In computer interface design, a pie menu (also known as a radial menu) is a circular context menu where selection depends on direction. A pie menu is made of several "pie slices" around an inactive center and works best with stylus input, and well with a mouse. Pie slices are drawn with a hole in the middle for an easy way to exit the menu.
Pie menus work well with keyboard acceleration, particularly four and eight item menus, on the cursor keys and the number pad. A goal of pie menus is to provide a smooth, reliable gestural style of interaction for novices and experts.
A slice can lead to another pie menu; selecting this may center the mouse cursor in the new menu. A marking menu is a variant of the technique where the menu is not shown when the interaction is short, allowing for faster operation times when several gestures are chained to make a selection through the first and subsequent menus.
As a kind of context menus, pie menus are often context-sensitive, showing different options depending on what the mouse cursor was pointing at when the menu was requested.
The first documented radial menu is attributed to a system called PIXIE in 1969. Some universities explored alternative visual layouts.
In 1986, Mike Gallaher and Don Hopkins together arrived independently to the concept of a context menu based on the angle to the origin where the exact angle could be passed as a parameter to a command, and the radius could be used to trigger a submenu.
The first performance comparison to linear menus was performed at 1988 showing an increase in performance of 15% less time and a reduction of selection errors.
For the novice, pie menus are easy because they are a self-revealing gestural interface: They show what you can do and direct you how to do it. By clicking and popping up a pie menu, looking at the labels, moving the cursor in the desired direction, then clicking to make a selection, you learn the menu and practice the gesture to "mark ahead" ("mouse ahead" in the case of a mouse, "wave ahead" in the case of a dataglove). With a little practice, it becomes quite easy to mark ahead even through nested pie menus.
For the expert, they're efficient because—without even looking—you can move in any direction, and mark ahead so fast that the menu doesn't even pop up. Only when used more slowly like a traditional menu, does a pie menu pop up on the screen, to reveal the available selections.
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