AS3 - Endless Vertically Scrolling Background - actionscript-3

Question

As my first AS3 project, I'm developing a simple space shooter/bullet hell game. I looked online for a way of producing an infinite background scrolling effect and found this working code.
public function backgroundScroll (evt:Event):void
{
background1.y -= scrollspeed;
background2.y -= scrollspeed;
if (background1.y <= -background2.height)
{
background1.y = background2.y + background2.height;
}
else if (background2.y <= -background2.height)
{
background2.y = background1.y + background1.height;
}
}
This causes the background to infinitely scroll upwards from the bottom of the screen without fail, but no matter what I try, I cannot cause it to instead infinitely scroll down from the top of the screen. Any help would be greatly appreciated.

Answer 1

Code seems to be works fine, i think you have some trubles with names and with events.
How is code working :
enter image description here
Make sure that :
1) You have add ENTER_FRAME listener
this.addEventListener(Event.ENTER_FRAME, backgroundScroll );
2) background1 & background1 is names of objects, that you need

Answer 2

I assume you have two backgrounds that are larger than the height of your stage (so they would switch when the are going off-screen at the bottom. Note that the "- background1.height" parts have changed as well
// at the beginning you set the backgrounds one below another
background1.y = 0;
background2.y = background1.height;
public function backgroundScroll (evt:Event):void
{
background1.y += scrollspeed;
background2.y += scrollspeed;
if (background1.y >= stage.stageHeight)
{
// the background is below the visible stage area, put it above the other background
background1.y = background2.y - background1.height;
}
else if (background2.y >= stage.stageHeight)
{
background2.y = background1.y - background2.height;
}
}