Creating your first project – the Pendulum example

Let's create an openFrameworks project, which draws a moving pendulum in 2D, consisting of a ball dangled on a rubber segment. The example is based on the emptyExample project in openFrameworks. Perform the following steps to create the project:

1. Copy the emptyExample project's folder into the folder intended for holding your applications (like apps/myApps), and rename it to Pendulum.

2. Go inside the Pendulum folder and open this project in your development environment (emptyExample.sln for Visual Studio, emptyExample.xcodeproj for Xcode, or emptyExample.workspace for Code::Blocks).

3. Open the file testApp.h in the development environment, and in the testApp class declaration add the declarations for the pendulum's center of suspension and the ball's position and velocity:

   ofPoint pos0;        //Center of suspension
   ofPoint pos;         //Ball's position
   ofPoint velocity;    //Ball's velocity

   Here ofPoint is the openFrameworks' class for holding point coordinates, it has x and y members (we will study it in Chapter 2, Drawing in 2D).

4. Open the file testApp.cpp, and fill the body of the testApp::setup() function definition:

   void testApp::setup(){
     //Set screen frame rate
     ofSetFrameRate( 60 );
   
     //Set initial values
     pos0 = ofPoint( 512, 300 );
     pos = ofPoint( 600, 200 );
     velocity = ofPoint( 100, 0 );
   }

   In this function we set the frame rate to 60 frames per second, and we also set initial values for all three points.

5. Now fill the body of the testApp::update() function definition:

   void testApp::update(){
     //Constants
     float dt = 1.0 / 60.0;         //Time step
     float mass = 0.1;              //Mass of a ball
     float rubberLen = 200.0;       //Segment's length
     float k = 0.5;                 //Segment's stiffness
     ofPoint g( 0.0, 9.8 );         //Gravity force
   
     //Compute Hooke's force
     ofPoint delta = pos - pos0;
     float len = delta.length();   //Vector's length
     float hookeValue = k * (len - rubberLen);
     delta.normalize();            //Normalize vector's length
     ofPoint hookeForce = delta * (-hookeValue);
   
     //Update velocity and pos
     ofPoint force = hookeForce + g;  //Resulted force
     ofPoint a = force / mass;        //Second Newton's law
     velocity += a * dt;              //Euler method
     pos += velocity * dt;            //Euler method
   }

   This function updates velocity and pos, using Newton's second law and the Euler method. For such a purpose, we compute the force acting on a ball as a sum of Hooke's force between the ball, suspension point, and gravity force.

   Tip

   The details on the Euler method can be seen in the Defining the particle functions section in Chapter 3, Building a Simple Particle System. The information on the Newton's second law, Hooke's force, and gravity force can be seen at the following links:

   • http://en.wikipedia.org/wiki/Newton's_laws_of_motion

   • http://en.wikipedia.org/wiki/Hooke's_law

   • http://en.wikipedia.org/wiki/Gravitational_field

6. Finally, fill the body of the testApp::draw() function definition:

   void testApp::draw(){
     //Set white background
     ofBackground( 255, 255, 255 );  
   
     //Draw rubber as a blue line
     ofSetColor( 0, 0, 255 );                 //Set blue color
     ofLine( pos0.x, pos0.y, pos.x, pos.y );  //Draw line
   
     //Draw ball as a red circle
     ofSetColor( 255, 0, 0 );                 //Set red color
     ofFill();                                //Enable filling
     ofCircle( pos.x, pos.y, 20 );            //Draw circle
   }

   Here we set a white background, draw a rubber as a blue line from pos0 to pos, and also draw a ball as a red circle. Note that we use the ofFill() function, which enables openFrameworks' mode to draw filled primitives (circles, rectangles, and triangles). See more details on these drawing functions in Chapter 2, Drawing in 2D.

7. Run the project. You will see the animation of a moving ball:

   

Play with numerical values in the setup() and update() functions and see how it affects the dynamics of the pendulum.