The iPad stroking gestures are kind of like pie menus in how you use them, but unlike pie menus in how they aren't "self revealing", with a way to show you what gestures are possible and what they do (i.e. a pop-up menu).

But there's another fundamental difference between pie menus and gestures, something I was trying to get at by coining the term "gesture space", defined here (I'd love to know if other people have come up with a better term or more rigorous definition):

https://medium.com/@donhopkins/gesture-space-842e3cdc7102

>Gesture Space: The space of all possible gestures, between touching the screen / pressing the button, moving along an arbitrary path (or not, in the case of a tap), and lifting your finger / releasing the button. It gets a lot more complex with multi touch gestures, but it’s the same basic idea, just multiple gestures in parallel.

Excerpt from OLPC Sugar Discussion about Pie Menus:

https://medium.com/@donhopkins/olpc-sugar-pie-menu-discussio...

>I think it’s important to trigger pie menus on a mouse click (and control them by the instantaneous direction between clicks, but NOT the path taken, in order to allow re-selection and browsing), and to center them on the exact position of the mouse click. The user should have a crisp consistent mental model of how pie menus work (which is NOT the case for gesture recognition). Pie menus should completely cover all possible “gesture space” with well defined behavior (by basing the selection on the angle between clicks, and not the path taken). In contrast, gesture recognition does NOT cover all gesture space (because most gestures are syntax errors, and gestures should be far apart and distinct in gesture space to prevent errors), and they do not allow in-flight re-selection, and they are not “self revealing” like pie menus.

>Pie menus are more predictable, reliable, forgiving, simpler and easier to learn than gesture recognition, because it’s impossible to make a syntax error, always possible to recover from a mistaken direction before releasing the button, they “self reveal” their directions by popping up a window with labels, and they “train” you to mouse ahead by “rehearsal”.

Excerpt from a Hacker News discussions:

https://news.ycombinator.com/item?id=16615023

>Swiping gestures are essentially like invisible pie menus, but actual pie menus have the advantage of being "Self Revealing" [5] because they have a way to prompt and show you what the possible gestures are, and give you feedback as you make the selection.

>They also provide the ability of "Reselection" [6], which means you as you're making a gesture, you can change it in-flight, and browse around to any of the items, in case you need to correct a mistake or change your mind, or just want to preview the effect or see the description of each item as you browse around the menu.

>Compared to typical gesture recognition systems, like Palm's graffiti for example, you can think of the gesture space of all possible gestures between touching the screen, moving around through any possible path, then releasing: most gestures are invalid syntax errors, and they only recognizes well formed gestures.

>There is no way to correct or abort a gesture once you start making it (other than scribbling, but that might be recognized as another undesired gesture!). Ideally each gesture should be as far away as possible from all other gestures in gesture space, to minimize the possibility of errors, but in practice they tend to be clumped (so "2" and "Z" are easily confused, while many other possible gestures are unused and wasted).

>But with pie menus, only the direction between the touch and the release matter, not the path. All gestures are valid and distinct: there are no possible syntax errors, so none of gesture space is wasted. There's a simple intuitive mapping of direction to selection that the user can understand (unlike the mysterious fuzzy black box of a handwriting recognizer), that gives you the ability to refine your selection by moving out further (to get more leverage), return to the center to cancel, move around to correct and change the selection.

>Pie menus also support "Rehearsal" [7] -- the way a novice uses them is actually practice for the way an expert uses them, so they have a smooth learning curve. Contrast this with keyboard accelerators for linear menus: you pull down a linear menu with the mouse to learn the keyboard accelerators, but using the keyboard accelerators is a totally different action, so it's not rehearsal.

>Pie menu users tend to learn them in three stages: 1) novice pops up an unfamiliar menu, looks at all the items, moves in the direction of the desired item, and selects it. 2) intermediate remembers the direction of the item they want, pop up the menu and moves in that direction without hesitating (mousing ahead but not selecting), looks at the screen to make sure the desired item is selected, then clicks to select the item. 3) expert knows which direction the item they want is, and has confidence that they can reliably select it, so they just flick in the appropriate direction without even looking at the screen.

https://news.ycombinator.com/item?id=7262849

>Pie menus completely saturate the entire possible gesture space with usable and accessible commands: there is no such thing as a syntax error, and you can always correct any gesture to select what you want, no matter how bad it started out, or cancel the menu, by moving around to the desired item or back to the center to cancel.

>Handwriting and gesture recognition does not have this property, and it can be quite frustrating because you can't correct or cancel mistakes, and dangerous because mistakes can be misinterpreted as the wrong command. Most gestures are syntax errors. Blind gesture recognition doesn't have a good way to prompt and train you with the possible gestures, which only cover a tiny fraction of the possible gesture space. All the rest of the space of possible gestures is wasted, and interpreted as a syntax error (or worse, misinterpreted as the wrong gesture), instead of enabling the user to correct mistakes and reselect different gestures.

>Even "fuzzy matching" of gestures trades off gestural precision with making it even harder to cancel or correct a gesture, without accidentally being misinterpreted as the wrong gesture. That's not the kind of an interface you would want to use in a mission critical application such as a car or airplane.