Door that fades away as you get close too it - how?

Sorry if this should be posted in the scripts or other area, I have no idea how this is done so I don't really know where else to ask but in the build area.

I was at a SIM today that has what appears to be red glowing lines (transparent) acting as a gate between the mall area and each store. (its a sci-fi looking theme)

You can see into the store past the red lines. As you walk toward the red lines, they just fade away. You don't really pass through them... then as you are in the store, they appear again.

My guess is this is done via a transparent image, a thin PRIM that allows passage through it... and perhaps a script that controls the visibility of the texture?

Does anyone know how this is done? I'd like to take this concept and use it for something else.

It looks to me like its a simple script that causes the texture to change with the proximity of an avatar. Anyone know of this type of script?

My guess is it changes from the bars to transparent and back.

I will post over in the scripting area.