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Exclusive: Inside The Making Of  Portal
Exclusive: Inside The Making Of Portal
January 10, 2008 | By Staff

January 10, 2008 | By Staff
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The new January 2008 issue of Game Developer magazine has an exclusive creator-written article on the making of Valve's Portal, and Gamasutra has extracts from the piece, revealing how the game's antagonist GLaDOS was originally a placeholder, and how level-of-detail technical constraints were overcome.

The postmortem, written by Valve's Kim Swift, Erik Wolpaw, and Jeep Barnett, is described by the Game Developer editors as follows:

"From the game's university-created roots through to its Orange Box-ed release, Portal was an exercise in creativity. Here, three members of the eight-person team come together to discuss Valve's iterative playtesting process, the power of simple storytelling, and clever ways to present new ideas to a mass-market audience."

The Creation Of GLaDOS

In this first extract, the Valve team discuss the evolution of the game's unique story and dialog, thanks to Psychonauts co-writer and OldManMurray.com veteran Wolpaw:

"Before the writing started, we met with Erik and discussed our list of narrative constraints. Since at the time we were using some Half-Life art assets, and because we wanted to leave ourselves the option of someday using the portal gun in a Half-Life game, we decided that the story should in some way connect to the Half-Life universe.

Practically speaking, we didn't have sufficient time or staffing to add any human characters, which would have required an impressive amount of animation work and scene choreography. That meant the story had to be expressed without the benefit of any visible extra characters.

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."


Overcoming Technical Issues

In this second extract, the team talks about why the complex nature of the portal system provides particular technical challenges. Having firstly discussed collision detection issues, they move on to other major issues:

"Another problem we ran across was the need to change distance-based systems such as level of detail (LOD) for models, because with our game, distance is relative to the portal locations.

This means that the distance calculations became a choice of three lines connecting two points, rather than just one line. Also, line of sight can pass through a single portal more than once to reach its target.

The Source Engine does many pre-computed visibility optimizations for culling. Allowing users to bridge visibility leaves with portals added another level of complexity.

For better rendering, we implemented a stencil buffer drawing method for portal views, which gave us a lot of flexibility for handling the portal recursion depth. This allowed us to render an infinitely deep number of portals (limited only by performance), which made our "infinite" hallways look pretty neat.

Stencil drawing also helped us solve the problem of integrating properly with other technology in the Source engine like HDR blooming. Since we have to render our scenes an additional two times for our portals we poured a lot of our effort into making portals render as fast as possible, such as special view frustum culling based on the portal's edges, and render list optimizations for portal drawing."


The full postmortem, including much more insight into the game's development, is now available in the January 2008 issue of Game Developer magazine.

The issue also includes a number of other major features, including a postmortem of Midway's Stranglehold by Brian Eddy, the 2007 Front Line Award winners, and a GDC 2008 editor's preview - plus tool reviews, special sections, and regular technical columns from Bungie's Steve Theodore, Neversoft co-founder Mick West, Lucasarts' Jesse Harlin, and Sinistar creator Noah Falstein.

Yearly print and digital subscriptions to Game Developer are now available, and all digital subscriptions now include web-browsable and downloadable PDF versions of the magazine back to May 2004, as well as the digital version of the Game Career Guide special issue.

In addition the January 2008 issue of Game Developer is available in digital form (viewable in a web browser, and with an associated downloadable PDF), and as a physical single-issue copy.


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