This example is for Processing 3+. If you have a previous version, use the examples included with your software. If you see any errors or have suggestions, please let us know.


The redraw() function makes draw() execute once. In this example, draw() is executed once every time the mouse is clicked.

float y;
// The statements in the setup() function 
// execute once when the program begins
void setup() {
  size(640, 360);  // Size should be the first statement
  stroke(255);     // Set line drawing color to white
  y = height * 0.5;

// The statements in draw() are executed until the 
// program is stopped. Each statement is executed in 
// sequence and after the last line is read, the first 
// line is executed again.
void draw() { 
  background(0);   // Set the background to black
  y = y - 4; 
  if (y < 0) { y = height; } 
  line(0, y, width, y);  

void mousePressed() {