img:=PPM(640,480);
foreach q in ([0..3]){		//quarter of screen
   n:=(8).shiftLeft(q);         //number of bars
   w:=640/n;			//width of bar (pixels)
   foreach b in ([0..n-1]){	//for each bar...
      c:=(255.0/(n-1).toFloat() * (if(q.isOdd) n-1-b else b)).toInt();
      c:=c.shiftLeft(16) + c.shiftLeft(8) + c;   //RGB color = gray
      foreach y in ([(3-q)*120 .. (3-q+1)*120-1]){  // flip image vertically
         img.line(w*b,y, w*(b+1)-1,y, c);
      }
   }
}
img.write(File("foo.ppm","wb"));