User interface

A user interface, or UI, is the specialized space where humans and machines interact.

This interaction serves a dual purpose for both parties. The human aims to operate and control the machine effectively. Meanwhile, the machine must provide feedback to the operator. This feedback is essential because it aids the human in the decision-making process. Designers focus on usability, which is the goal of making a machine easy, efficient, and enjoyable to use. High usability means a user provides minimal input to reach a desired output. It also means the machine minimizes undesired outputs sent to the user.

Interfaces function through several organized layers. The primary layer is the human-machine interface, or HMI. This layer connects the human to physical input hardware and output hardware. Input hardware includes devices like keyboards, mice, or game pads. Output hardware includes computer monitors, speakers, and printers. A device that implements this HMI is known as a human interface device, or HID. In some advanced settings, interfaces may bypass physical movement entirely. These are called brain-computer interfaces (BCIs) or brain-machine interfaces (BMIs). They use electrodes to link the brain directly to the machine.

We can also categorize interfaces by the human senses they engage. A tactile UI uses touch, while a visual UI uses sight. Other types include auditory (sound), olfactory (smell), equilibria (balance), and gustatory (taste) interfaces. When an interface interacts with two or more senses, it is a composite user interface (CUI). The most common CUI is the graphical user interface (GUI). A GUI combines tactile and visual elements to display graphics. If you add sound to a GUI, it becomes a multimedia user interface (MUI).

Composite user interfaces are further divided into three distinct categories. Standard CUIs use common tools like mice and monitors. Virtual interfaces create a virtual reality by blocking out the real world. Augmented interfaces create an augmented reality by adding digital elements to the real world. Some theoretical systems aim to interact with all human senses. These are called qualia interfaces, named after the theory of qualia. Scientists even classify these by the number of senses used. For example, a "3S" standard CUI might use sight, sound, and smell. A "4S" virtual reality interface might use smell and touch alongside other senses.

Designing these systems requires deep scientific knowledge. Engineers use ergonomics, which studies how tools fit the human body. They also use psychology to understand human behavior. These fields are often called human factors engineering (HFE) or usability engineering (UE). To build these interfaces, experts rely on computer science disciplines. These include computer graphics, operating systems, and programming languages. In complex industrial settings, the HMI is often computerized. When this happens, it is specifically called a human-computer interface.

The history of these interfaces began with the batch era from 1945 to 1968. During this time, computing power was extremely scarce and expensive. Users had to adapt to the machine rather than the machine adapting to them. Input was provided using punched cards or paper tape.

Users prepared these cards on bulky keypunch machines. Once submitted, a job might sit in a queue for hours or even days. There was no real-time interaction with the computer. Some very early machines even required users to toggle programs in binary code using console switches.

After 1969, the command-line user interface (CLI) became the dominant method. These evolved from the monitors used in batch systems. A CLI uses a series of request-response transactions. Users type textual commands using a specialized vocabulary. This method drastically reduced latency from days to mere seconds. This speed allowed users to receive near-real-time feedback. Because of this, software became much more exploratory and interactive. However, CLIs still required a heavy mnemonic load. This means users had to invest significant time and effort to learn the specific commands.