Trapping the player is defined by two things:
1) The player is not dead.
2) The player can't do anything to complete your test.

The problem is, the player is taken out of the game experience every time they have to untrap themself using noclip or quickload. That's just no fun at all, and not everyone knows about noclip or quickload. Expect your test subjects at some point to report this problem with one of your tests.

This guide is here not to condemn your test for allowing the player to get trapped, but to provide some PeTI solutions for the most common cases where the player can get trapped.

This is the most obvoius case of allowing the test subject to get trapped. Unfortunately, people are prone to overlooking even this. There are a few solutions available, one of the most prominent being to fill the pit with deadly goo. If the pit is one (maybe two) blocks deep, repulsion gel will work too, as long as it doesn't create unintended solutions.


Another solution involves laserfields (see right), but there is a small problem. If they stand on the edge, the player can't just climb out like in real life. The player can only jump, and it's too high of a ledge for the player to jump out of. The solution: position the laser field so that it's all the way at the top instead of in the middle.


Yet another solution allows the test subject to portal out or use a faith plate to escape. Usually this is the case if you want your pit to be used for flinging. However there is one subtle danger:


You might think that the test subject can just portal out, but there's really a fizzler in the way! The player is thus not able to escape. To remedy this without a faith plate, just use a button to disable the fizzlers. I should warn you, however, that unintended solutions are likely if you choose not to use a faith plate.



Cubes and Pits



Yes, even a cube can get trapped. Here the test subject can't safely reach it, and the test is thus impossible. The solution: allow your cubes to be respawned with the push of a button, where the player can reach them.

Another solution involves using fizzlers to control where the cube is allowed to go. In the example above, fizzlers would instantly remedy the situation.

By the way, notice here that the laserfields are properly positioned. The test subject can never get trapped in the test; it's just impossible because the cube can get trapped.

Here's another example (left) and solution (right):



Again, notice the positioning of the laser field: it's recessed to prevent an unintended solution.

Similar to pits, the case is usually where the test subject is on some island surrounded by a "moat area" (e.g. deadly goo) and can't escape. There are two solutions:



On the left, you will see one solution: a hard light bridge (or excursion funnel) with a timed button leading to a place where the test subject can finish the test. On the right, you will see a faith plate leading to a place where the test subject can finish the test. Because there is no timer with the faith plate, it is less likely to cause frustration among your test subjects.

Timed laser fields or panels in front of an area where the player can enter and not get out are a problem. If the test subject is in the wrong place when the timer is done, they're trapped.

The screenshot on the right shows the case of getting trapped: the test subject hits the button, tries to get the cube, but then the timer runs out of time and the player is behind laser bars. Remember: the test subject is not a criminal!

Now let's look at a couple of solutions.


This first solution has a fizzler that will let the test subject out, but not the cube. It also has a faith plate so that the test subject will not enter the area without hitting the button. While this is a legitamite solution, there is an unintended way to solve the test. The test subject can throw the cube through the laserfield, and then use the escape route, grab the cube, and finish the test.


To counter that, use a cube respawning mechanism like you see in the test on the left.

Sometimes this problem occurs: you have a cube and you need to coat it with repulsion gel for some purpose, but then you need to put it on a button in order to solve the test. If you forget about cleansing gel/water, you've made a bad test. Besides using cleansing gel/water, you can also make your cubes respawnable with the push of a button.

To show how easy this is to fix (the solution is on the right):



By the way, repulsion gel can and has been used like this to prevent unintended solutions, such as in Mevious' Remorse. This is not the same thing as creating an unsolvable test.

Since excursion funnels are one-way, it's possible for the player to get trapped at the end of one. You might not even see it to begin with. Here's a tip: look at what all of the portalable surfaces are directly facing. If it's not another portalable surface, the exit, or anything deadly, chances are that the test subject can get trapped. Here's a couple of solutions:



On the left, you'll see that adding portalable surfaces are one option. You can even add them so that the player heads in a different direction than before. Either way, you have to watch out for any unintended solutions that may arise. On the right, you have a simpler option: just kill the player with a laserfield for going too far.

A very basic test, but loaded with problems. From the Portal commentary:

"A problem we came across with virtually any puzzle involving boxes and doors, was that the players could portal the boxes to the other side of the door, thereby trapping themselves in a room with buttons, but no boxes."

There are two ways to do this:



On the left (call this Case One), the cube is on the entrance side, but the test subject is on the exit side. On the right (call this Case Two), the cube is on the exit side, but the test subject is on the entrance side. In both cases, both portals are on the test subject's side.

Solution One: make your cubes respawnable



This will prevent Case Two from happening because the player is now able to respawn the cube. The problem is that Case One can still happen (see right).






Solution Two: add an escape route



The secure escape route, if it's not a faith plate, consists of a timed door/panel so that the test subject only heads in one direction, and a fizzler to prevent any cubes from passing through (as well as erase any portals).


This time, Case One is prevented from happening since the player can always go back to the entrance room. However, if you just use an escape route and nothing else, you have to take great care. If you have a fizzler in your test and the cube is not respawnable, the test subject can fizzle your cube and the test cannot be completed (left image).

Even if your cubes are respawnable, if there's no button to respawn them, Case Two can happen (right image).

The proper solution: make your cubes respawnable with the push of a button AND add an escape route

With everything in place, both cases will be prevented, and the test subject will always be able to complete your test.