Touchpad pressure-based touch detection
libinput uses the touchpad pressure values and/or touch size values to detect whether a finger has been placed on the touchpad. This is Information provided by the kernel and combines with a libinput-specific hardware database to adjust the thresholds on a per-device basis. libinput uses these thresholds primarily to filter out accidental light touches but the information is also used for some Palm detection.
Most devices only support one of either touch pressure or touch size. libinput uses whichever is available but a preference is given to touch size as it provides more specific information. Since most devices only provide one type anyway, this internal preference does not usually matter.
Pressure and touch size thresholds are not directly configurable by the user. Instead, libinput provides these thresholds for each device where necessary. See Debugging touchpad pressure ranges for instructions on how to adjust the pressure ranges and Debugging touch size ranges for instructions on how to adjust the touch size ranges.
Information provided by the kernel
The kernel sends multiple values to inform userspace about a finger touching
the touchpad. The most basic is the
EV_KEY/BTN_TOUCH boolean event
that simply announces physical contact with the touchpad. The decision when
this event is sent is usually made by the kernel driver and may depend on
device-specific thresholds. These thresholds are transparent to userspace
and cannot be modified. On touchpads where pressure or touch size is not
available, libinput uses
BTN_TOUCH to determine when a finger is
Many contemporary touchpad devices provide an absolute pressure axis in
BTN_TOUCH. This pressure generally increases as the pressure
increases, however few touchpads are capable of detecting true pressure. The
pressure value is usually related to the covered area - as the pressure
increases a finger flattens and thus covers a larger area. The range
provided by the kernel is not mapped to a specific physical range and
often requires adjustment. Pressure is sent by the
for single-touch touchpads or
ABS_MT_PRESSURE on multi-touch capable
touchpads. Some devices can detect multiple fingers but only provide
Some devices provide additional touch size information through
ABS_MT_TOUCH_MAJOR/ABS_MT_TOUCH_MINOR axes and/or
ABS_MT_WIDTH_MAJOR/ABS_MT_WIDTH_MINOR axes. These axes specifcy
the size of the touch ellipse. While the kernel documentation specifies how
these axes are supposed to be mapped, few devices forward reliable
information. libinput uses these values together with a device-specific
Device quirks entry. In other words, touch size detection does not work
unless a device quirk is present for the device.