Portal 2 - Development - Design

Design

Early in its development, the development team planned to exclude portals from Portal 2. For five months, Valve focused on a gameplay mechanic called "F-Stop", the specifics of which as of January 2013 are unknown outside of Valve, because the developers considered using it for a new game. Though the new mechanics prompted some positive feedback, every playtester expressed disappointment at the omission of portals. Following a report in Kotaku of some leaked aspects of the game's plot then under consideration, Newell directed the team to reconsider its plans for Portal 2, including the lack of portals.

Johnson stated that Valve's aim was not to make Portal 2 more difficult than its predecessor, but instead to produce "a game where you think your way through particular parts of the level, and feel really smart when you solve it." Portal 2 allows the player to take incremental steps to understand the game's mechanics, an approach that led to two basic types of test chamber. The first type, which Valve calls "checklisting", provides a relatively safe environment for player to experiment with a new gameplay concept; the second type combines elements in new ways to force the player to think laterally, providing challenging and rewarding puzzles. Chambers were first developed through whiteboard via isometric drawings. The developers performed a sanity check on the chamber before crafting simple levels with a software tool called the Hammer level editor. Extensive play-testing ensured the solutions to each chamber were neither too obvious nor too difficult, and observed alternative solutions discovered by playtesters. Based on play-testing results, the design team retained these alternative solutions or blocked them if they were considered too easy. These versions were sent back for further play-testing to verify that the new elements did not prevent players from finding solutions; further iterations between artists and playtesters occurred until such issues were resolved. Some elements from Portal were modified to suit Portal 2; whereas players of Portal would be familiar with the game mechanics, novice players required some training, as would players of Portal for some game elements. For example, the energy spheres used in the first game were replaced with lasers, which provided immediate feedback and reducing the in-game training time.

The designers built several of Portal 2's early chambers by applying decay, collapse, and overgrowth to Portal chambers to give Portal players a sense of nostalgia and a sense of how much time had passed in the game world. The design team replaced low resolution textures from the first game with higher-resolution, dirty textures that the new game engine could support. The middle section of the single-player campaign takes place in large in-game spaces in which most surfaces are unable to accept portals, which forces players to find creative ways to cross them. Much of the architecture in these sections was inspired by photographs of industrial complexes, including CERN, NASA, and the abandoned Soviet space program. According to writer Jay Pinkerton, in the game's final section, in which Wheatley controls the Aperture facility, "the level designers just had a blast" creating deranged chambers reflecting Wheatley's stupidity. The designers recognized that solving puzzles would mentally tire players, so they inserted occasional "experiences" to provide a respite and to advance the plot.

Portal 2 integrates a game mechanic from Tag Team's Tag: The Power of Paint: paint-like gels that impart special properties to surfaces or objects they coat. Impressed by Tag, Valve hired its creators to develop the mechanic further and only later decided to include it in Portal 2. Valve's vice-president of marketing said, "the decision to combine their tech with Portal 2 came naturally". Journalists have likened Tag to Narbacular Drop, the DigiPen student project that became Portal. The Repulsion (jumping) and Propulsion (running) gels in Portal 2 originated in Tag. Using the third Tag gel, which allows the character to walk on any coated surface regardless of gravity, gave playtesters motion sickness. It was replaced by Conversion gel, which integrates with the portal mechanic. The gels give the player more control over the environment, which increased the challenge for the puzzle designers. The gels are rendered using fluid dynamics routines specially developed at Valve by the former Tag Team.

Portal 2 uses advanced rendering techniques developed for Left 4 Dead 2 for rendering pools of liquid; Portal 2 combines "flowing" surface maps to mimic the motion of water with "debris flow" maps and random noise to create realistic, real-time rendering of water effects.

The cooperative gameplay originated from players' requests and from anecdotes of players working together on the same computer or console to solve the game's puzzles. Wolpaw likened this to players working together on the same computer to solve point-and-click adventure games. The cooperative campaign was also inspired by Valve's Left 4 Dead cooperative games, in which players enjoyed discussing their personal experiences with the game when they had finished playing it. While the single player campaign in Portal 2 is designed to avoid frustrating the player, the cooperative levels focus on coordination and communication, and Valve recognizes they are much more difficult than the single-player puzzles. Valve did not include timed puzzles in the single-player campaigns in Portal and Portal 2, but found that their inclusion in the cooperative mode is effective and gives players a positive feeling after they successfully plan and execute difficult maneuvers. Each puzzle chamber in the cooperative mode requires four portals to solve to prevent puzzles being solved by the actions of only one player. As soon as a playtester discovered a way to complete a puzzle with one set of portals, the level was sent back to the designers for further work. Except in a few cases, Valve designed the chambers so that both players would remain in sight of each other to promote communication and cooperation. Some of the puzzle chambers were designed asymmetrically; one player would manipulate portals and controls to allow the other player to cross the room, emphasizing that the two characters, while working together, are separate entities. The designers soon realized that the ability to tag surfaces with instructional icons for one's partner was a necessary element, since they found this to be more effective for cooperation than simple, verbal instructions.

Valve considered adding a competitive mode to Portal 2. According to Wolpaw, the mode resembled a variation of speedball, in which one team would try to transport a ball from one side of the playing field to the other using portals, while the other team would attempt to stop them with their own use of portals. Matches would commence with this objective in mind, but quickly descended into chaos. Valve realized that people enjoyed solving puzzles with portals more and therefore they focused on the cooperative mode.