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But what kind of barriers and boundaries are we talking about, then? Well, throughout the game the player is drawing upon real-world experiences and will instinctively know, without the game having to tell them, whether a barrier is passable or not; a large brick wall, canyon, or cliff face for example.
These are barriers that tend to make sense to the style of play and when they work well, we see something akin to that of figure 1, where the player can clearly see the boundary and immediately understands that it cannot be passed or what must be done to circumnavigate it.
Figure 1 - Good relationship between barrier, visibility and affordance
So what about other barrier types? If we again look at our real-word experiences, a player can come up to a doorway in a game and instantly know that it should open and close. How it does this - i.e. pulled or pushed - is irrelevant in current games.
If a player comes to a clear pathway, then this path affords continued walking - as it does in real life. However, the problem we have is when this affordance breaks down and what was expected by the player does not happen.
Yet, the question needs to be asked - if the player does come up to a barrier which breaks the affordance rules, is it really that important? It's not as if the game is technically broken at all. The answer is an indefatigable "yes". What we find happens is that the player starts carrying out a trial-and-error routine, and when that fails the player will start to become frustrated at the game itself - something which should be avoided at all costs.
In figure 2 we have a scene in the excellent Halo 3 whereby the player is at a bunch of rocks. The game allows the player to jump onto the first rock and because the next rock is of a similar height, it offers the affordance that we can jump onto it and get past.
However the game does not allow this and the player spent a good few minutes jumping up, falling down and retrying over and over to get past.
Figure 2 - Impassable boundary in Halo 3
So now we have an apparent path which is visible to us. Because the game allows the player to jump over objects and allowed them to proceed through a preceeding area in a similar fashion, the player now has a mental model as to what can be done.
So, in the player's eyes, these rocks afford being able to be jumped on and passed. However this affordance has simply broken down and only serves to frustrate the player.
What we now have is shown in figure 3.
Figure 3 - Breakdown between affordance and barrier
The player is able to see the barrier and attempts to pass it but for no explained reason, this barrier is impassable. The danger of what might happen next can be seen in figure 4.
Figure 4 - No more relationships between barrier, visibility and affordance
What has happened here is that the player now has a harder time seeing the difference between any of the barriers simply because they are unsure as to which ones they are able to interact with and which ones they are not. The player now has no real idea what they can and cannot do and will often spend many a frustrating moment trying out pointless things - derailing the flow of the game and distracting the player from the full experience the game offers.
In figure 5 we have another example of how the player's passage can be blocked by the classic invisible wall. This example is from a scene from the superb Battlefield 2. What we have here is an area which is identical to a number of other places on this type of map.
There are a number of trees, but there is clearly a path in between them to the other side. In fact, simply to get to this point the player was required to navigate through a path which looked identical in every manner.
However, upon reaching this point, the player comes to an abrupt halt and is unable to proceed. We engaged a test player to test this environment. He attempted to get past it in various ways, including crawling through the path, but the barrier held.
During the play testing session we conducted, the player responded extremely negatively and stated that it was things like this he hated in games - which often means he would not buy them as a result.
Figure 5 - Breakdown between affordance and barrier in Battlefield 2
These invisible barriers fail both the visibility and affordance rules completely. Firstly, by simply being invisible, it by definition means the player cannot see the barrier, and when the player comes across a barrier like this, there is no legitimate explanation as to why their character meets with resistance.
Naturally, the concept of visibility is instantly thrown out of the window. But what about the affordance rule? Truth be told, how can something which apparently does not exist afford anything? Here, we have a complete breakdown in the game, because how is a player expected to understand why this area suddenly, and for no reason, does not allow passage?
These two examples have shown barriers which are meant to keep the player in a certain area but fail (as credible barriers) simply because of how they break the player's expectations about what can or cannot be done.