Introduction

This article is an exploration of interaction. It is likely to appeal most to designers with a particular interest in the low-level mechanics of basic actions. It does not intend to set out facts and figures, rather its intent is to pose questions, provide suggestions, and present possibilities. The focus is on the abstract, where exploitation of interaction is not considered or considered only in moderation where necessary. A glossary is included in the appendix.

In this article, one button is the limitation on interaction. Using this limitation, the reader can then draw their own conclusions about how these ideas may be exploited on multi-button interaction systems.

By beginning with the most fundamental interaction, it is possible to carefully explore and experiment with it. By the end of this section, it should be apparent just how many applications there are for such a basic interaction. Whatever is discovered will be applicable to elements of most games, since they use one or more buttons. This is the basic currency of interactivity.

Let's begin with looking at our button:

Our button has two states: Pressed and released. How can this be used?

There are an infinite number of answers to this, so let's begin with some basic actions of early computer or arcade games.

• Movement (the Player Toy's movement around the playfield)
• Attacking (the execution of an aggressive move or construction and deployment of a projectile)
• Activation (a change in state of one or more Toys or playfield elements)

All of the above might be considered actions of the Player Toy, and for the moment this will be the focus.

Taking movement as our first study, it's customary to assign multiple buttons - or as is more often the case now, an analog control - to movement, as this is usually intuitive to the Player. However, the purpose of this section is to explore the diversity of options available for manipulating a Player toy with one button. What is possible?

Movement

There are a surprising number of applications. Here are some options:

Option #1

• Player toy experiences gravity.
• Player toy collides with ground.
• Player toy jumps (to fixed height) when the button is pressed.

Option #2

• Player toy moves to next fixed position each time the button is pressed.

Option #3

• Player toy moves forward a step when the button is pressed.
• Player toy rotates when button is not pressed.

Option #4

• Player toy occupies position B when the button is pressed.
• Player toy occupies position A when the button is not pressed.

Option #5

• Player toy moves forward continuously when the button is not pressed.
• Player toy stops when the button is not pressed.
• Player toy rotates when the button is not pressed.

Option #6

• Player toy moves down continuously when the button is not pressed.
• Player toy moves up continuously when the button is pressed.

Option #7

• Player toy moves forward continuously regardless of button state.
• Player toy rotates counter-clockwise continuously when the button is not pressed.
• Player toy rotates clockwise continuously when the button is pressed.

Option #8

• Player toy experiences gravity.
• Player toy collides with ground.
• Player toy lowers trajectory when the button is pressed.
• Player toy increases jump power when the button is pressed.
• Player toy jumps to fixed height when the button is released.
• Player toy jumps horizontally with speed proportional to button held time when the button is released.

	Movement options (interactive Flash demo)

Of course, this is not a complete list. Each possibility holds a number of sub-possibilities each of which are open to many different exploitations. If natural rules are added to the system, such as inertia, gravity, and perhaps even torque, the number of ways each can be exploited increases still further.

It is also worth noting at this stage the powerful, yet often underused, significance that time can play in interaction. In many of the examples above, holding down the button affected a continuous action over time, which is similar to applying an upward force on an analog joystick to move a Player Toy forward in the world. However, the last example is different.

The last example has two systems at work. The first part is the continuous action again: the raising (or lowering) of the trajectory based on whether the button is held down (or released). The second part is quite different. The jumping of the toy or firing of a projectile happens on release of the button. This has an important effect on the player. They will know that, once they have pressed down the button, they are committed to jumping/firing at some point in the future. With the added element of trajectory involved, there are the mechanics for a simple skill based game. This system was employed in DMA's Wild Metal Country for the firing mechanism

	Wild Metal Country

Timing might also be used in the first three options in our table above. Let's give it a go:

Option #1b

• Player toy stops when the button is pressed.
• Player toy jumps to a height which is proportional to held time when the button is released.

Option #2b

• Player toy selector cycles through positions with highlighter when the button is pressed.
• Player toy jumps to selected position when the button is released.

Option #3b

• Player toy stops rotating when the button is pressed.
• Player toy jumps forward in proportional to held time when the button is released.
• Player toy rotates continuously when the button is not pressed

Attacking

Many of the same mechanisms in the movement example can be used in firing. Simple shooter systems will be looked at in this article, along with examples and variations where it is useful.

Deconstruction Note: It might be useful to think of what shooting is. From a design perspective, firing is the creation of a toy (projectile) that is positioned at the exit point of a weapon. The projectile toy is then given a direction to move in and continues moving until it hits something it is designed to read as a viable target (i.e. a meanie toy or the playfield). It can be taken for granted, but can provide our player with some original experiences by questioning what is taken for granted.

For instance: what if the projectile toy was not created at the point of firing, but was instead part of the player toy, so that the Toy got lighter as it ran out of ammunition. This could be taken even further by suggesting the player toy was entirely constructed of ammunition. What would happen when it ran out? Game over? Does the player become the last shot? What natural and supernatural rules affect the toy?

1. First the most basic system of all: The event of the button changing state (from released to pressed) triggers the shot. The player has to release the button in order to fire again. In other words, the duration that the player holds down the button has no effect. The event (firing) is triggered only when the button goes from released to pressed.

	Button state changing in action (interactive Flash demo)

This is how Space Invaders works, except it has the further restriction (in the original version) that you will be unable to fire again until the first missile has hit something, or exited the screen.

	Space Invaders

1. A slightly more advanced interaction would be the auto-fire ability. In this situation, the character on screen will continually shoot while the button is pressed and will stop firing when released. This is how Ikaruga and many other shooters work. It is a simple system that has an impressive visual effect, and focuses the play experience on the Player Toy movement, rather than the accuracy of shooting.
   
   

   	Shoot and release action (interactive Flash demo)

2. The next step is to change the effect or power of a shot in proportion to the time the button is pressed and consequently, the shot would only fire when the button is released. This might be called a "charged shot", as the analogy is that the time spent holding the button down is the action of channeling power into the shot over time.
   
   

   	Shot charging (interactive Flash demo)

This basic variation has huge consequences. In the mind of the player, a "charged shot" has invested in it time and risk. The shot is now much more important than before, and the player will be that much more intent on using it well.

R-Type used this to great effect and became a classic game which many others tried to copy.

	R-Type

1. Any attribute could be selected to vary with time, rather than just the power of shot. The spread of fire could be varied. With this system a short tap might produce a wide, weak shot and a held button might produce a tight, focused, powerful shot. Perhaps the mode of shot could be changed entirely in discrete stages. A tap could be a simple low power shot. Held for longer and then released, it might split into three shots. Held longer still, it could invoke homing shots. There is a great deal of variety to be found in trying out different approaches.

Although some of the options above may seem more powerful, this does not mean they are better. Consideration should be given to the game as a whole. Treasure's Ikaruga, for instance, has a very simple shooting system, and its binary mode player toy really makes it stand out. Treasure knows not to overload the player with unnecessary options, and they focus on the original aspects of their design to make them really shine.