Game Mechanics (Portal 2)

From Valve Developer Community
Jump to: navigation, search

A game mechanic is an interaction between the player(s) and one or more game elements.

P2 mechanics 00.png

The mechanics of the Portal series each revolve around moving the player or items from one place to another in a non-intuitive manner. In order to complete the task the puzzle must pass through one or more "goal states." The mechanics are used to tie each goal state in the puzzle together.


One way of breaking down the mechanics of Portal 2 is to look at each puzzle like an abstract graph, with nodes connecting each state. The connections between the nodes are the mechanics. Most will be two-way, meaning the state can be easily reversed. Others will be one way - in order to return to a previous state the puzzle must past through intermediate states.


Doors have three purposes. First, to signify the beginning and end of a level. Second, to indicate that two puzzles are not related to eachother. Finally, doors can be used as a temporary obstacle within a puzzle. Each of these kinds of doors should be different enough in appearance to prevent confusion among players.

For example, levels in the underground portion of Portal 2 begin and end with push-bar type doors and a variety of doors in between, depending on context.

Doors should be set up so as to prevent any attempt to wedge test items between them.

Disassemblers, Elevators, etc.

These are the actual entry and exit points for levels. What they are isn't as important from a mechanical point of view since they simply form the in-game explanation for how the player gets to the next level.

Crushers, Slime, Pits, and Shredders

In single player these are used to signify major errors, since a re-load is usually required between deaths, and should therefore be relatively easily avoided. Conversely, in cooperative levels it does not take very long to recover from them so they may be liberally applied. These can actually be helpful to players in certain situations where it is too difficult to return to an earlier part of the puzzle if the current solution isn't working out.

Emancipation Grid and Pneumatic Diversity Vent

The Emancipation Grid keeps test items from entering another puzzle or part of a puzzle. It also helps indicate when one puzzle ends when it is placed next to an exit door.

Unfortunately, the Pneumatic Diversity Vent was cut from Portal 2 as a test element so its appearance is again relegated to providing replacement material for the player.

It's important to test levels so that vital test material isn't permanently lost, meaning that the player will have to reload in order to finish the puzzle.


Besides forming the walls of test chambers, white and extended panels are used to indicate likely portal or gel placements. Extended panels help the player understand the puzzle without necessarily giving away the solution, as well as offering hints for tutorial puzzles. In certain situations (such as the faith plate and tilted panel combination) they can be used to ensure portal use and position during a puzzle.

Toward that end, the info_placement_helper is a handy tool for proper portal placement. It ensures that portals are placed exactly, so that players don't end up hitting things the designer didn't mean for them to, lasers hit their mark on the first try, and other tedium reducing effects. Having to fire portals just right isn't fun. On the other hand, "force placement" helpers should generally be avoided unless absolutely required. The default description of this keyvalue is very misleading: it has nothing to do with portal placement rules, but rather removes the "cooldown" period after using the helper, forcing portals to lock to it no matter what. Because placement helpers are invisible, this behavior can be somewhat frustrating for players if a portal surface appears larger but actually only allows portals to be placed in a specific location.

Remember that a good puzzle should give the player a moment of epiphany without making them feel stupid or frustrating them to the point of giving up. The best way to frustrate a player is to leave no indication whatsoever of how to solve the puzzle. Too many portal surfaces can lead to trial and error gameplay, which is tedious and boring. Not enough portal surfaces makes the puzzle too obvious.

Glass walls

Allow the player to see another part of the puzzle while preventing immediate access. These help the player understand the mechanics of the puzzle. It's very important to remember the difference between giving away the solution and allowing the player to see what they are doing.

They are especially important in cooperative mode, and are used extensively in the maze-like trust puzzles.

Grates and Glass-With-A-Hole-In-It

Allows the player to see as well as get through using portals. This is a way of ensuring portal state in the middle of a puzzle.

Buttons and switches

Buttons and switches are iconic pieces of portal gameplay. Their use primarily directs the movement of players and test material within a puzzle.

  • The small button requires a player to visit a point in the puzzle. This allows the designer to be sure of a player's position or keep the player away from another position at a certain stage in the solution.
  • The simultaneous switch in cooperative mode requires both players to visit two points, and thus functions similarly to the small button.
  • The super button requires a player or material to spend time at a point, usually ensuring player or material position for several steps in the puzzle.

Indicator Lights

These show the player how certain mechanics work. They are particularly important where what is activated cannot be seen from where it is activated from. For example: if a button opens a door on the other side of a wall, use indicator lights to associate the two. Otherwise the button may appear to be useless.

Ideally, place the actuator right next to the thing it activates. This alleviates the need for indicator lights, but isn't practical for all tests.

Moving platforms

Including vertically moving lifts, horizontally moving platforms, and other elements, platforms are state dependent level geometry. A self moving platform introduces an element of timing to the puzzle, while the platform that moves in response to a button or other mechanic may be used in the same way as a door. They may also be used as trick floors, dropping a player or item after a certain amount of time.

Platforms can be thought of as one or two-way halls, and other more complex structures. They are arguably more versatile than many of the flashier mechanics.

Aerial Faith Plates

The aerial plates serve allow the player or materials to move from one part of the puzzle to another without necessarily allowing movement in the opposite direction. They are almost identical to one way doors, except more exciting. The aerial faith plate can be made portable by allowing a portal in its launch path.


