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334c3faa7b
And link the software buttons sentence to the t440 page. Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
114 lines
5.2 KiB
Text
114 lines
5.2 KiB
Text
/**
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@page palm_detection Palm detection
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Palm detection tries to identify accidental touches while typing.
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On most laptops typing on the keyboard generates accidental touches on the
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touchpad with the palm (usually the area below the thumb). This can lead to
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cursor jumps or accidental clicks.
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Interference from a palm depends on the size of the touchpad and the position
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of the user's hand. Data from touchpads showed that almost all palm events on a
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Lenovo T440 happened in the left-most and right-most 5% of the touchpad. The
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T440 series has one of the largest touchpads, other touchpads are less
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affected by palm touches.
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@section palm_exclusion_zones Palm exclusion zones
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libinput enables palm detection on the edge of the touchpad. Two exclusion
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zones are defined on the left and right edge of the touchpad.
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If a touch starts in the exclusion zone, it is considered a palm and the
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touch point is ignored. However, for fast cursor movements across the
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screen, it is common for a finger to start inside an exclusion zone and move
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rapidly across the touchpad. libinput detects such movements and avoids palm
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detection on such touch sequences.
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Each exclusion zone is divided into a top part and a bottom part. A touch
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starting in the top part of the exclusion zone does not trigger a
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tap (see @ref tapping).
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In the diagram below, the exclusion zones are painted red.
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Touch 'A' starts inside the exclusion zone and moves
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almost vertically. It is considered a palm and ignored for cursor movement,
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despite moving out of the exclusion zone.
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Touch 'B' starts inside the exclusion zone but moves horizontally out of the
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zone. It is considered a valid touch and controls the cursor.
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Touch 'C' occurs in the top part of the exclusion zone. Despite being a
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tapping motion, it does not generate an emulated button event. Touch 'D'
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likewise occurs within the exclusion zone but in the bottom half. libinput
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will generate a button event for this touch.
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@image html palm-detection.svg
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@section trackpoint-disabling Palm detection during trackpoint use
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If a device provides a <a
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href="http://en.wikipedia.org/wiki/Pointing_stick">trackpoint</a>, it is
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usually located above the touchpad. This increases the likelihood of
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accidental touches whenever the trackpoint is used.
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libinput disables the touchpad whenever it detects trackpoint activity for a
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certain timeout until after trackpoint activity stops. Touches generated
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during this timeout will not move the pointer, and touches started during
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this timeout will likewise not move the pointer (allowing for a user to rest
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the palm on the touchpad while using the trackstick).
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If the touchpad is disabled, the @ref t440_support "top software buttons"
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remain enabled.
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@section disable-while-typing Disable-while-typing
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libinput automatically disables the touchpad for a timeout after a key
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press, a feature traditionally referred to as "disable while typing" and
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previously available through the
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[syndaemon(1)](http://linux.die.net/man/1/syndaemon) command. libinput does
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not require an external command and the feature is currently enabled for all
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touchpads but will be reduced in the future to only apply to touchpads where
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finger width or pressure data is unreliable.
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Notable behaviors of libinput's disable-while-typing feature:
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- Two different timeouts are used, after a single key press the timeout is
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short to ensure responsiveness. After multiple key events, the timeout is
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longer to avoid accidental pointer manipulation while typing.
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- Some keys do not trigger the timeout, specifically some modifier keys
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(Ctrl, Alt, Shift, and Fn). Actions such as Ctrl + click thus stay
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responsive.
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- Touches started while typing do not control the cursor even after typing
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has stopped, it is thus possible to rest the palm on the touchpad while
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typing.
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- Physical buttons work even while the touchpad is disabled. This includes
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@ref t440_support "software-emulated buttons".
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Disable-while-typing can be enabled and disabled by calling
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libinput_device_config_dwt_set_enabled().
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@section thumb-detection Thumb detection
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Many users rest their thumb on the touchpad while using the index finger to
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move the finger around. For clicks, often the thumb is used rather than the
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finger. The thumb should otherwise be ignored as a touch, i.e. it should not
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count towards @ref clickfinger and it should not cause a single-finger
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movement to trigger @ref twofinger_scrolling.
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libinput uses two triggers for thumb detection: pressure and
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location. A touch exceeding a pressure threshold is considered a thumb if it
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is within the thumb detection zone.
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@note "Pressure" on touchpads is synonymous with "contact area", a large
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touch surface area has a higher pressure and thus hints at a thumb or palm
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touching the surface.
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Pressure readings are unreliable at the far bottom of the touchpad as a
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thumb hanging mostly off the touchpad will have a small surface area.
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libinput has a definitive thumb zone where any touch is considered a resting
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thumb.
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@image html thumb-detection.svg
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The picture above shows the two detection areas. In the larger (light red)
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area, a touch is labelled as thumb when it exceeds a device-specific
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pressure threshold. In the lower (dark red) area, a touch is labelled as
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thumb if it remains in that area for a time without moving outside.
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*/
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