Shows a simple modification of gravity in Box2D

box2dModifyingGravity.cpp

#ifndef FOOTEST_H
#define FOOTEST_H

class FooTest : public Test
{
public:
    // List of shapes
    b2Body *dynamicBody; // The players shape
    b2Body* lowerground; // The horizontal platform
    b2Body* sideground; // The vertical platform
    bool isGravitySideWays;
    FooTest() {
        
        b2Vec2 gravity(0, -9.81); //normal earth gravity, 9.8 m/s/s straight down!
        isGravitySideWays = false; // Gravity is not sideways
        
        m_world->SetGravity(gravity);
        
        //Draw the "player" shape
        b2BodyDef myBodyDef;
        myBodyDef.type = b2_dynamicBody; //this will be a dynamic body
        myBodyDef.position.Set(0, 20); //set the starting position
        myBodyDef.angle = 0; //set the starting angle
        
        dynamicBody = m_world->CreateBody(&myBodyDef);
        
        b2CircleShape circleShape;
        circleShape.m_p.Set(0, 0); //position, relative to body position
        circleShape.m_radius = 1; //radius
        
        b2FixtureDef boxFixtureDef;
        boxFixtureDef.shape = &circleShape;
        boxFixtureDef.density = 1;
        dynamicBody->CreateFixture(&boxFixtureDef);
        
        //Draw the ground
        myBodyDef.type = b2_staticBody; //this will be a static body
        myBodyDef.position.Set(-5, 15); //slightly lower position
        lowerground = m_world->CreateBody(&myBodyDef); //add body to world
        
        b2PolygonShape hfloor;
        hfloor.SetAsBox(10,1);
        
        b2FixtureDef LongFixtureDef;
        LongFixtureDef.shape = &hfloor;
        LongFixtureDef.density = 1;
        
        //Draw the side "Ground"
        
        myBodyDef.type = b2_staticBody;
        myBodyDef.type = b2_staticBody; //this will be a static body
        myBodyDef.position.Set(15, 30); //slightly off to the right, and up a bit
        sideground = m_world->CreateBody(&myBodyDef); //add body to world
        
        b2PolygonShape sideFloor;
        sideFloor.SetAsBox(1,10);
        
        b2FixtureDef sideFixtureDef;
        sideFixtureDef.shape = &sideFloor;
        sideFixtureDef.density = 1;
        
        
        lowerground->CreateFixture(&LongFixtureDef); //add fixture to body
        sideground->CreateFixture(&sideFixtureDef);
    } //do nothing, no scene yet
    
    void Step(Settings* settings)
    {
        //run the default physics and rendering
        Test::Step(settings);
        
        b2Vec2 d1 = dynamicBody->GetPosition(); // Position of the player
        b2Vec2 d2 = lowerground->GetPosition(); // Position of the horizontal ground
        b2Vec2 d3 =  sideground->GetPosition(); // Position of the sideways ground
        d2 = d1 - d2;    //Distance between the horizontal ground and the player
        d3 = d1 - d3;    // Same as above, but for the side platform
        // Compare the lengths, and see which is shorter
        if(d2.Normalize() < d3.Normalize())
        {
            //Closer to d2, which is lowerground, so gravity acts down
            isGravitySideWays = false;
            m_world->SetGravity(b2Vec2(0,-9.81)); // This should be the normal to the surface that you want gravity to be acting towards
        
        }
        else
        {
            //Closer to d2, which is sideground, so gravity acts sideways
            isGravitySideWays = true;
            m_world->SetGravity(b2Vec2(9.81,0)); // Same as above, should actually be the normal to the lower ground surface.
        }


    }
    
    static Test* Create()
    {
        
        return new FooTest;
    }
    
    void Keyboard(unsigned char key)
    {
        float jump = 3000;
        float side = 350;
        b2Vec2 t;
        // Deal with user input
        switch (key)
        {
            case 'w':
                //apply force upwards
                //If user is closer to the side wall, then gravity acts sideways and can be set in the x direction
                if (isGravitySideWays){
                    t.Set(-1*jump, 0);
                }
                else
                {
                    t.Set(0,jump);
                }
                dynamicBody->ApplyForce( t, dynamicBody->GetWorldCenter() );
                break;
            // Set the moving sideways vectors, based on the current orientation
            case 'a':
                // apply force left
                if (isGravitySideWays)
                {
                    t.Set(0, -1*side);
                }
                else
                {
                    t.Set(-1*side,0);
                }
                dynamicBody->ApplyForce( t, dynamicBody->GetWorldCenter() );
                break;
            case 'd':
                //Go right
                if (isGravitySideWays)
                {
                    t.Set(0, side);
                }
                else
                {
                    t.Set(side,0);
                }
                dynamicBody->ApplyForce( t, dynamicBody->GetWorldCenter() );
                break;
                
            default:
                //run default behaviour
                Test::Keyboard(key);
        }
    }
    
};
#endif