Pointing device

A pointing device is a human interface device used to input spatial data.

This data includes continuous and multi-dimensional information about position and movement. These devices allow users to interact with graphical user interfaces (GUI) or CAD systems. By using physical gestures, a person can control a computer. The movement of the device is echoed on the screen by a pointer or cursor. Common gestures include pointing, clicking, and the "drag and drop" action. While many devices exist, the term "mouse" is often used as a metaphor for any device that moves a cursor.

Scientists and engineers classify these devices using several specific features. One way to group them is by direct versus indirect input. A direct-input device, like a finger on a touch screen, places the pointer at the same physical position as the input.

An indirect-input device, such as a mouse or joystick, translates movement to the screen from a different location. Another classification is absolute versus relative movement. Absolute-movement devices, like a stylus, provide a consistent mapping between a point in the input space and a point on the screen. Relative-movement devices, like a mouse, map displacement to the cursor's position relative to its starting point.

Pointing devices also differ in how they handle force and movement. An isotonic device is movable and measures its displacement, such as a pen or a human arm. An isometric device is fixed and measures the force acting upon it, such as a trackpoint.

Some devices are elastic, meaning they increase force resistance as they are moved, like a joystick. We can also distinguish between position control and rate control. A position-control device, like a touch screen, directly changes the pointer's position. A rate-control device, like a trackpoint, changes the speed and direction of the pointer's movement.

Complexity can be measured by degrees of freedom (DOF). This term refers to the number of independent ways a device can move or rotate. A standard computer mouse has two degrees of freedom, moving along the x- and y-axes. In contrast, a Wiimote has six degrees of freedom. It tracks movement along the x, y, and z-axes, as well as rotation around those axes.

Other devices, like a 3D mouse, provide even more complex spatial input for specialized tasks.

Bill Buxton introduced a taxonomy to classify devices by their dimensions and sensed properties. This model is rooted in the human motor and sensory systems. It distinguishes between mechanical intermediary devices, like a stylus, and touch-sensitive devices.

Buxton also described a three-state model for how devices interact with systems. The three states are out of range, tracking, and dragging. A mouse is typically in a tracking state when moved without a button press. It enters a dragging state when a button is pressed during movement.

A stylus on a graphics tablet can use all three states: out of range when lifted, tracking when touching, and dragging when extra pressure is applied.

Fitts's law is a predictive model used to understand human-computer interaction. This law predicts the time required to move to a target area. It states that time is a function of the ratio between the distance to the target and the width of the target. In other words, a large button near the cursor is faster to click than a small button far away.

Designers use this law to improve user interfaces. They make interactive elements larger and place important buttons in accessible spots. They also consider that edges and corners of a screen can act as "pins" for the cursor, making them faster to reach.

Understanding these devices involves managing the Control-Display (CD) gain. The CD gain is the proportion between movements in the control space and the display space. For example, a physical mouse might move a different distance than the cursor on the screen. Users can often adjust these settings. High gains make it easier to reach distant targets, but they make selecting specific targets harder. Low gains make selection easier but take more time. Modern operating systems, like macOS and Windows, use mechanisms to adapt this gain to the user's movement velocity.