3D objects (a house, a tree, etc) that will have collisions. In terms of complexity, the physics engine should be like World of Warcraft. We don't need friction, bouncing behaviour or anything more complex like joints, etc. Just gravity. I have managed to implement terrain physics so far by casting a ray downwards, but it does not take into account possible 3D objects. Note that these 3D objects need to have convex collisions, so our artists create a 3D house and the player can walk inside but can't walk through the walls. How do I implement proper collision detection with 3D objects like in World of Warcraft? Do I need an advanced physics engine? I read about Physijs which looks cool, but I fear that it may be overkill to implement that for our game. Also, how does WoW do it? Do they have a separate raycasting system for the terrain? Or do they treat the terrain like any other convex mesh? A screenshot of our game so far:

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• creating a contact listener with sprites from different classes

  - by wilM

  I've been trying to set a contact listener that creates a joint on contact between two sprites which have their own individual classes. Both sprites are inheriting from NSObject and their are initialized in their parentlayer (init method of HelloWorldLayer.mm). It is quite straightforward when everything is in the same class, but in a case like this where sprites have their own classes how will it be done. Please Help, I've been at it for days.

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• How does braking assist of car racing games work?

  - by Ayush Khemka

  There are a lot of PC car racing games around which have this unique driving assist which helps brake your car so that you can safely turn it. While in some games it just an 'assist', it will just help your car brake but won't ensure a safe turn. While in others, the braking assist will help you get a safe turn. I was wondering on what could be the algorithm that is followed to achieve it. A very basic algorithm I could think of was, Pre-determine the braking distance of an ideal car for every turn of the track, depending on the radius of the turn, and then start braking the car accordingly. For example, for a turn of less than 90o, the car would start braking automatically at 50m distance from the start of the turn. A more advanced algorithm, which would ensure a safe turn, could be Pre-determine the speed of the car at the start of each turn, individually for each track, turn and car. Also, pre-determine the deceleration rate of each car individually, which varies because of the car's performance. The braking assist would keep recording the speed of the car at a certain instant of time. Start braking the car appropriately so that the car gets to the exact speed needed at the start of the turn. For example, let the speed of a particular car at the start of a turn 43m in radius, be 120km/h. Let the deceleration rate of the car be 200km/h2. If, at some instant of time, the speed of the car is 200km/h, then the car would automatically start braking at 400m from the start of the turn.

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• Unity - Mecanim & Rigidbody on Third Person Controller - Gravity bug?

  - by Celtc

  I'm working on a third person controller which uses physX to interact with the other objects (using the Rigidbody component) and Mecanim to animate the character. All the animations used are baked to Y, and the movement on this axis is controlled by the gravity applied by the rigidbody component. The configuration of the falling animation: And the character components configuration: Since the falling animation doesn't have root motion on XZ, I move the character on XZ by code. Like this: // On the Ground if (IsGrounded()) { GroundedMovementMgm(); // Stores the velocity velocityPreFalling = rigidbody.velocity; } // Mid-Air else { // Continue the pre falling velocity rigidbody.velocity = new Vector3(velocityPreFalling.x, rigidbody.velocity.y, velocityPreFalling.z); } The problem is that when the chracter starts falling and hit against a wall in mid air, it gets stuck to the wall. Here are some pics which explains the problems: Hope someone can help me. Thanks and sory for my bad english! PD.: I was asked for the IsGrounded() function, so I'm adding it: void OnCollisionEnter(Collision collision) { if (!grounded) TrackGrounded(collision); } void OnCollisionStay(Collision collision) { TrackGrounded(collision); } void OnCollisionExit() { grounded = false; } public bool IsGrounded() { return grounded; } private void TrackGrounded(Collision collision) { var maxHeight = capCollider.bounds.min.y + capCollider.radius * .9f; foreach (var contact in collision.contacts) { if (contact.point.y < maxHeight && Vector3.Angle(contact.normal, Vector3.up) < maxSlopeAngle) { grounded = true; break; } } } I'll also add a LINK to download the project if someone wants it.

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• Unity3d Gravity script issues

  - by Joseph Le Brech

  I'm try this script out http://wiki.unity3d.com/index.php/Gravity and I'm having some issues with it (it seemed to work when I tried it with an old version of unity) the first issue is of collision, the objects (in my case spheres) will stick into each other rather than just touch. and the second is that when the objects collide one of the objects with continue it's trajectory. I'm thinking of rewriting the script from scratch unless someone can explain what's wrong with the script that i've got.

