[Graham McAllister, director of user experience studio Vertical Slice, examines the conundrum of creating a twin stick shooter for the touch-only iOS, breaking down the dos and don'ts, and examining a number of popular games to see what approaches work best.]

What is a Twin Stick Shooter?

Twin stick shooters are a genre of game which use two controls, typically operated by the thumbs. One control dictates the character's movement, the other the direction of shooting.

Importantly, these can be operated independently, i.e. it's possible to move in one direction, and shoot in another.

The camera angle in these games is either from directly overhead, or overhead and from a slight angle. The games are essentially 2D, i.e. there is no forward (or z-axis) movement.

Control Layout

Movement is almost always on the left-hand side, shooting on the right; some games allow the options to switch this for lefties. This layout is also consistent with twin stick shooters on Xbox 360 and PS3.

Minigore on iPhone introduces the controls very clearly.

All Games are Not Equal

With only two touch screen controls to implement, you may think that all twin stick shooters are created equal, however it turns out that subtle changes in design can greatly affect the player experience.

Terminology

No standard definitions exist to describe the interactions we're about to introduce, so we'll try to keep them as descriptive as possible.

Most of the terminology will be defined when we introduce the four components, however they all have one feature in common, the virtual joystick region (VJR hereafter).

Within this region, thumb contact is registered as control input, if the player's thumb touches the screen outside of this area, it is not registered as player input.

The Four Components

There are four main design decisions to consider when implementing the controls for your iOS twin stick shooter.

1. Static or dynamic controls
2. Controls always visible
3. Controls active outside the VJR
4. Play to border

These four components are combined to form alternative control implementations.

Before we take a look at the main varieties and show examples of games which use each particular combination, let's describe the four components.

For clarity, our examples will focus on the movement control, but all comments apply to the shooting control also. Each component is really a design decision with two options, so we'll cover each alternative.

Component 1: Static or Dynamic Controls

Static Controls. This approach fixes the location of the touch controls, typically they'll be within easy thumb reach of the corners of the iOS device.

Later we'll see how component three, Controls Active Outside The VJR, extends the usability of the static control approach.

Static controls - fixed at corners of the device.

Dynamic Controls. This approach centers the controls at the point where the player touches the screen, i.e. the controls can change spatial location depending on where the player's thumb makes contact.

Crucially, however, there is one important factor in how dynamic controls are implemented, which brings us on to the second component.

Dynamic controls - controls center where the player taps.