doc/palm-detection.dox

/**
@page palm_detection Palm detection

Palm detection tries to identify accidental touches while typing.

On most laptops typing on the keyboard generates accidental touches on the
touchpad with the palm (usually the area below the thumb). This can lead to
cursor jumps or accidental clicks.

Interference from a palm depends on the size of the touchpad and the position
of the user's hand. Data from touchpads showed that almost all palm events on a
Lenovo T440 happened in the left-most and right-most 5% of the touchpad. The
T440 series has one of the largest touchpads, other touchpads are less
affected by palm touches.

@section palm_exclusion_zones Palm exclusion zones

libinput enables palm detection on the edge of the touchpad. Two exclusion
zones are defined  on the left and right edge of the touchpad.
If a touch starts in the exclusion zone, it is considered a palm and the
touch point is ignored. However, for fast cursor movements across the
screen, it is common for a finger to start inside an exclusion zone and move
rapidly across the touchpad. libinput detects such movements and avoids palm
detection on such touch sequences.

Each exclusion zone is divided into a top part and a bottom part. A touch
starting in the top part of the exclusion zone does not trigger a
tap (see @ref tapping).

In the diagram below, the exclusion zones are painted red.
Touch 'A' starts inside the exclusion zone and moves
almost vertically. It is considered a palm and ignored for cursor movement,
despite moving out of the exclusion zone.

Touch 'B' starts inside the exclusion zone but moves horizontally out of the
zone. It is considered a valid touch and controls the cursor.

Touch 'C' occurs in the top part of the exclusion zone. Despite being a
tapping motion, it does not generate an emulated button event. Touch 'D'
likewise occurs within the exclusion zone but in the bottom half. libinput
will generate a button event for this touch.

@image html palm-detection.svg

@section trackpoint-disabling Palm detection during trackpoint use

If a device provides a <a
href="http://en.wikipedia.org/wiki/Pointing_stick">trackpoint</a>, it is
usually located above the touchpad. This increases the likelihood of
accidental touches whenever the trackpoint is used.

libinput disables the touchpad whenever it detects trackpoint activity for a
certain timeout until after trackpoint activity stops. Touches generated
during this timeout will not move the pointer, and touches started during
this timeout will likewise not move the pointer (allowing for a user to rest
the palm on the touchpad while using the trackstick).
If the touchpad is disabled, the @ref t440_support "top software buttons"
remain enabled.

@section disable-while-typing Disable-while-typing

libinput automatically disables the touchpad for a timeout after a key
press, a feature traditionally referred to as "disable while typing" and
previously available through the
[syndaemon(1)](http://linux.die.net/man/1/syndaemon) command. libinput does
not require an external command and the feature is currently enabled for all
touchpads but will be reduced in the future to only apply to touchpads where
finger width or pressure data is unreliable.

Notable behaviors of libinput's disable-while-typing feature:
- Two different timeouts are used, after a single key press the timeout is
  short to ensure responsiveness. After multiple key events, the timeout is
  longer to avoid accidental pointer manipulation while typing.
- Some keys do not trigger the timeout, specifically some modifier keys 
  (Ctrl, Alt, Shift, and Fn). Actions such as Ctrl + click thus stay
  responsive.
- Touches started while typing do not control the cursor even after typing
  has stopped, it is thus possible to rest the palm on the touchpad while
  typing.
- Physical buttons work even while the touchpad is disabled. This includes
  software-emulated buttons.

@section thumb-detection Thumb detection

Many users rest their thumb on the touchpad while using the index finger to
move the finger around. For clicks, often the thumb is used rather than the
finger. The thumb should otherwise be ignored as a touch, i.e. it should not
count towards @ref clickfinger and it should not cause a single-finger
movement to trigger @ref twofinger_scrolling.

libinput uses two triggers for thumb detection: pressure and
location. A touch exceeding a pressure threshold is considered a thumb if it
is within the thumb detection zone.

@note "Pressure" on touchpads is synonymous with "contact area", a large
touch surface area has a higher pressure and thus hints at a thumb or palm
touching the surface.

Pressure readings are unreliable at the far bottom of the touchpad as a
thumb hanging mostly off the touchpad will have a small surface area.
libinput has a definitive thumb zone where any touch is considered a resting
thumb.

@image html thumb-detection.svg

The picture above shows the two detection areas. In the larger (light red)
area, a touch is labelled as thumb when it exceeds a device-specific
pressure threshold. In the lower (dark red) area, a touch is labelled as
thumb if it remains in that area for a time without moving outside.

*/