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Optical versus mechanical mice
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Buttons
In contrast to the motion-sensing mechanism, the mouse's buttons have changed little over the years, varying mostly in shape, number, and placement. Engelbart's very first mouse had a single button; Xerox PARC soon designed a three-button model, but reduced the count to two for Xerox products. Apple reduced it back to one button with the Macintosh in 1984, while Unix workstations from Sun and others used three buttons. Commercial mice usually have between one and three buttons, although in the late 1990s some mice had five or more.
The two-button mouse has become the most commonly available design. As of 2007 (and roughly since the mid-1990s), users most commonly employ the second button to invoke a contextual menu in the computer's software user interface, which contains options specifically tailored to the interface element over which the mouse pointer currently sits. By default, the primary mouse button sits located on the left-hand side of the mouse, for the benefit of right-handed users; left-handed users can usually reverse this configuration via software. (As of 2007, the most common mice actually have three buttons, with a middle scroll-wheel (see below) that includes a switch function — but they look like and are often referred to as "two-button".)
On systems with three-button mice, pressing the center button (a middle click) often conveniently maps a commonly-used action or a macro. In the X Window System, middle-clicking by default pastes the contents of the primary buffer at the pointer's position. Many users of two-button mice emulate a three-button mouse by clicking both the right and left buttons simultaneously. Middle-clicks often provide the functionality of an extra button.
Additional buttons
Manufacturers have built mice with five or more buttons. Depending on the user's preferences and software environment, the extra buttons may allow forward and backward web-navigation, scrolling through a browser's history, or other functions. As with similar features in keyboards, however, not all software supports these functions. The additional buttons become especially useful in computer games, where quick and easy access to a wide variety of functions (for example, weapon-switching in first-person shooters) can give a player an advantage. Because software can map mouse-buttons to virtually any function, keystroke, application or switch, extra buttons can make working with such a mouse more efficient and easier.
In the matter of the number of buttons, Douglas Engelbart favored the view "as many as possible". The prototype that popularized the idea of three buttons as standard had that number only because "we could not find anywhere to fit any more switches".
Wheels. The scroll wheel, a notably different form of mouse-button, consists of a small wheel that the user can rotate to provide immediate one-dimensional input. Usually, this input translates into "scrolling" up or down within the active window or GUI-element. The scroll wheel can provide convenience, especially when navigating a long document. The scroll wheel also often includes a pressure detector or switch, a de facto third (center) button. Under many Microsoft Windows applications, appropriate pressure on the wheel activates autoscrolling, and in conjunction with the control key (Ctrl) may give the capability of zooming in and out; applications that support this feature include Adobe Reader, Microsoft Word, Internet Explorer, Opera, Mozilla Firefox and Mulberry.
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Manufacturers may refer to scroll-wheels by different names for branding purposes; Genius, for example, usually brand their scroll-wheel-equipped products "Netscroll".
Mouse Systems introduced the scroll-wheel commercially in 1995, marketing it as the Mouse Systems ProAgio and Genius EasyScroll. However, mainstream adoption of the scroll wheel mouse did not occur until Microsoft released the Microsoft IntelliMouse in 1996. It became a commercial success in 1997 when the Microsoft Office application suite and the Internet Explorer browser started supporting its wheel-scrolling feature. Since then the scroll wheel has become a standard feature of many mice-models. Some newer mouse models have two wheels, separately assigned to horizontal and vertical scrolling. Designs exist which make use of a "rocker" button instead of a wheel — a pivoting button that a user can press at the top or bottom, simulating "up" and "down" respectively. A more recent form of mouse wheel, the tilt-wheel, features in some of the higher-end Logitech and Microsoft mice, such as the Logitech G5. Tilt wheels are essentially conventional mouse wheels that have been modified with a pair of sensors articulated to the tilting mechanism. These sensors are mapped, by default, to horizontal scrolling.
A third variety of built-in scrolling device, the scroll ball, essentially consists of a trackball embedded in the upper surface of the mouse. The user can scroll in all possible directions in very much the same way as with the actual mouse, and in some mice, can use it as a trackball. Mice featuring a scroll ball include Apple's Mighty Mouse and the IOGEAR 4D Web Cruiser Optical Scroll Ball Mouse.
