int x=0;
void setup(){
  size(400, 400);
  stroke(0);
}
void draw(){
  background(0);
  line(200, 200, 200, mouseY);
  line(200, mouseY, mouseX, mouseY);
  line(mouseX, mouseY, mouseX, 200);
  line(mouseX, 200, 200, 200);
}
void mousePressed(){
  x=x+100;
  stroke(0, x, 0);
  saveFrame();
}