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• Point inside Oriented Bounding Box?

  - by Milo

  I have an OBB2D class based on SAT. This is my point in OBB method: public boolean pointInside(float x, float y) { float newy = (float) (Math.sin(angle) * (y - center.y) + Math.cos(angle) * (x - center.x)); float newx = (float) (Math.cos(angle) * (x - center.x) - Math.sin(angle) * (y - center.y)); return (newy > center.y - (getHeight() / 2)) && (newy < center.y + (getHeight() / 2)) && (newx > center.x - (getWidth() / 2)) && (newx < center.x + (getWidth() / 2)); } public boolean pointInside(Vector2D v) { return pointInside(v.x,v.y); } Here is the rest of the class; the parts that pertain: public class OBB2D { private Vector2D projVec = new Vector2D(); private static Vector2D projAVec = new Vector2D(); private static Vector2D projBVec = new Vector2D(); private static Vector2D tempNormal = new Vector2D(); private Vector2D deltaVec = new Vector2D(); private ArrayList<Vector2D> collisionPoints = new ArrayList<Vector2D>(); // Corners of the box, where 0 is the lower left. private Vector2D corner[] = new Vector2D[4]; private Vector2D center = new Vector2D(); private Vector2D extents = new Vector2D(); private RectF boundingRect = new RectF(); private float angle; //Two edges of the box extended away from corner[0]. private Vector2D axis[] = new Vector2D[2]; private double origin[] = new double[2]; public OBB2D(float centerx, float centery, float w, float h, float angle) { for(int i = 0; i < corner.length; ++i) { corner[i] = new Vector2D(); } for(int i = 0; i < axis.length; ++i) { axis[i] = new Vector2D(); } set(centerx,centery,w,h,angle); } public OBB2D(float left, float top, float width, float height) { for(int i = 0; i < corner.length; ++i) { corner[i] = new Vector2D(); } for(int i = 0; i < axis.length; ++i) { axis[i] = new Vector2D(); } set(left + (width / 2), top + (height / 2),width,height,0.0f); } public void set(float centerx,float centery,float w, float h,float angle) { float vxx = (float)Math.cos(angle); float vxy = (float)Math.sin(angle); float vyx = (float)-Math.sin(angle); float vyy = (float)Math.cos(angle); vxx *= w / 2; vxy *= (w / 2); vyx *= (h / 2); vyy *= (h / 2); corner[0].x = centerx - vxx - vyx; corner[0].y = centery - vxy - vyy; corner[1].x = centerx + vxx - vyx; corner[1].y = centery + vxy - vyy; corner[2].x = centerx + vxx + vyx; corner[2].y = centery + vxy + vyy; corner[3].x = centerx - vxx + vyx; corner[3].y = centery - vxy + vyy; this.center.x = centerx; this.center.y = centery; this.angle = angle; computeAxes(); extents.x = w / 2; extents.y = h / 2; computeBoundingRect(); } //Updates the axes after the corners move. Assumes the //corners actually form a rectangle. private void computeAxes() { axis[0].x = corner[1].x - corner[0].x; axis[0].y = corner[1].y - corner[0].y; axis[1].x = corner[3].x - corner[0].x; axis[1].y = corner[3].y - corner[0].y; // Make the length of each axis 1/edge length so we know any // dot product must be less than 1 to fall within the edge. for (int a = 0; a < axis.length; ++a) { float l = axis[a].length(); float ll = l * l; axis[a].x = axis[a].x / ll; axis[a].y = axis[a].y / ll; origin[a] = corner[0].dot(axis[a]); } } public void computeBoundingRect() { boundingRect.left = JMath.min(JMath.min(corner[0].x, corner[3].x), JMath.min(corner[1].x, corner[2].x)); boundingRect.top = JMath.min(JMath.min(corner[0].y, corner[1].y),JMath.min(corner[2].y, corner[3].y)); boundingRect.right = JMath.max(JMath.max(corner[1].x, corner[2].x), JMath.max(corner[0].x, corner[3].x)); boundingRect.bottom = JMath.max(JMath.max(corner[2].y, corner[3].y),JMath.max(corner[0].y, corner[1].y)); } public void set(RectF rect) { set(rect.centerX(),rect.centerY(),rect.width(),rect.height(),0.0f); } // Returns true if other overlaps one dimension of