Animation - making it independant of processor speed.

The frameRate (fps) method enables the programmer to specify the the number of frames per second (fps) to be displayed. It is tempting to use this to control the speed of animation but it is not guaranteed to give the same animation speed on all computers. The problem is that if the computer cannot achieve the desired frames per second (fps) then it will simply to the best it can, it means the same sketch may run at different speeds on different computers. One solution is to set a low fps but then the animation may become jerky, especially for fast moving objects.
The solution is to measure the elapsed time between frames and use this to control the animation speed.
If you run the following sketch, you wil see three balls moving across the screen and wrapping off the right-hand-side. The red ball is traveling at 50, the green at 100 and blue at 200 pixels a second.

// horizontal positions
float xR, xG, xB; 
// horizontal speeds
float speedR, speedG, speedB;
// distance to move the ball before moving back to the start
float trackDist;
// The system time for this and the previous frame (in milliseconds)
int currTime, prevTime;
// The elapsed time since the last frame (in seconds)
float deltaTime;

void setup() {
  size(800, 140);
  trackDist = width - 20;
  xR = 0;
  xG = 0;
  xB = 0;
  speedR = 50;
  speedG = 100;
  speedB = 200;
  // Initialise the timer
  currTime = prevTime = millis();

void draw() {
  // get the current time
  currTime = millis();
  // calculate the elapsed time in seconds
  deltaTime = (currTime - prevTime)/1000.0;
  // rember current time for the next frame
  prevTime = currTime;
  // Update the positions of the three balls
  xR = updatePosition(xR, speedR, deltaTime);
  xG = updatePosition(xG, speedG, deltaTime);
  xB = updatePosition(xB, speedB, deltaTime);
  // Start drawing
  fill(255, 0, 0);
  ellipse(xR + 10, 40, 20, 20);
  fill(0, 255, 0);
  ellipse(xG + 10, 70, 20, 20);
  fill(0, 0, 255);
  ellipse(xB + 10, 100, 20, 20);

// Calculates the new position based on current position, speed 
// and elapsed time. Also move the ball back to the start when
// it reaches the right-hand-side
float updatePosition(float x, float speed, float elapsedTime) {
  x += speed * elapsedTime;
  if (x > trackDist)
    x -= trackDist;
  return x;