Note: this is a "vintage" tip for older computers/systems.
Gestures are an alternative way to control your computer. Basically, the user moves the mouse in a particular pattern, usually while holding down a particular mouse button and/or key. This saves pressing awkward keyboard shortcuts or moving the mouse up to the menus (especially useful if you have a big screen).
Some programs (such as Opera) have had this functionalty for a while but it is more useful when applied system wide. To make gestures useful you really need a three button mouse. The bad news is that for a long time Macs come with a one button mouse by default. The good news is that they now ship with a 4 button mouse, and replacements cost very little anyway. Any PC USB mouse will work without drivers in Mac OS X.
Gestures can be used in Mac OS X if you install an input manager called "Cocoa Gestures". This is available from bitart. If you install this all Cocoa programs will then get a "Cocoa Gestures" menu item in the application menu which will allow configuring that program for gestures. Note that you don't have to restart or even log out to activate this (isn't Mac OS X wonderful?) but you will need to re-launch any programs that were running when Cocoa gestures was installed. the installation involves copying a folder into the input managers folder in your library, so its really easy.
After installing, run your Cocoa program and select Cocoa gestures. Click the enable button, select the mouse button to use, and start entering gestures. For example a move to the left in Chimera would logically mean previous page. I use up-right-down (sort of like an "n") for new tab.
There are some limitations in the current version (1.1). Most seriously, as the name suggests, it only works with Cocoa programs. Cocoa programs include Mail (but not Entourage or Eudora), OmniWeb and Chimera (but not Internet Explorer, Mozilla, Netscape or iCab), Path Finder (but not the normal Finder), Acquisition, Address Book, Disk Copy, FileXaminer, Help Viewer, iCal, iChat, iOrganizeX, LDapper, Network Utility, OmniDictionary, OmniWeb, PhotoStickies, PrefEdit, Preview, Print Center, Simon, System Preferences, Terminal, TextEdit, Ultralingua, UnicodeChecker, Watson and Wcalc. Those are the Cocoa programs I regularly use. I'm sure there are more.
Another limitation is that only horizontal and vertical movements are supported. Curves and diagonals are translated as the nearest sequence of lines. Also, only menu commands can be intiated by a gesture, so programs like iCal, which have a lot of buttons and dialogs, are hard to control.
Although these limitations can be annoying, gestures are very addicitve. I still find myself gesturing when usng Internet Explorer (for example) on someone else's computer without this capability and wondering why nothing happens!
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