Processing 101

mousePressed() is called once after every time a mouse button is pressed.
mouseReleased() is called every time a mouse button is released.
mouseClicked() is called once after a mouse button has been pressed and then released.
mouseMoved() is called every time the mouse moves and a mouse button is not pressed
mouseDragged() is called once every time the mouse moves and a mouse button is pressed.
mouseButton is the variable that stores which button is pressed. Its values are LEFT, RIGHT, or CENTER depending on which button is pressed.
mousePressed is true if a mouse button is pressed and false if a button is not pressed.
mouseX and mouseY contains the current horizontal/verticle coordinate of the mouse.
pmouseX and pmouseY contains the previous horizontal/verticle coordinate of the mouse


//draw a from(20,20) to mouse point as mouse moves
void draw()
{
  background(204);
  line(20, 20,mouseX, mouseY);
}

//draw a from(20,20) to a point where mouse is pressed 
void draw()
{
  background(204);
  smooth();
}

void mousePressed() {
  if(mouseButton == LEFT){
    line(20, 20,mouseX, mouseY);
    noLoop();
  } 
  else{
    loop();
  }
}

Draw points along the way mouse is dragged.


boolean dragged = false;
int x =0;
int y =0;
void setup(){
  background(255);
  smooth();
 
}
void draw(){
if(dragged){
   point(x,y); 
 }
 dragged = false;
}

void mouseDragged(){ 
    dragged = true;
     x = mouseX;
     y = mouseY;
}

Try: Draw line along the way mouse is clicked.

Input-Mouse