Input Models Flashcards
Models
- Models are representations of phenomena that help us understand how something works (or how it will work)
- as simple as possible
- as complex as necessary
- Models are never perfect
- Models are based on assumptions
- Models are useful for specific phenomena, and not useful for other
- Fitts’ Law models performance of aimed movement
- Assumptions
- Target is known in advance (no search time)
- Reachable in uninterrupted movement (no steps)
- Fitts’ Law holds for pointing
- Direct and Indirect
- But does not hold for (e.g.) drawing
Fitts’ Law applied to 2D Pointing
- MT = a + b * log2(D/W + 1)
- Task: Move pointer onto the target as quickly as possible
- Distance: From current cursor position to center of target
- Width: Across target, in the direction of movement
Example: Select the Square
- MT = a + b * log2 (D/W + 1)
- Calculate the average movement time for the cursor to select the square
- a = 50ms, b = 100ms/bit
- MT = 50 + 100 (log2(6/2+1))
- MT = 450
Which menu design is most efficient?
Crossing versus Pointing
Pointing: tolerance (width) in the direction of movement (collinear)
- Can only control and correct at the end of the movement
Crossing: tolerance (width) orthogonal to direction of movement
- Can control and correct all the way towards the goal
Crossing versus Pointing 2
- Pointing
- Crossing object boundary and stopping on object
- Example: mouse-over to select hover state
- Crossing
- Crossing through an object for selection
- Removes constraint to stop with object boundaries
Advantages of Crossing-based Selection
- Selection of objects that are “thin”
- Multi-selection: move across multiple objects to select
- e.g., Swiping across keys on a keyboard
- Good with input devices that have no button to click
- Mid-air pointing (Kinect etc); Head pointing in 3D world
-Pen/Stylus: Crossing with pen on surface; don’t need to lift pen to tap
- Good when movement is jittery, or tracking noisy
- e.g., hand tremor in mid-air pointing, controllers with low precision
Objects at the Display Edge
- When an object touches the edge, selecting it becomes a crossing task
- W: defined by object boundary
- e.g. Apple Menu
- When an object does not touch the edge, selecting it is a pointing task
- W: defined by direction of mov.
- e.g., MS Taskbar
Corners and Edges
- Application menu bar: In window or at top of screen?
- Access to key controls and menus in display corners
Steering Law
- Time to drive through a narrow road
- Time depends on path length and width along the way
- Path integral over 1/W; approximately by summing up: T = a + b * S/W
Steering a Mouse through a Tunnel
- A straight road … with distance D and width W
Steering Law Definition (Accot-Zhai)
MT = a + b * D/W
ID = D/W
- The movement time (MT) to acquire a target through a tunnel is a function of the length (D) and width (W) of the tunnel
- MT: Movement time
- a and b: constants dependent on the pointing system
- D: distance, i.e. length of the tunnel
- W: width of the tunnel
Tunnels in the User Interface
- Pointing to ‘Find’
- ID = log2(D/W)
- Steering through the tunnel to the submenu
- ID = D/W
- Pointing in submenu
Steering between objects
Keystroke-Level Model (KLM)
- KLM defines a set of basic input operators with time estimates for their execution
- Physical motor operators
- Pressing a key, Pointing with the mouse, Draw a line
- Move hand from one device to another
- Mental operator
- Mental preparation of a motor action
- Also to model time to make a decision, on to look for something
- System response
- Time the user must wait for a response
KLM
KLM Example
