int i,dx,dy;
int direction;
float distance;

void setup(){
  size(300,300);
  direction = 1;
}
void loop(){
  background(255,255,255);
  
  dx = mouseX - i;
  dy = mouseY - i;
  distance = sqrt(dx*dx + dy*dy);
  
  stroke(255-direction,0,i);
  line(0,distance,distance,i);
  i += direction;

  if(i > 300 || i<0){
    direction = direction*-1;
  }
}