A PIE MENU COOKBOOK
Techniques for the Design of Circular Menus

Don Hopkins

ABSTRACT

Pie menus are used for making selections from items displayed on the computer screen, by pointing and clicking at the desired one with a mouse. The regions of the menu are shaped like the slices of a pie, laid out in a circle around the menu center. 

The click of a mouse button invokes a menu, which pops up on the screen positioned so that the cursor is centered in the small inactive region in the menu center. The active target regions are all adjacent to the cursor, but in different directions.  Pie menus are fast, because it only takes a small amount of cursor movement to point at one of the regions, and they are accurate, because the wedge shaped regions all have large areas. 

The circular layout of pie menus makes them very appropriate for certain tasks. Complementary items can be placed in opposite directions, and spatially oriented items can be put in their appropriate directions. Experienced users can select from familiar pie menus without looking at the menu, and can even mouse ahead into menus faster than the computer can update the screen.  When the user selects by mousing ahead into a menu, suppressing the menu display can speed up interaction considerably. 

The cursor distance from the menu center can be increased to get more angular precision, for accurate directional selection. It can also be used as an argument to the selection, as a continuous analog value, or a discrete sub-selection. 

Users can benefit from commonly used pie menus if they are designed to be easy to learn and use. A window management pie menu with its spatially oriented items in appropriate directions is an example of such a menu. A font selection menu using direction to select font style, and distance to select point size, is an example of how the two-dimensional aspect of pie menus can be exploited. 

A user should be able to discern the function of a pie menu by looking at it. A simple, intuitive, consistent look for visually representing the meaning and function of a pie menu can help to create an easy to use user interface.  Pie menus can also be designed so that they have a good kinesthetic feel to them, they do not require a lot of wasted mouse movement, and the directions are easier to select, and well matched with the input device. 



			LIST OF ILLUSTRATIONS

Figure	Title
1.	Pie Menu Schematic
2.	Sun Workstation, with a mouse
3.	Default linear frame menu
4.	Default frame menu in pie menu form
5.	Window management pie menu
6.	Stretch submenu
7.	Move submenu
8.	Zap submenu
9.	Times-Roman font style/size submenu
10.	Helvetica font style/size submenu



			     INTRODUCTION

This report describes the design of efficient and easy to use pie menu selection techniques. Pie menus are a fast, accurate way of using a mouse to select from items displayed on the screen. They have several advantages over traditional linear menus, and many practical applications. 

This report describes characteristics and distinctions of pie menus and gives examples of pie menu applications that take advantage of these qualities. It suggests general principles and heuristics for effective pie menu design, that can be applied to other applications. 


1. CHARACTERISTICS AND DISTINCTIONS OF PIE MENUS

1.1. DEFINITION
Pie menus are a fast, accurate way of selecting commands from a list of items shown on the screen, by using a mouse to point and click at the desired item. 

Clicking the mouse's menu button pops a pie menu up on the screen, centered on the cursor location. The menu items are positioned in a circle around the cursor, which is initially located in a small round inactive region at the menu center. Each item is adjacent to the cursor, but in a different direction. Moving the cursor in the direction of one of the items and clicking the button again selects it from the menu. 

The selection is defined by the direction of relative cursor motion between the clicks of the mouse button. Since the target regions of the menu items are shaped like the slices of a pie, the accuracy of selection becomes more precise as the cursor is moved farther away from the menu center.  The distance of movement is independent of the direction, so it may serve to modify the selection. 

1.2. DIRECTION
Menu designers can take advantage of the directional aspect of pie menus in several ways.  

CIRCULAR LAYOUT
Their circular layout is very appropriate for certain applications. Spatially oriented items can be placed in their corresponding directions, pairs of complementary items can be placed in opposite directions, and orthogonal items can be placed at right angles. Other natural and intuitive arrangements are possible, such as compasses and clock dials. (Hopkins, et al, 1987) 

SELECTING WITHOUT LOOKING
Because pie menus are based on direction of relative mouse movement, they do not require a lot of visual attention to use. An experienced pie menu user can make selections fairly reliably from a familiar, reasonably sized menu, without even looking at the display! 

MOUSE AHEAD
One of the nicest advantages of pie menus is that they work very well with "mouse ahead". Mouse ahead is when the user gives commands to the computer with the mouse more quickly than the computer can process, but instead of ignoring the commands, the computer buffers them, and processes them all in order.  Because it possible to use a pie menu without seeing it, experienced pie menu users are able to mouse ahead through several levels of nested menus without waiting for them to be displayed.	 

