News Mar 20: R.U.B.E v1.7 released. Instanciable objects!
Feb 21: Added RUBE sample loader for Cocos2d-X v3.4.
Feb 17: Funny Fists released in iOS App store and Google Play store. (YouTube video)
May 25: R.U.B.E v1.6 released. Samplers!
May 12: "On-the-fly" resource updating for Cocos2d-x
Apr 22: New blog post: Inverted pendulum control
Apr 7: On-screen logging class for Cocos2d-x
Mar 22: Downhill Supreme 2 released for iOS and Android
Jan 2: YouTube video: Making soft-body wheels in RUBE
Oct 6: Check out Supplyfront RTS, my 7dRTS entry continued.
Sep 26: R.U.B.E v1.5 released. Customizable item labels, snap-to-grid, export filtering, full-text help search.
Sep 18: Added RUBE sample loader for Cocos2d-X.
Sep 16: Updated RUBE sample loader for Cocos2d v2.
Aug 12: RUBE loader for Nape by Zeh Fernando
Aug 7: Added RUBE sample loader for SFML.
Jul 30: Try my MiniLD 7dRTS entry.
Jul 24: Added physics-driven particles tutorial.
Jul 20: New blog post: rendering solid ground (as in Downhill Supreme)
Jul 18: R.U.B.E v1.4 released. Command-line interface for batch jobs, hotkeys for scripts, better script management.
May 22: Downhill Supreme is out now! (iOS)
Apr 2: R.U.B.E v1.3 released. Collision bitplane editing, Cocos2d-iphone loader, usability improvements.
Mar 11: R.U.B.E v1.2 released. Now supports weld, friction, motor joints and automatic image reloading.
Mar 10: RUBE loader for libGDX by Tim Scott
Jan 28: New blog post: a functional combustion engine!
Jan 22: New blog post: wind tunnel
Jan 20: Added explosions tutorial.
Jan 16: Added buoyancy tutorial.
Jan 16: AndEngine sample project to load RUBE scene by Bart Hirst
Jan 14: R.U.B.E v1.1 released, now supports custom properties.
Jan 1: All basic tutorials are now available in Chinese at Huge thankyou to @OhCoder!
Dec 23: Discussion forums have been set up
Dec 4: R.U.B.E v1.0 is ready !
Dec 2: YouTube video: Box2D pendulum clock
Nov 26: New blog post: rocket platform thingy
Nov 25: Video of R.U.B.E scenes in Chipmunk.
Nov 23: A sample project to load R.U.B.E scenes into Chipmunk physics! Details here.
Nov 14: An XCode sample project to load R.U.B.E scenes on iOS is now available. Details here.
Nov 11: A Java version of b2dJson is now available, based on JBox2D.
Nov 6: A Javascript version of b2dJson based on box2dweb is now available. Demo here!
Nov 2: The full specification of the JSON format used by b2dJson can be found here: b2dJson file structure
Oct 28: YouTube video: 2-minute ragdoll in R.U.B.E Box2D editor
Sep 29: YouTube video: R.U.B.E Box2D editor usage example

Box2D C++ tutorials - Custom gravity

Last edited: July 14 2013

Chinese version -> 中文

Specifying a different gravity for each body

A question that comes up quite often is how to make certain bodies ignore gravity, while other bodies still obey gravity. This is really easy, so now that we know about forces let's try it. Start with the same scene as in the last topic, with three identical bodies.
  //class member variable to keep track of three bodies
    b2Body* bodies[3];
    FooTest() {
      //body definition
      b2BodyDef myBodyDef;
      myBodyDef.type = b2_dynamicBody;
      //shape definition
      b2PolygonShape polygonShape;
      polygonShape.SetAsBox(1, 1); //a 2x2 rectangle
      //fixture definition
      b2FixtureDef myFixtureDef;
      myFixtureDef.shape = &polygonShape;
      myFixtureDef.density = 1;
      //create identical bodies in different positions
      for (int i = 0; i < 3; i++) {
        myBodyDef.position.Set(-10+i*10, 20);
        bodies[i] = m_world->CreateBody(&myBodyDef);
      //a static floor to drop things on
      myBodyDef.type = b2_staticBody;
      myBodyDef.position.Set(0, 0);
      polygonShape.SetAsEdge( b2Vec2(-15,0), b2Vec2(15,0) );
Custom gravity Since gravity is really just the same as applying a linear force downwards every time step, all we need to do to counteract it is apply the same force upwards. The required force is relative to the mass of the body:
  //in the Step() function
  //cancel gravity for body 1 only
  bodies[1]->ApplyForce( bodies[1]->GetMass() * -m_world->GetGravity(), bodies[1]->GetWorldCenter() );
Custom gravity The same technique can be used to apply gravity in any direction you like, if you need to have something walking on walls or the ceiling for example.

Important: This gravity-cancelling force should be applied before the first time step. If you have the ApplyForce call after the world Step call in your main loop, the body will get one time step at normal gravity and it will have a chance to move down a tiny bit before the gravity is cancelled.

Note: the above is a typical solution for the v2.1.2 release of Box2D used for these tutorials. As of v2.2.1 each body has a 'gravity scale' to strengthen or weaken the effect of the world's gravity on it. This removes the need to manually apply a force every frame. You can set this in the body definition when you create it, or use:
  //Box2D v2.2.1 onwards
  body->SetGravityScale(0);//cancel gravity (use -1 to reverse gravity, etc)

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