float x = 0;  float y = 0;   float dx = random(10)-5; float dy = random(20)-10;
  float u = 30; float v = 400; float du = random(10)-5; float dv = random(20)-10;
  int i = 0;
void setup(){
  size( 400, 400 );
  stroke( #00FF00 );
  strokeWeight( 4 );
  background( #000000 );
}
public void draw(){
  stroke( (2*i)%255, i%255, 127-i%127, 255-i*i%255 );
  line( x, y, u, v );
  x += dx; y += dy; u+= du; v+= dv;
  if (x<0) dx = abs(dx); else if (x>400) dx = -abs(dx);
  if (y<0) dy = abs(dy); else if (y>400) dy = -abs(dy); 
  if (u<0) du = abs(du); else if (u>400) du = -abs(du);
  if (v<0) dv = abs(dv); else if (v>400) dv = -abs(dv);
  if (i < 2000) i++; else noLoop();
}