Dasher Special Needs Guide

There are several ways of starting and stopping Dasher. Pressing a button (for example, the left mouse button or the space bar) is one option. But if you can not press any buttons, it's possible to start and stop using only continuous gestures: in the options menu, select “start on position”; and switch on “control mode”;. When control mode is switched on, the Dasher alphabet includes a special Control node (a bit like an Esc key on a keyboard), within which various control functions are available. When you are inside the control node, Dasher moves more slowly than normal, for safety. The control node options include `pause' and `stop'. Use `pause' if you are half-way through writing something, and want to pause for a moment. Use `stop' when you have finished. Pause and stop produce the same behaviour, except stop may cause other automatic actions, such as `speak on stop', or `copy the text on stop'.

When Dasher is paused or stopped, it can be restarted using any of the starting methods that are enabled. If `start on position' is enabled, then whenever Dasher is stopped a sequence of large targets will be displayed; you restart Dasher by pointing at (or looking at) the first (red) target, then the second (yellow) target. (We use two targets in sequence to make it difficult to start Dasher by accident.)

Many trackers have `smoothing' options, which determine the frequency with which the mouse position is updated; these options are normally used to smooth and damp down the mouse motion. For Dasher, we don't want such smoothing. We like instant, live, raw and jerky mouse coordinates. If there is a `smoothing' control, turn it right down.

The `gain' (sometimes called the `speed') of the head-tracker is also an important setting to adjust. Some trackers' gains can be adjusted in software. You can also adjust the gain by changing the geometry of your tracker: if you move the tracked dot from your forehead to the brim of a baseball cap, for example, then you roughly double the gain. Sitting closer to the tracker may also increase the gain. Find a gain setting that is comfortable. I like high gain because it allows me to steer with very small head motions.

For good results with gaze trackers, we strongly recommend that the gaze-tracker be made to be as responsive as possible. Many trackers have `smoothing' options, which determine the frequency with which the mouse position is updated and the number of successive gaze images used to estimate the mouse position. These options are normally used to smooth and damp down the mouse motion. For Dasher, we don't want such smoothing. We like instant, live, raw and jerky mouse coordinates. When you are navigating, your eye moves very quickly to the target you are interested in, and we want Dasher to respond instantly. The ideal settings for Dasher may be very different from the ideal settings for other software. Ask your eyetracker manufacturer to make it easy to change the settings when switching application.

Dasher has several options designed for use with gaze-trackers. We recommend using eyetracker mode (under Options/Preferences/Control). In this mode, the dynamics of Dasher are slightly different from standard dynamics, making error-correction easier by gaze.

If your gaze-tracker's calibration drifts with time, for example when your head moves, then you should select the Autocalibrate eyetracker feature. When this feature is switched on, Dasher keeps track of your steering and infers the vertical calibration error, and corrects for it. You can see this correction taking effect by noticing the vertical offset between the mouse position as displayed by Dasher (by the tip of the red line) and the gaze-tracker's mouse position (shown by the system's mouse cursor).

To avoid difficulties with the mouse being bounded by the top and bottom of the screen, we recommend choosing a window size for Dasher that is not full-screen in size. Place the Dasher window so that there is a margin above and below the Dasher canvas.

Technical note: As well as through mouse emulation, Dasher is able to receive tracking information from gaze trackers, head trackers, or similar systems directly by means of a system socket. This option can be configured in the 'Input Device' section of the 'preferences' dialogue.

Some ways of conveying information make use of the timing of gestures. However, some people can't make gestures at a required instant. For example, spastics find it very difficult to do an action `exactly now!'

If time-critical gestures are not an option, go to Section 7.3 ― `Timeless' choices of Dasher.

If you can convey information by precisely timed gestures, go to section Section 7.4 ― Button Dashers that exploit timing.

In click mode, you position the mouse pointer where you want to go, then press a button when you are ready. Dasher then zooms in on the position you chose.

Alternatively, if you have a touch screen, a single touch on the screen initiates a zoom to that position.

How many different switches, keys, or buttons can you easily operate?

When started, Dasher will zoom towards a point towards the top or bottom of the display. A short button press will switch the point of zoom to the oposite side. Text can be entered by pressing the button when the desired phrase reaches the edge of the display.

Dasher zooms continuously towards the centre of the screen, with the two buttons being used to shift the display up and down. The buttons should be pressed whenever the desired text is aligned with the two markers.

In either dynamic mode, there are three options for error correction: using an additional button, long presses or multiple presses. In all cases these actions will switch to a mode where Dasher unzooms at a fixed rate. In this mode, one more press will stop Dasher and a second press will return Dasher to forward zooming.