DISPLAY SUPPRESSION
With mouse ahead display suppression, a menu is not displayed on the screen if the user mouses ahead through it quickly enough. If the user completes the selection before the menu can be displayed, it is no longer necessary for the user to see the menu, if feedback is not required, or if acting on the selection provides feedback. Experienced users can learn to "chunk" multiple selection actions into single gestures that can be performed quickly and automatically (Buxton, 1986), and can utilize mouse ahead display suppression more often than a novice user.  This feature speeds up interaction considerably if the user is fast or the system is unresponsive. 

1.3. DISTANCE
The other important characteristic of a pie menu is the way in which the cursor distance effects the menu selection. 

SELECTION SPEED
To select any item, the cursor has only to move out of radius of the small inactive region in the menu center, where the menu was invoked. An item is selected by moving the cursor into its pie slice shaped active region, outside of the inactive region.  They are all equally distant from the cursor's initial position, but in different directions.  The active regions actually extend past the menu window edge, to the edge of the screen. 

In an experiment comparing pie menus with traditional linear menus, Jack Callahan has shown that novice users could make selection from eight item pie menus faster, and with fewer errors, than from traditional linear menus. (Callahan, et all, 1988) 

Fitt's law (Card, 1983) states that positioning time is dependant on the distance and target size.  Pie menus are fast because the distance to each target region is small, and the size of each target area is large. 

INCREASED ANGULAR PRECISION
As the cursor moves farther away from the menu center, to the wider regions of the slices, cursor motion has less effect on the direction from the menu center. The target regions extend out past the edges of the menu window to the screen edge, so the user can move the cursor out as far as needed to get the angular precision desired. 

DISTANCE AS AN ARGUMENT
The cursor's distance from the menu center can also be used as an additional input. It can serve as a continuous numeric argument to the menu selection, or it can select between one of several discrete sub-items of the selected slice. 

2. APPLICATION EXAMPLES
Pie Menus have been implemented for the NeWS window system (Gosling, 1985) by Don Hopkins, using an object oriented PostScript programming package (Densmore, 1986). This section will describe two pie menu applications developed with this system, as a result of research into the design of quick, intuitive, reliable, simple to learn, easy to remember interaction techniques.  

2.1. WINDOW MANAGEMENT MENU
In a graphical environment supporting multiple overlapping windows on the screen, such as the NeWS window system, pie menus can be a very effective interaction technique for window management tasks. They can be used to issue commands to move, resize, and manipulate windows, invoke programs, and control the environment in many ways.  

In NeWS, windows generally have a border around them called the frame. When the cursor is in a window's frame, pressing the mouse's menu button pops up a frame menu of window management commands. 

The standard frame menu, in linear format, is illustrated in figure 3. The items are grouped in more or less logical order.  The "Move" item lets the user reposition the window with the mouse. "Move Constrained" allows user to indicate either the x or y axis with the next mouse click, and then to position the window horizontally or vertically with the following click.  "Top" brings the window to the top of the stack of overlapping windows, and "Bottom" puts it on the bottom.  "Zap" destroys the window. "Resize" prompts the user to specify two corners of a rectangle, to which the window is moved and reshaped.  "Stretch corner" and "Stretch edge" allow the user to indicate either a corner or an edge with the next mouse click, and then to reposition it with the following click. "Close" replaces the window with a small icon representing the window. "Redisplay" erases and redraws the graphics in the window. 

A pie menu with the same functions in the same order as the standard frame menu is illustrated in figure 4. The pie menu has a larger area than the linear menu, and the logical label grouping does not especially take advantage of the circular layout.  

The goal was to design a pie menu with the functionality of the standard frame menu, but with its items labeled and arranged so that the menu would be easy to learn and use. 

Figure 5 is an illustration of a window management pie menu designed and implemented in object oriented PostScript, for the NeWS window system. The following is an explanation of the rationale behind the choice of functions, labels, and positions. 

TOP & BOTTOM
The up and down arrows correspond to the "Top" and "Bottom" items of the standard frame menu, that bring the window to the top or bottom of the stack. Since they are complementary, they are positioned opposite of one another. They are at the top and bottom pie menu slices because of how frequently they are used, and their obvious spatial orientation. 

CLOSED EYE
The label in the upper left slice of the menu (which many people think is a clam or a hamburger) is actually a closed eye.  It corresponds to the standard frame menu's "Close" item, which closes the window down into an icon. When the window is already in its iconic state, the eye in the menu appears open, and selecting it opens the icon up into a window.  The closed and opened eyes suggest the putting to sleep and waking up of the window, but it actually looks that way for the sake of a visual pun. 

PAINTING HAND
The painting hand icon in the upper right slice corresponds to the standard frame menu's "Redisplay" command. The hand balances with the eye, to give the menu a more symmetric and anthropomorphic look. 

