A touchpad is an input device commonly used in laptop computers. They are used to move the cursor, using motions of the user's finger. They substitute for a computer mouse. Touchpads vary in size but are rarely made larger than 50 cm² (about 8 in²)
Touchpads commonly operate by sensing the capacitance of a finger, or the capacitance between sensors. Capacitive sensors are laid out along the horizontal and vertical axis of the touchpad. The location of the finger is determined from the pattern of capacitance from these sensors. This is why they will not sense the tip of a pencil or even a finger in a glove. Moist or sweaty fingers can be problematic for those touchpads that rely on measuring the capacitance between the sensors.
Touchpads are relative motion devices. That is, there is no isomorphism from the screen to the touchpad. Instead, relative motion of the user's fingers causes relative motion of the cursor. The buttons below or above the pad serve as mouse standard buttons. You can also click by tapping your finger on the touchpad, and drag with a tap following by a continuous pointing motion (a click-and-a-half).
Some touchpads also have "hotspots": locations on the touchpad that indicate user intentions other than pointing. For example, on certain touchpads, moving your finger along the right edge of the touch pad will control the scrollbar and scroll the window that has the focus vertically. Moving the finger on the bottom of the touchpad often scrolls in horizontal direction.
Some touchpads can emulate multiple mouse buttons by either tapping in a special corner of the pad, or by tapping with two or more fingers.
Touchpads are primarily used in portable laptop computers, because the usual mouse device requires a flat table adjacent to the keyboard not always available away from the office. But touchpads have some advantages over the mouse, particularly that the pad's position is fixed relative to the keyboard, and very short finger movements are required to move the cursor across the display screen. Some computer users prefer them for such reasons, and desktop keyboards with built-in touchpads are available from specialist manufacturers.
Touchpads have also recently appeared in Apple's iPod. The main control interface of for menu navigation in all of the iPods (except the shuffle) use a Synaptics touchpad. Creative Labs also uses a touchpad in their Nomad Jukebox Zen line with the Zen Touch and Zen Micro.
Synaptics provide many features in their drivers for free, including many advanced programming abilities. Their drivers will work with all their touchpads (which are usually incorporated by notebook makers), and will also provide some programmability for normal mice. Features include variable scrolling areas, corner tap areas which act as virtual buttons, and the ability to start any application or perform a keyboard sequence from a button, whether real or virtual. This means any of their products have up to six 'buttons'. Their drivers are Windows certified.
Cirque Corporation retail individual touchpads. Unlike Synaptics, Cirque blocks the features of their touchpad drivers depending on which model you buy. So for example, if you buy one of their cheaper touchpads, you will not have perhaps the zoom function, even though technically there is nothing stopping them from enabling the feature. Their features are also not as advanced as Synaptics, and they also have some basic problems, such as jumping cursors and not being able to perform a double click and drag (which will select text a word at a time). However they are the only people selling a USB touchpad at this time. Their drivers are not Windows certified, and thus many users have complained of various problems.
Alps Electric Corporation license their hardware from Cirque Corporation, but write their own drivers. Their drivers are windows certified, and thus do not suffer from the myriad of problems that plague Cirque users. They all have simple features such as edge scrolling, and also some advanced features such as corner programmability. They also have an icon on the taskbar, which is missing from Cirque drivers. They are mostly found in Sony notebooks, who make a rebranded driver.