[Editors Note: In the downloadable Marathon map tutorial you will be reading the following on a terminal aboard a moving vehicle - gls 12/12/2000] Hopefully, while you were running around trying to shoot out the windows and kill the Bobs, you looked out the window and noticed that this ship seems to be moving. This effect is the second lesson.

(If you grabbed the weapons before coming to this terminal, you just missed the first effect lesson.)

Let me explain how the effect works:

1. The ground is given a texture with the "Texture Mode" of "Fast Vertical Slide". (Horizontal slides will also work, but you'd have to aim the ship 90 degrees differently.)
2. Thin polygons are made on the ground that the "Bar" aliens will travel down. To either side of these "route" polygons are "Monster Impassable" polygons, to ensure that the li'l fellas stay somewhat on track.
3. At the end of their route is a "goal" polygon, and the bar aliens are set to be activated by the goal, which causes them to scurry towards it.
4. The aliens are given a speed of 8 in the Physics File to match the speed of the "Fast Vertical Slide". I'm not even going to get anymore into the creation of an alien to use for this effect. That's a little beyond the scope of this tutorial... Or, am I just lazy - ?

There's a few more fine details to explain. Let's go take a look behind the scenes...

[Editor's note: The terminal you are reading teleports you to "C" on Figure 1 where you watch "bar" aliens march towards "A" - gls 12/12/2000]

You're standing on "C" at the moment, in an area of the map that can't be seen from the ship. This area was added purely for the tutorial. Since the map has extensively long viewing distances, I couldn't allow you to run around the open area freely to explore.

"A" is a series of goal polygons which the bar aliens are trying to reach. Platforms kill 'em before they have a chance to get there. This is crucial, or they might start walking backwards toward the ship, throwing off the whole effect.

"B" denotes some extra polygon edges I added to their route. On occasion, the platforms won't kill them, so they decide to run away. I found that plugging in an extra polygon edge (out of view from the ship) stops them, and they return to the goal to be slaughtered.

I'll note here that a good way to ruin the effect is to allow the player to be able to kill the aliens. Without solid, transparent windows, the player could shoot them, and their corpses would stay static on the moving ground. Not good: It looks bad, and it might block the route for other aliens.

You'll also want to space out the aliens enough that one won't trigger the kill-platforms while another alien is on his tail. This causes them to run away sometimes, too. They do sem to have minds of their own - kudos to Bungie.

Here's a look at the polygon types.

This illustrates how I used the polygons to channel the aliens to their death away from view of the player while standing in the ship.

Another note: To ensure that the aliens get moving when the level starts, almost every polygon in the ship is a "Dual Monster Trigger" polygon. The platform the player starts on is obviously the most important, anyway.

I don't think this effect is particularily difficult to grasp, although it took me quite some time to figure out how to stop the bar aliens from coming back into view. I tried everything from hostile aliens firing at them, to deep pits that they fall into. Nothing else worked quite as well as this method.