STRETCH
Selecting "Stretch..." pops up a submenu with pictures of the four edges and four corners that may be stretched (figure 6).  Selecting one of these will start the stretching process immediately, allowing the user to position the edge or corner with the next mouse click. This pie menu is easy to remember and fast to use, because selecting a corner or edge is a spatially oriented task that can take advantage of the pie menu's circular layout, and can exploit mouse ahead display suppression well. 

MOVE
The "Move..." submenu has symbols for 4 different ways to move the window (figure 7). To the right is a symbol for unconstrained movement. Since that one is used more often than the others, it is in the same direction as the "Move..." item is in the frame menu, so the user does not have to change the direction of cursor motion to select it. 

To the left is the symbol for horizontally constrained movement, and the symbol on the top is for vertically constrained movement. Since these two are orthogonal spatially oriented tasks, they are positioned at right angles to each other, each on the appropriate axis. 

Selecting the closed eye label on the bottom of the menu will close the window to an icon, and then let the user position the icon with the next mouse click. When the window is already an icon, the eye is opened, and its function is to open the icon and let the user position the window. 

The ellipses after the labels "Stretch..." and "Move..." indicate that those items invoke submenus. The two items are opposite of each other on the horizontal axis, because they both invoke commonly used submenus. 

RESHAPE
The "Reshape!" item to the lower left is the same as the "Resize" item of the standard frame menu. The exclamation point after the label indicated that it is a command, acted upon immediately after being selected. The positions of the next two mouse clicks specify the rectangle to which the window is reshaped. 

ZAP
The "Zap?" item to the lower right pops up a confirmation submenu to make sure the user really means to destroy the window (figure 8). The question mark after the label indicated that the item requires confirmation. To confirm the selection, the user must choose the "nuke" symbol. The positive confirmation is located at the top of the menu, in a different direction than the "Zap?" item is in the frame menu, so that selecting it requires a change in the direction of mouse movement.  

2.2. FONT SELECTION MENU
The second pie menu application that will be described is a set of nested font selection menus (figures 9 and 10).  They are used to choose between one of four font families, and to select the style and size of the font.  The initial menu is labeled with the font family names, "Helvetica", "Courier", "Times-Roman", and "Hershey", shown in the appropriate fonts. 

Selecting any one of these pops up a two dimensional submenu to select the style and point size of the font.  The style is selected by the direction, and the point size is controlled by the distance. 

The style/size submenus are labeled with four different styles in the top, bottom, left, and right slices. labeled in the appropriate font and style. (Normal (the font family name), Italic or Oblique, Bold, and Bold Italic or Bold Oblique.) The style submenus are orthogonal (except for the Hershey family, which has a different set of styles), so they are simpler to remember. 

When the cursor is moved into one of the slices, a point size label is shown in the inner part of the selected wedge. The label is dynamically displayed in the point size selected by the cursor distance, and in the font style selected by the direction.  As the cursor is moved out farther, the label grows larger.  This animated feedback allows the user to see exactly what she will get, before making a selection. 

The style/size submenus are designed so that the style is the primary selection, to which the size is an argument.  If the two dimensional style/size submenus were instead arranged as size/style submenus, using direction to select the point size, and distance to select the style, it would be more difficult to change the point size without changing the style. 

3. DESIGN PRINCIPLES
This section will expound some principles of practical, intuitive, efficient menu design. Because user interface design is a balanced science and art, it is up to the menu designer to decide which heuristics are applicable to the task at hand. It is hoped that these will stimulate the imagination, and suggest other ways in which the unique characteristics of pie menus may be exploited. 

3.1. VISUAL REPRESENTATION OF FUNCTION
One challenge is to develop a simple, consistent look for visually representing the meaning and function of a pie menu. The graphics and text of the menu, labels, and feedback should convey useful information about the meaning of the items, and the way in which direction and distance are interpreted. 

DIRECTION
The cursor's direction from the menu center can be interpreted as a discrete or continuous input. 

DISCRETE DIRECTION
Cursor direction can be used to select between several discrete choices. The fewer choices there are, the easier the menu is to select from, because each choice get a bigger slice of the pie. The slices should be delimited to show their bounds, and labeled to show their function.  

The labels do not have to be confined within their respective slices, but it is important that no two labels overlap, and that it's obvious which label is associated with which slice.  Textual names or graphical icons can be used as item labels.  Horizontal text labels are better than slanted or vertical text, which is hard to read and can take up a lot of room. Graphical labels can be cryptic and unrecognizable to the inexperienced user, but they can be compact, meaningful, and interesting looking, if well designed. 

CONTINUOUS DIRECTION
Direction can also be used as a continuous analog input, so that the menu behaves like a dial.  A menu should have a scale indicating its range of values, or give dynamic feedback of the currently selected value. 