this. private boolean overlaps1Way(OBB2D other) { for (int a = 0; a < axis.length; ++a) { double t = other.corner[0].dot(axis[a]); // Find the extent of box 2 on axis a double tMin = t; double tMax = t; for (int c = 1; c < corner.length; ++c) { t = other.corner[c].dot(axis[a]); if (t < tMin) { tMin = t; } else if (t > tMax) { tMax = t; } } // We have to subtract off the origin // See if [tMin, tMax] intersects [0, 1] if ((tMin > 1 + origin[a]) || (tMax < origin[a])) { // There was no intersection along this dimension; // the boxes cannot possibly overlap. return false; } } // There was no dimension along which there is no intersection. // Therefore the boxes overlap. return true; } public void moveTo(float centerx, float centery) { float cx,cy; cx = center.x; cy = center.y; deltaVec.x = centerx - cx; deltaVec.y = centery - cy; for (int c = 0; c < 4; ++c) { corner[c].x += deltaVec.x; corner[c].y += deltaVec.y; } boundingRect.left += deltaVec.x; boundingRect.top += deltaVec.y; boundingRect.right += deltaVec.x; boundingRect.bottom += deltaVec.y; this.center.x = centerx; this.center.y = centery; computeAxes(); } // Returns true if the intersection of the boxes is non-empty. public boolean overlaps(OBB2D other) { if(right() < other.left()) { return false; } if(bottom() < other.top()) { return false; } if(left() > other.right()) { return false; } if(top() > other.bottom()) { return false; } if(other.getAngle() == 0.0f && getAngle() == 0.0f) { return true; } return overlaps1Way(other) && other.overlaps1Way(this); } public Vector2D getCenter() { return center; } public float getWidth() { return extents.x * 2; } public float getHeight() { return extents.y * 2; } public void setAngle(float angle) { set(center.x,center.y,getWidth(),getHeight(),angle); } public float getAngle() { return angle; } public void setSize(float w,float h) { set(center.x,center.y,w,h,angle); } public float left() { return boundingRect.left; } public float right() { return boundingRect.right; } public float bottom() { return boundingRect.bottom; } public float top() { return boundingRect.top; } public RectF getBoundingRect() { return boundingRect; } public boolean overlaps(float left, float top, float right, float bottom) { if(right() < left) { return false; } if(bottom() < top) { return false; } if(left() > right) { return false; } if(top() > bottom) { return false; } return true; } public static float distance(float ax, float ay,float bx, float by) { if (ax < bx) return bx - ay; else return ax - by; } public Vector2D project(float ax, float ay) { projVec.x = Float.MAX_VALUE; projVec.y = Float.MIN_VALUE; for (int i = 0; i < corner.length; ++i) { float dot = Vector2D.dot(corner[i].x,corner[i].y,ax,ay); projVec.x = JMath.min(dot, projVec.x); projVec.y = JMath.max(dot, projVec.y); } return projVec; } public Vector2D getCorner(int c) { return corner[c]; } public int getNumCorners() { return corner.length; } public boolean pointInside(float x, float y) { float newy = (float) (Math.sin(angle) * (y - center.y) + Math.cos(angle) * (x - center.x)); float newx = (float) (Math.cos(angle) * (x - center.x) - Math.sin(angle) * (y - center.y)); return (newy > center.y - (getHeight() / 2)) && (newy < center.y + (getHeight() / 2)) && (newx > center.x - (getWidth() / 2)) && (newx < center.x + (getWidth() / 2)); } public boolean pointInside(Vector2D v) { return pointInside(v.x,v.y); } public ArrayList<Vector2D> getCollsionPoints(OBB2D b) { collisionPoints.clear(); for(int i = 0; i < corner.length; ++i) { if(b.pointInside(corner[i])) { collisionPoints.add(corner[i]); } } for(int i = 0; i < b.corner.length; ++i) { if(pointInside(b.corner[i])) { collisionPoints.add(b.corner[i]); } } return collisionPoints; } }; What could be wrong? When I getCollisionPoints for 2 OBBs I know are penetrating, it returns no points. Thanks

