this.alpha and showpoly

[Engine]

I cannot figure out how to change the alpha on polygons, could anyone help me? I want to use polygons with the particle engine to mess around but I can't seem to figure it out.

PHP Code:

function ShowPolyTest(){
  showpoly(200,{22,22,22,25,25,25,25,22});
  this.alpha = 0.5;
  with(findimg(200)){
      this.alpha = 0.5;
  }
}