/*

  This example highlights one of the interesting aspects of Polar coordinates;
  as theta increases, object moving only along theta will never leave the screen.
  
*/

// The radius at which our object will move
float r;

// basic components of movement
float theta;
float theta_vel;
float theta_accel;

void setup() {

    // setup the environment
    size(300, 300);
    colorMode(RGB, 255, 255, 255, 255);
    background(0);
    
    // give all of our basic values a starting value
    r = 75.0f;
    theta = 0.0f;
    theta_vel = 0.0f;
    theta_accel = 0.00001f;
}

void draw() {
    
    // Again; we'll be generating values that swing between positive
    // and negative, so we'll reset the center of the screen (0, 0), to
    // the actual center of the screen.
    translate(width / 2, height / 2);
    rectMode(CENTER);
    
    // fade out the background
    fill(0, 1);
    rect(0, 0, width, height);
    
    // calculate our position, cosine is related to the horizontal, sine to the vertical
    float x = r * cos(theta);
    float y = r * sin(theta);
    
    // draw our circle
    noStroke();
    fill(0, 0, 255);
    ellipse(x, y, 20, 20);
    
    // apply our forces to theta
    theta_vel += theta_accel;
    theta += theta_vel;
}