5.4 KiB
AAC sub-spec — The access model
Normative refinement of SPEC §5. Reference: aac_core::{AccessMethod, MotorProfile, ScanConfig, ScanMode, DwellConfig}.
Every access method is a first-class input. The method and its tuning travel with the user, not the board: a board authored for eye-gaze MUST still open under touch or a switch (SPEC §5.3).
1. Access method — AccessMethod
DirectTouch (default) · EyeGaze · HeadTracking · SwitchScan { switches: u8 } · Joystick · Bci · PartialVoice · Pointer. SwitchScan carries the number of switches available (1, 2, or more); a 1-switch user relies on auto-scan timing, a 2-switch user steps and selects manually.
2. Motor profile — MotorProfile
The user's physical capabilities, used to adapt targets and timing: fine_motor and sustained_press (Full | Limited | None), tremor_level (None | Mild | Moderate | Severe), min_target_mm: f32, optional max_reach_mm, and preferred_side. A client SHOULD derive target size and dwell/scan timing from this profile and MUST NOT render a target smaller than min_target_mm.
3. Switch scanning — ScanConfig
A conforming scanner MUST support at minimum the Linear, RowColumn, and Group modes of ScanMode (also defined: Block, and inverse scanning). It MUST honor:
scan_rate_ms— highlight dwell per step;start_delay_ms— delay before auto-scan begins after a press;max_cycles— complete cycles before stopping (0= infinite);auto_restart— whether scanning restarts after a selection.
What moves the highlight is advance (ScanAdvance): Auto (the scan rate; a press selects), Step (the person's switch moves it one step, another selects — two-switch step scanning), or Hold (it moves at the scan rate while the switch is held, and letting go selects — inverse scanning). With Step and Hold, max_cycles does not apply: the scan never stops under the person. resume_from_last starts each pass at the unit (row, group, block or cell, or the group beside the grid) that held the last choice, on the same board; it only turns the visiting order. A third and fourth switch MAY move the highlight back one step (retreat) and leave a row for the rows (back_out). The groups beside the grid — the message bar (bar) and the navigation, Back, Home and Help (nav) — are placed with BarPlacement (After | Before | Off) each, the bar first when both are on one side (SPEC §5.5, §5.6). The switches themselves — which key, gamepad button or touch is which — are the person's (SPEC §5.7) and are kept by the client, not in ScanConfig.
scan_rate_ms is the person's to set (SPEC §5.1). adaptive_rate MUST default to false. A rate recommended, proposed or applied from measured responses MUST follow SPEC §5.2: at least 10 recorded responses, at least max(⌊100·M/65⌋, P95) of their response times, clamped to [300, 5000] ms. Without opt-in the scanner MAY only propose that rate (proposed_scan_rate_ms, which offers nothing within 10% of the current rate); with opt-in it MAY apply it. A client that measures responses MUST time them from the moment a highlight appears to the press.
4. Dwell / eye-gaze — DwellConfig
Gaze selection MUST use a configurable dwell time (base_dwell_ms, with the dwell in force in effective_dwell_ms) with visible progress feedback (DwellFeedback) and a tolerance_px radius within which gaze may wander without resetting. A selection MUST NOT fire before the effective dwell elapses.
A dwell rests on a DwellTarget: a cell, or a control beside the grid (a button of the bar or the navigation, or of an open panel; SPEC §5.6), which always takes the person's own dwell. With look_away, a target just selected is not selected again until gaze (or the pointer) has left it; chosen_otherwise ends a dwell when a tap, a click or a switch chose instead (SPEC §5.8). A pointer (a head mouse, an eye tracker that moves the pointer) dwells as gaze does, when the person chooses it: an ordinary mouse is the same input.
The dwell is the person's to set (SPEC §5.1; the reference's set_dwell_ms, clamped to [150, max_dwell_ms]). fatigue_adaptation — the reference's name for automatic dwell adjustment — MUST default to false. Proposals (proposed_dwell_ms) follow SPEC §5.3: one ladder step longer when more than 20% of recent selections were undone; one step shorter when fewer than 5% were undone and more than 20% of attempts were abandoned late (gaze left after at least half the dwell); never from fewer than 10 selections, never shorter than 300 ms. A client counts an undo when the person removes a word within a few seconds of selecting it by dwell. The dwell engine reads a monotonic clock that MUST work on the client's platform (the reference uses web-time, since std::time::Instant panics in browsers).
5. Universality
No access method MAY be a precondition for opening or reading a board (SPEC §5, item 3). Choosing an access method is best done by the person with their team, and it is never a prerequisite for starting (SPEC §11); the standard guarantees that whichever method is chosen is a first-class, fully-specified input — never a bolt-on. A client SHOULD start the chosen method as soon as it is chosen (scanning over the board on screen, the camera for gaze) and stop what it no longer needs — in particular, release the camera when gaze is no longer in use.