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• How could you parallelise a 2D boids simulation

  - by Sycren

  How could you program a 2D boids simulation in such a way that it could use processing power from different sources (clusters, gpu). In the above example, the non-coloured particles move around until they cluster (yellow) and stop moving. The problem is that all the entities could potentially interact with each other although an entity in the top left is unlikely to interact with one in the bottom right. If the domain was split into different segments, it may speed the whole thing up, But if an entity wanted to cross into another segment there may be problems. At the moment this simulation works with 5000 entities with a good frame rate, I would like to try this with millions if possible. Would it be possible to use quad trees to further optimise this? Any other suggestions?

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• How do I make good guy attacks only hit bad guys and vice versa?

  - by tieTYT

  My game has many different type of good guys and many different type of bad guys. They will all be firing projectiles at each other but I don't want any accidental collateral damage to occur for either alignment. So bad guys should not be able to hit/damage other bad guys and good guys should not be able to hit/damage other good guys. The way I'm thinking of solving this is by making it so that the Unit instance (this is javascript, btw), has an alignment property that can be either good or bad. And I'll only let collision happen if the class Attack boolean didAttackCollideWithTarget(target) return attack.source.alignment != target.alignment and collisionDetected(attack.source, target) This is pseudo-code, of course. But I'm asking this question because I get the sense that there might be a much more elegant way to design this besides adding yet another property to my Unit class.

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• Follow point of interest by applying torque

  - by azymm

  Given a body with an orientation angle and a point of interest or targetAngle, is there an elegant solution for keeping the body oriented towards the point of interest by applying torque or impulses? I have a naive solution working below, but the effect is pretty 'wobbly', it'll overshoot each time, slowly getting closer to the target angle - undesirable effect in my case. I'd like to find a solution that is more intelligent - that can accelerate to near the target angle then decelerate and stop right at the target angle (or within a small range). If it helps, I'm using box2d and the body is a rectangle. def gameloop(dt): targetAngle = get_target_angle() bodyAngle = get_body_angle() deltaAngle = targetAngle - bodyAngle if deltaAngle > PI: deltaAngle = targetAngle - (bodyAngle + 2.0 * PI) if deltaAngle < -PI: deltaAngle = targetAngle - (bodyAngle - 2.0 * PI) # multiply by 2, for stronger reaction deltaAngle = deltaAngle * 2.0; body.apply_torque(deltaAngle); One other thing, when body has no linear velocity, the above solution works ok. But when the body has some linear velocity, the solution above causes really wonky movement. Not sure why, but would appreciate any hints as to why that might be.

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• How to move a sprite automatically using a physicsHandler in Andengine?

  - by shailenTJ

  I use a DigitalOnScreenControl (knob with a four-directional arrow control) to move the entity and the entity which is bound to a physicsHandler. physicsHandler.setEntity(sprite); sprite.registerUpdateHandler(physicsHandler); From the DigitalOnScreenControl, I know which direction I want my sprite to move. Inside its overridden onControlChange function, I call a function animateSprite that checks which direction I chose. Based on the direction, I animate my sprite differently. PROBLEM: I want to automatically move the sprite to a specific location on the scene, say at coordinates (207, 305). My sprite is at (100, 305, which means it has to move down by 107 pixels. How do I tell the physicsHandler to move the sprite down by 107 pixels? My animateSprite method will take care of animating the sprite's downward motion. Thank you for your input!

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• Is There a Center of the Universe?

  - by Akemi Iwaya

  From our earliest days mankind has debated what should be defined as the center of the universe, a ‘definition’ that continues to evolve as our technology and knowledge of the universe improves. Follow along with mankind’s historical progress on defining the center of the universe and learn some great facts about the debate with this terrific TEDEducation video. Is there a center of the universe? – Marjee Chmiel and Trevor Owens [YouTube]     