Turrets function the same as in Portal 1, and can be thought of as either temporary or permanent obstructions, giving the player the choice of portalling past them, dropping objects on top to topple them or portalling underneath them to tip them over, but preventing them from simply walking past certain areas.

Thermal Discouragement Beam and catcher

This pair of devices act in a manner similar to the button and box scheme. The primary difference is that box and receptacle need not be in the same place to form a closed circuit. Moreover, using portals the box can be moved out of the system. The box isn't strictly necessary either, as portals can be used to redirect the beam.


Used as stepladders, beam blockers, beam redirectors (and therefore weapons), shields [1], and to hold down buttons. Consider these uses when providing them as, while they may be important for the puzzle, they can also be used in unintended ways.

Hard Light Surface

Hard light surfaces may be used as portable walls or as portable two-way halls. Make sure to consider both uses in designing puzzles. Don't use a hard light surface when an excursion funnel will do.

Excursion Funnel

The excursion funnel is a one-way hall, similar to the aerial faith plate. Can be converted into two-way halls with clever use of actuators such as buttons.


Tag: The Power of Paint brought state to panels, creating a whole new set of interesting puzzle mechanics. With gel, panels can have four different states in the same puzzle: non-portal-able, portal-able, repulsion, and propulsion.

Gel passes through emancipation grids and grates.

Conversion Gel

When incorporating conversion gel into a puzzle two approaches can be taken. The easiest way, and possibly safest in terms of play, is to assume that every surface will become portal-able. The other way is to set the puzzle up so that gel can only be applied to specific surfaces, usually with portal-able surfaces opposite to the desired surface. The second method may require a lot of testing, while the former can create very difficult puzzles as the player simply dumps gel everyplace with no particular goal in mind.

Repulsion Gel

Allows players to convert surfaces (including hard light bridges and walls) into aerial plate-like devices. Care should be taken to prevent unintended use of repulsion gel.

Propulsion Gel

Converted surfaces allow the player to move faster. May be used in combination with ramps and/or repulsion gel to create aerial faith plate-like mechanics or as part of the solution to puzzles involving crushers, trick floors, etc.


Inform the player of their relative position in a string of tests, what a given test will involve, and that the player is taking a test.

Incorporating Mechanics in Design

Basic puzzle design may involve only one intermediate goal such as "put the box on the button, go out the door" so the final state of the puzzle is the only thing that needs to be considered. More advanced puzzles involve a series of state changes in order to complete. Consider the following steps:

  1. Retrieve box from ledge via portals.
  2. Place box on button to invert funnel direction.
  3. Use portals to funnel to the other side of the chasm.
  4. Funnel box off of button to invert funnel direction.
  5. Funnel self up another ledge to exit.

The box, funnel, and button go through several intermediate states to complete the puzzle. Also worth noting is that the puzzle does not end up in a steady state as with the simple box-button combination.

  • This puzzle is the layout of Test chamber 03, Chapter 8

Bottom Up Design

One way of tackling puzzle design for Portal is to set up several puzzle states that the player will pass through on their way to the final solution. Once these states have been established, use the mechanics to ensure that the player follows the path implied by the puzzle states.

Using this approach it isn't necessary to have a complete picture of what the final puzzle will be, making it an excellent way to get started designing levels quickly. Instead, the designer decides which states the puzzle will pass through and then builds the level up to fit that pattern. The following is an example workflow:

1. Begin by writing out a table showing the variable elements involved and what they will be doing (or having done to them) at each stage in the puzzle.

Elements Step: 1 2 3 4 5 6 7
Small Button Pressed
Reflector Cube Hidden Hidden Dropped Reflecting Reflecting On Button On Button
Large Button Depressed Depressed Depressed
Trap Door Open Open Open
Exit Door Open

Don't worry about making the table make sense until you've finished it. Think of this part as the blueprint of the blueprint. The process of making it make sense is where the final design will come from.

2. Start Hammer (or the Puzzle Maker) and put together the space where the puzzle will be solved. For now, ignore the table and focus on making a clean architectural layout.

3. Implement the table in the architecture. As you do this an idea of what the puzzle should be should start to form, and you'll start to find ways of fixing the table so it follows the actual solution.

4. Test everything. Try and solve the puzzle in unintended ways. These alternative solutions often suggest other designs.

5. Repeat until satisfied.

6. Clean everything up so that it sparkles - add the lights, the sounds, the music (metaphorically).

Top Down design

This is the approach most beginning designers (of all sorts) instinctively use, as well as many inspired professionals. It's the de-facto method of building what's already in mind. The corollary of that is if there is no idea then there is no product. It is also well suited to mechanic central design, where a mechanic is introduced and everything else follows from it.

The most important thing in top down design is not losing sight of the mental picture, which is easy to do when there are several steps involved in putting the idea into practice. There are many, many steps in Portal level design.

The second most important thing is making sure that what seemed like a good idea at the time is actually a good idea. Bounce it off of a friend to see what they think of it and note down any criticisms. If they don't get it, explain. This will also help crystallize the idea.

1. Quickly sketch the idea out on a whiteboard, piece of paper, back of an envelope, whatever. Ignore the details and focus on the central idea.

2. Open up Hammer and put together a draft. Test the main theme out to see if it's worth pursuing.

3. Make adjustments and repeat.

4. Add the details and upload!