A week after the meeting, Erik came back with some sample dialog he'd recorded using a text-to-speech program. It was a series of announcements that played over the newly-christened "relaxation vault" that appears in Portal's first room.

Everyone on the team liked the funny, sinister tone of the writing, and so Erik continued to write and record announcements for other chambers, while still searching for the story proper.

At some point, however, it became apparent that these announcements were providing playtesters with the incentive to keep playing that we'd been looking for all along.

Better yet, in the sterile, empty test chamber environment, players were actually becoming attached to the alternately soothing and menacing computer guide. We'd found the narrative voice of Portal.

After this insight, the rest of Portal's story fell into place quickly. The facility would be owned by Aperture Science, the scrappy, unethical scientific rival of Half-Life's Black Mesa.

The guide, now named GLaDOS, would simply talk to players throughout their experience -- praising them, taunting them, and, whenever possible, trying to make them feel guilty for the nonstop acts of defiance and mayhem that game players are conditioned to commit routinely in game environments.

Our hope was that by the end of the Portal, players would know GLaDOS better than any boss monster in the history of gaming. Though we knew at some point the player would have to meet and destroy her, we thought it would be even more satisfying if players got a chance to cause her some emotional pain along the way.

Even though you literally break her to pieces at the end, the entire game is a long process of tearing her down; she becomes increasingly more vitriolic and desperate as the player progresses.

What started out as a seemingly burdensome constraint -- a total lack of human NPCs -- eventually turned into one of the strongest parts of the game. Navigating the environment is Portal's primary gameplay challenge; In effect, the environment is your enemy. GLaDOS's disembodied omnipresence gives that enemy a voice and personality.

Porting Portals

These successes are not to say that the game was a cakewalk. The first challenge we faced was to take the portal technology we'd developed for Narbacular Drop and make it work in Valve's Source engine.

Source is a huge code base, and while it offers a lot more functionality than our homespun Narbacular Drop engine, it required that we undertake a massive redesign.

When we first started working on Portal, we tried to get a hacked version of portals up and running as quickly as possible so we could begin testing our maps. This system basically involved a teleport and the camera/monitor system that already existed in Source.

Quickly, we realized that we needed a more robust method for rendering the portals and allowing the player and other objects to move seamlessly between them. This required us to dig a little deeper into the Source engine's rendering and physics code, and we had to program our own portal system.

Basically, we had to tell the Source physics system to make a temporary hole on only one side of a wall, and that everything behind the portal is connected to geometry in another part of the map. Getting this to work and optimizing the solutions to run in real-time was a major challenge.

After we implemented the bare bones of getting a working portal system we had to figure out how to tackle some of the more complicated problems. For instance, how do you deal with one of our weighted cubes sitting halfway in a portal?

The player and other objects needed to be able to interact with both sides of the weighted cube and have that interaction be convincing. This also comes with interesting edge cases such as an object being able to actually collide with itself.