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• Jumping Vs. Gravity

  - by PhaDaPhunk

  Hi i'm working on my first XNA 2D game and I have a little problem. If I jump, my sprite jumps but does not fall down. And I also have another problem, the user can hold spacebar to jump as high as he wants and I don't know how to keep him from doing that. Here's my code: The Jump : if (FaKeyboard.IsKeyDown(Keys.Space)) { Jumping = true; xPosition -= new Vector2(0, 5); } if (xPosition.Y >= 10) { Jumping = false; Grounded = false; } The really simple basic Gravity: if (!Grounded && !Jumping) { xPosition += new Vector2(1, 3) * speed; } Here's where's the grounded is set to True or False with a Collision Rectangle MegamanRectangle = new Rectangle((int)xPosition.X, (int)xPosition.Y, FrameSizeDraw.X, FrameSizeDraw.Y); Rectangle Block1Rectangle = new Rectangle((int)0, (int)73, Block1.Width, Block1.Height); Rectangle Block2Rectangle = new Rectangle((int)500, (int)73, Block2.Width, Block2.Height); if ((MegamanRectangle.Intersects(Block1Rectangle) || (MegamanRectangle.Intersects(Block2Rectangle)))) { Grounded = true; } else { Grounded = false; } The grounded bool and The gravity have been tested and are working. Any ideas why? Thanks in advance and don't hesitate to ask if you need another Part of the Code.

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• How Do Guns Work In Space? [Video]

  - by Jason Fitzpatrick

  Why don’t astronauts fall back to Earth? What happens if you shoot a gun in space? How big of a squirt gun would you need to put out the sun? Don’t end your day with these pressing questions unanswered. [via Boing Boing] HTG Explains: Why Do Hard Drives Show the Wrong Capacity in Windows? Java is Insecure and Awful, It’s Time to Disable It, and Here’s How What Are the Windows A: and B: Drives Used For?

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• Do I need path finding to make AI avoid obstacles?

  - by yannicuLar

  How do you know when a path-finding algorithm is really needed? There are contexts, where you just want to improve AI navigation to avoid an object, like a space -ship that won't crash on a planet or a car that already knows where to steer, but needs small corrections to avoid a road bump. As I've seen on similar posts, the obvious solution is to implement some path-finding algorithm, most likely like A*, and let your AI-controlled object to navigate through the path. Now, I have the necessary skills to implement a path-finding algorithm, and I'm not being lazy here, but I'm still a bit skeptical on if this is really needed. I have the impression that path-finding is appropriate to navigate through a maze, or picking a path when there are many alternatives. But in obstacle avoidance, when you do know the path, but need to make slight corrections, is path finding really necessary? Even when the obstacles are too sparse or small ? I mean, in real life, when you're driving and notice a bump on the road, you will just have to pick between steering a bit on the left (and have the bump on your right side) or the other way around. You will not consider stopping, or going backwards. A path finding would be appropriate when you need to pick a route through the city, right ? So, are there any other methods to help AI navigation, except path-finding? And if there are, how do you know when path-fining is the appropriate algorithm ? Thanks for any thoughts

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• PhysicsMouseJoint problem in andengine + Box2d

  - by Nikhil Lamba

  What we can remove from this code i.e from PhysicsMouseJointExample to remove the functionality of drag and drog of sprite but i need all functionality except this only user move the sprite with some force and velocity of fling but user can't move the ball as like drag and drop like moving a finger on screen and sprite move with finger plz plz help me I am Using Below method for Mouse Joint CODE : public MouseJoint createMouseJoint(final IShape pFace, final float pTouchAreaLocalX, final float pTouchAreaLocalY) { final Body body = (Body) pFace.getUserData(); final MouseJointDef mouseJointDef = new MouseJointDef(); final Vector2 localPoint = Vector2Pool.obtain((pTouchAreaLocalX - pFace.getWidth() * 0.5f) / PhysicsConstants.PIXEL_TO_METER_RATIO_DEFAULT, (pTouchAreaLocalY - pFace.getHeight() * 0.5f) / PhysicsConstants.PIXEL_TO_METER_RATIO_DEFAULT); this.groundBody.setTransform(localPoint, 0); mouseJointDef.bodyA = this.groundBody; mouseJointDef.bodyB = body; mouseJointDef.dampingRatio = 0.95f; mouseJointDef.frequencyHz = 30; mouseJointDef.maxForce = (200.0f * body.getMass()); mouseJointDef.collideConnected = true; mouseJointDef.target.set(body.getWorldPoint(localPoint)); Vector2Pool.recycle(localPoint); return (MouseJoint)mPhysicsWorld.createJoint(mouseJointDef); }

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• How do you stop OgreBullet Capsule from falling over?