DISTANCE
Distance can be used as a continuous analog input, or it can select between one of several discrete "layers" of sub-choices within a slice. A scale, delimiters, or interactive feedback should be used to indicate the value selected by the distance. 

3.2. LABEL ARRANGEMENT
For many applications, a well thought out label arrangement can help to make a pie menu intuitive, learnable, and efficient to use.  This effort is mainly worthwhile with menus that will always have the same number of items, however. (Changing the number of choices in a pie menu changes the direction the choices are in, and requires the user to look at the menu to be sure of the selection direction.) By choosing appropriate and convenient directions, the menu designer can strive to make a menu with a mnemonic layout and a good kinesthetic feel. 

INTUITIVE
A pie menu is intuitive if there is an obvious, well known, or natural correspondence between the meaning of the items and their directions, the visual look of the labels and feedback, and the kinesthetic feel of mouse movement and clicks. 

LEARNABLE
The learnability of a pie menu can be enhanced by taking advantage of arrangements that the user is already familiar with, such as the positions of the hours on a clock face. A menu with orthogonal submenus, such as the font menu, is simpler to learn than one without such organization. 

GOOD KINESTHETIC FEEL
There are several ways to design a pie menu to have a good kinesthetic feel. Since these heuristics may not all be compatible with each other when applied to the design of a particular menu, the menu designer is encouraged to try out the feel of several different layouts first hand. Intellectualization and hand waving cannot replace prototyping and the actual evaluation of hands on experience. 

Some input devices favor certain directions of movement, making them easier to indicate than others (Buxton, 1986). In the experiment comparing pie menus with linear menus (Callahan, et al, 1988), it was found that the top and bottom items of an eight-item pie menu could be chosen faster than the other six, by novices using an optical mouse.  Therefore, putting commonly used items in the top and bottom slices of a pie menu can make it more kinesthetically efficient. 

Placing a commonly used submenu items in the same direction as the submenu is in its parent menu will enable the user to choose that item without changing the direction of mouse movement. Since the unconstrained movement command in the "Move..." submenu (figure 7) is the most commonly used item, it is placed to the right, in the same direction as the "Move..."  item in the window management pie menu (figure 5). 

Some commands will require the user to move the mouse to a new point on the screen and click the mouse button after making a menu selection.  Try to place such items in directions corresponding to the way the user would usually want to move the cursor after making the selection. For example, the corner and edge items in the "Stretch..." submenu are placed in the directions corresponding to the actual corners and edges of the window (figure 6). After the selection is made, the user moves the cursor to a new location for the specified corner or edge, and clicks the button. The new location is often  in the same direction as the item, in which case there is less wasted mouse movement. 

Another such example is that the vertical and horizontal constrained window movement commands in the "Move..." submenu are the top and left items of the menu, on the axis corresponding to types of movement they select (figure 7). To change a window's vertical or horizontal position, the user selects one of the constrained movement items, and points and clicks at a new vertical or horizontal position, which is commonly in the same direction as the selected item. 

3.3. MENU SIZE
Screen space is a valuable resource, and pie menus with long labels or lots of items can take up a lot of space on the screen. 

MINIMIZING THE SIZE
One way to minimize the amount of screen space a menu takes up is to use compact labels.  Wide labels are best placed near the top and bottom of a menu, where they can fit inside a smaller radius.  Wide labels near the left and right sides of a menu tend to make it much larger. 

SUBMENUS
Another way of making menus smaller is to use fewer items in them. Submenus can be used to group together logically related items.  Using a small menu of nested submenus is often better than using one menu with many items.  A set of nested menus is easier to learn and use if the submenus are orthogonal, with similar item arrangements, since there are fewer different sets of directions for the user to remember. 

EIGHT ITEM MENUS
Eight is a convenient number of items to have in a pie menu. Four is an even nicer number. Eight and four item menus are reasonably sized, uncrowded, symmetric, and have convenient pairs of opposite directions.  Experienced pie menu users can swiftly and confidently mouse ahead thought such menus, with their eyes closed!  Another advantage designing eight and four item pie menus is that they can be easily used with a numeric or arrow keypad instead of a mouse. 



			      CONCLUSION

Pie menus can be a very efficient and accurate selection technique, especially if some thought and experimentation are put into their design. It is hoped that these suggestions will inspire menu designers to apply pie menus to their own applications, and to come up with more techniques to improve efficient interaction. 

An effective way to evaluate a particular menu design is to actually implement it and try it out for a while. The experimenter can actually learn quite a bit from unsuccessful designs. Powerful prototyping tools like the NeWS window system encourage an experimental approach to menu design, which results in applications based on actual hands-on experience, instead of speculation. 



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