Portal 2 is a first-person perspective puzzle game. Players take the role of Chell in the single-player campaign, as one of two robots—Atlas and P-Body—in the cooperative campaign, or as a simplistic humanoid icon in community-developed puzzles. These three characters can explore and interact with the environment. Characters can withstand limited damage but will die after sustained injury. There is no penalty for falling onto a solid surface, but falling into bottomless pits or toxic pools kills the player character immediately. When Chell dies in the single-player game, the game restarts from a recent checkpoint; in the cooperative game, the robot respawns shortly afterwards without restarting the puzzle. The goal of both campaigns is to maneuver through the Aperture Science facility. While most of the game takes place in modular test chambers with clearly defined entrances and exits, other parts occur in behind-the-scenes areas where the objectives are less clear.
The initial tutorial levels guide the player through the general movement controls and illustrate how to interact with the environment. The player must solve puzzles using the portal gun, formally known as the Aperture Science Handheld Portal Device, which can create two portals connecting two distant surfaces depicted as matte white, continuous, and flat. Characters can use these portals to move between rooms or to "fling" objects or themselves across a distance. Outlines of placed portals are visible through walls and other obstacles for easy location.
Game elements include Thermal Discouragement Beams (lasers), Excursion Funnels (tractor beams), and Hard Light Bridges, all of which can be transmitted through portals. Aerial Faith Plates launch the player or objects through the air and sometimes into portals. The player must disable turrets or avoid their line of sight. The Weighted Storage Cube has been redesigned, and there are new types: Redirection Cubes, which have prismatic lenses that redirect laser beams, and spherical Edgeless Safety Cubes. The heart-decorated Weighted Companion Cube reappears briefly. Early demonstrations included Pneumatic Diversity Vents, shown to transport objects and transfer suction power through portals, but these do not appear in the final game because the technology was not ready in time. All of these game elements open locked doors, help or hamper the character from reaching the exit.
Paint-like gels can be used to impart certain properties to surfaces or objects coated with them. Gels are dispensed from pipes and can be transported through portals. Players can use Orange Propulsion Gel to cross surfaces more quickly, blue Repulsion Gel to bounce from a surface, and white Conversion Gel to allow many surfaces to accept portals. Some surfaces, such as grilles, cannot be coated with a gel. Water can block or wash away gels, returning the surface or object to its normal state.
The game includes a two-player cooperative mode. Two players can use the same console with a split screen, or can use a separate computer or console; Microsoft Windows, Mac OS X, and PlayStation 3 users can play with each other regardless of platform; a patch provided in late 2012 added split-screen support for Windows and Mac OS X users under "Big Picture" mode. Both player-characters are robots that control separate portal guns and can use the other character's portals. Each player's portals are of a different color scheme, one is blue and purple and the other is orange and red. A calibration chamber separates the characters to teach the players to use the communication tools and portals. Most later chambers are less structured and require players to use both sets of portals for laser or funnel redirection, launches, and other maneuvers. The game provides voice communication between players, and online players can temporarily enter a split-screen view to help coordinate actions. Players can "ping" to draw the other player's attention to walls or objects, start countdown timers for synchronized actions, and perform joint gestures such as waving or hugging. The game tracks which chambers each player has completed and allows players to replay chambers they have completed with new partners.
Portal 2's lead writer Erik Wolpaw estimates each campaign to be about six hours long. Portal 2 contains in-game commentary from the game developers, writers, and artists. The commentary, accessible after completing the game once, appears on node icons scattered through the chambers. According to Valve, each of the single-player and cooperative campaigns is 2 to 2.5 times as long as the campaign in Portal, with the overall game five times as long.
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