  - by Nathan Baggs

  I've just started implementing bullet into my Ogre project. I followed the install instructions here: http://www.ogre3d.org/tikiwiki/OgreBullet+Tutorial+1 And the rest if the tutorial here: http://www.ogre3d.org/tikiwiki/OgreBullet+Tutorial+2 I got that to work fine however now I wanted to extend it to a handle a first person camera. I created a CapsuleShape and a Rigid Body (like the tutorial did for the boxes) however when I run the game the capsule falls over and rolls around on the floor, causing the camera swing wildly around. I need a way to fix the capsule to always stay upright, but I have no idea how Below is the code I'm using. (part of) Header File OgreBulletDynamics::DynamicsWorld *mWorld; // OgreBullet World OgreBulletCollisions::DebugDrawer *debugDrawer; std::deque<OgreBulletDynamics::RigidBody *> mBodies; std::deque<OgreBulletCollisions::CollisionShape *> mShapes; OgreBulletCollisions::CollisionShape *character; OgreBulletDynamics::RigidBody *characterBody; Ogre::SceneNode *charNode; Ogre::Camera* mCamera; Ogre::SceneManager* mSceneMgr; Ogre::RenderWindow* mWindow; main file bool MinimalOgre::go(void) { ... mCamera = mSceneMgr->createCamera("PlayerCam"); mCamera->setPosition(Vector3(0,0,0)); mCamera->lookAt(Vector3(0,0,300)); mCamera->setNearClipDistance(5); mCameraMan = new OgreBites::SdkCameraMan(mCamera); OgreBulletCollisions::CollisionShape *Shape; Shape = new OgreBulletCollisions::StaticPlaneCollisionShape(Vector3(0,1,0), 0); // (normal vector, distance) OgreBulletDynamics::RigidBody *defaultPlaneBody = new OgreBulletDynamics::RigidBody( "BasePlane", mWorld); defaultPlaneBody->setStaticShape(Shape, 0.1, 0.8); // (shape, restitution, friction) // push the created objects to the deques mShapes.push_back(Shape); mBodies.push_back(defaultPlaneBody); character = new OgreBulletCollisions::CapsuleCollisionShape(1.0f, 1.0f, Vector3(0, 1, 0)); charNode = mSceneMgr->getRootSceneNode()->createChildSceneNode(); charNode->attachObject(mCamera); charNode->setPosition(mCamera->getPosition()); characterBody = new OgreBulletDynamics::RigidBody("character", mWorld); characterBody->setShape( charNode, character, 0.0f, // dynamic body restitution 10.0f, // dynamic body friction 10.0f, // dynamic bodymass Vector3(0,0,0), Quaternion(0, 0, 1, 0)); mShapes.push_back(character); mBodies.push_back(characterBody); ... }

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• Perpendicularity of a normal and a velocity?

  - by Milo

  I'm trying to fake angular velocity on my vehicle when it hits a wall by getting the dot product of the normal of the edge the car is hitting and the vehicle's velocity: Vector2D normVel = new Vector2D(); normVel.equals(vehicle.getVelocity()); normVel.normalize(); float dot = normVel.dot(outNorm); dot = -dot; vehicle.setAngularVelocity(vehicle.getAngularVelocity() + (dot * vehicle.getVelocity().length() * 0.01f)); outNorm is the normal of the wall. The problem is it only works half the time. It seems no matter what, the car always goes clockwise. If the car should head clockwise: -------------------------------------- / / I want the angular velocity to be positive, otherwise if it needs to go CCW: -------------------------------------- \ \ Then the angular velocity should be negative... What should I change to achieve this? Thanks Hmmm... Im not sure why this is not working... for(int i = 0; i < buildings.size(); ++i) { e = buildings.get(i); ArrayList<Vector2D> colPts = vehicle.getRect().getCollsionPoints(e.getRect()); float dist = OBB2D.collisionResponse(vehicle.getRect(), e.getRect(), outNorm); for(int u = 0; u < colPts.size(); ++u) { Vector2D p = colPts.get(u).subtract(vehicle.getRect().getCenter()); vehicle.setTorque(vehicle.getTorque() + p.cross(outNorm)); }

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