Motor Skill Learning in User Interfaces via Discretized Pie Menus
An important area of human-computer interaction research focuses on pointer-based graphical user interfaces. The literature shows that designers must be sensitive to the subtleties resulting from the interface paradigm. Specifically, video game designers must understand the level of difficulty and thus the overall efficiency of a user interface. Can the player, in quick succession, efficiently perform many distinct actions using a traditional pointer-based GUI? Are there too many icons to place on one screen? What if the player uses a gamepad instead of a mouse?
This article provides a short overview of Fitts' Law and related material as a basis for discerning common pitfalls of pointer-based interfaces. Discretized pie menus are introduced as an alternative to the pointer interface. Following that is a discussion of its merits and demerits.
Although this article uses typical real time strategy games as an example, the ideas presented can be generalized to any traditional pointer-based game interface.
Common Pitfalls of Pointer-Based Interfaces
Fitts' Law is a logarithmic function describing the index of difficulty (ID) in movement based motor tasks [4, 5]. The function is defined as
where A is the amplitude or distance of movement and W is the width of the target where movement ends. In Figure 1, if a subject were to perform a one dimensional, horizontal movement from the start position (marked S), to any point within the rectangular region, then A is the distance to the rectangular region and W is the width of the rectangular region.
Many HCI researchers have used Fitts' Law  with one of the first uses emerging from the work of Card, English, and Burr . With the aid of Fitts' Law, Card et al. evaluated the performance of four devices in text selection on a CRT screen. Of the four devices (mouse, rate-controlled isometric joystick, step keys, text keys) they found the mouse to provide the best performance. Card et al. used Welford's formulation of Fitts' Law , which expressed the time required to make a hand movement. Given an index of difficulty (ID), it is possible to predict the movement time based on the following equation :
where a and b are constants depending on the type of device used (e.g., how long it takes to grasp the device).
Though Fitts' Law deals with movements in one dimension, it has been shown by Mackenzie  that the law can be extended to two dimensions. Mackenzie suggests that a more accurate strategy for determining W (target width) in two dimensions is to use either a) the "Smaller-Of" model where W is the smaller of the width or height or b) the " W' " model where the 1D nature of Fitts' Law is maintained by measuring W along the approach (to target) vector. Clearly, Fitts' Law is applicable to two dimensional user interfaces.
Note that the major factors in determining the difficulty of movement based actions is the amplitude (distance to target) and the width (size of target). These spatial factors contribute to subtle deficiencies in typical pointer-based interfaces.
Real time strategy games commonly implement an interface where players use a pointer to select icons representing actions or units. The result is an easy to learn interface requiring little explanation. However, the downside of such an interface emerges when players are required to perform several actions in quick succession. For example, if the player wishes to quickly build several turrets, he/she would repeatedly reposition the pointer from the game board to the menu icons and back. From the experiments performed by Card et al. , mouse movements over short distances (1-16 cm) required just over one second of positioning time! Given that data, a player wishing to build in quick succession would require a considerable amount of time.
Designers solved this problem by providing shortcut keys to frequently used actions or units. This alleviates the time cost associated with pointer based interfaces. Given Fitts' Law, A = 0 and so MT = a. The time to perform the action is simply the time it takes for the player to press a key.
Yet in resolving the time issue, new problems arise. Now a player must follow an unrelated input protocol requiring the memorization of shortcut keys. Over time, veteran players run the risk of forgetting these keys. Worse, this solution is not suitable to game platforms which do not include a keyboard (consoles and portable gaming systems). It also does not scale to games requiring many actions and/or units as it would either clutter the screen with icons or require additional pointer movements (i.e., submenus). One way to alleviate these shortcomings is to teach an interface relying on motor skills.