Mouse speed. The computer-industry often measures mouse sensitivity in terms of counts per inch (CPI), commonly expressed less correctly as dots per inch (DPI) — the number of steps the mouse will report when it moves one inch. If the default mouse-tracking condition involves moving the pointer by one screen-pixel or dot on-screen per reported step, then the CPI does equate to DPI: dots of pointer motion per inch of mouse motion. The CPI or DPI as reported by manufacturers depends on how they make the mouse; the higher the CPI, the faster the pointer moves with mouse movement. However, software can adjust the mouse sensitivity, making the cursor move faster or slower than its DPI. Current software can change the speed of the pointer dynamically, taking into account the mouse's absolute speed and the movement from the last stop-point. Different software may name the settings "acceleration" or "speed" — referring respectively to "threshold" and "pointer precision".
For simple software, when the mouse starts to move, the software will count the number of "counts" received from the mouse and will move the pointer across the screen by that number eof pixels (or multiplied by a factor f1=1,2,3). So, the pointer will move slowly on the screen, having a good precision. When the movement of the mouse reaches the value set for "threshold", the software will start to move the pointer more quickly; thus for each number n of counts received from the mouse, the pointer may move (f2 x n) pixels, where f2=2,3...10. Usually, the user can set the value of f2 by changing the "acceleration" setting.
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Operating systems sometimes apply acceleration, referred to as "ballistics", to the motion reported by the mouse. For example, versions of Windows prior to Windows XP doubled reported values above a configurable threshold, and then optionally doubled them again above a second configurable threshold. These doublings applied separately in the X and Y directions, resulting in very nonlinear response. For example one can see how the things work in Microsoft Windows NT. Starting with Windows XP OS version of Microsoft and many OS versions for Apple Macintosh, computers use a smoother ballistics calculation that compensates for screen-resolution and has better linearity.
A less common measure of mouse performance, the "Mickey", takes its name from Mickey Mouse. Not a traditional unit of measurement, it indicates merely the number of "dots" reported in a particular direction. Only when combined with the DPI of the mouse does the Mickey become an indication of actual distance moved. In the absence of acceleration, the Mickey corresponds to the number of pixels moved on the computer screen.
"Mice" and "mouses". The fourth (current as of 2006) edition of The American Heritage Dictionary of the English Language endorses both computer mice and computer mouses as correct plural forms for computer mouse. The form mice, however, appears most commonly, while some authors of technical documents may prefer either mouse devices or the more generic pointing devices. The plural mouses treats mouse as a "headless noun."
Accessories
Mousepad. The mousepad, the most common mouse accessory, appears most commonly in conjunction with mechanical mice, because in order to roll smoothly, the ball requires more friction than common desk surfaces usually provide. Special "hard mousepads" for gamers also exist.
Most optical and laser mice do not require a pad, and using pads with such models remains mostly a matter of personal taste. One exception occurs when the desk surface creates problems for the optical or laser tracking. Other cases may involve keeping desk or table surfaces free of scratches and deterioration; when the grain pattern on the surface causes inaccurate tracking of the pointer, or when the mouse-user desires a more comfortable mousing surface to work on and reduced collection of debris under the mouse.
Foot covers. Mouse foot-covers (or foot-pads) consists of low-friction or polished plastic. This makes the mouse glide with less resistance over a surface. Some higher quality models have teflon feet to reduce friction even further.
Wrist-rests. Cushioning pillows made from silicone gel, neoprene or other spongy material have also become popular accessories. The padding provides for a more natural angle of the wrist, in order to reduce fatigue and avoid excessive strain. Some people believe that improved mousing posture accounts for the popularity of wrist-rests.
Mice in the marketplace. Around 1981 Xerox included mice with its Xerox Star, based on the mouse used in the 1970s on the Alto computer at Xerox PARC. Sun Microsystems, Symbolics, Lisp Machines Inc., and Tektronix also shipped workstations with mice, starting in about 1981. Later, inspired by the Star, Apple Computer released the Apple Lisa, which also used a mouse. However, none of these products achieved large-scale success. Only with the release of the Apple Macintosh in 1984 did the mouse see widespread use.
The Macintosh design, commercially successful and technically influential, led many other vendors to begin producing mice or including them with their other computer products. The widespread adoption of graphical user interfaces in the software of the 1980s and 1990s made mice all but indispensable for controlling computers. As of 2000, Dataquest estimated annual world-wide sales of mice as worth USB 1.5 billion.
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