Thumb detection
Thumb detection tries to identify touches triggered by a thumb rather than a pointer-moving finger. This is necessary on Clickpads as a finger pressing a button always creates a new touch, causing misinterpretation of gestures. Click-and-drag with two fingers (one holding the button, one moving) would be interpreted as two-finger scrolling without working thumb detection.
libinput has built-in thumb detection, partially dependent on hardware-specific capabilities.
Thumb detection uses multiple approaches and the final decision on whether to ignore a thumb depends on the interaction at the time.
Thumb detection based on pressure or size
The simplest form of thumb detection identifies a touch as thumb when the pressure value goes above a certain threshold. This threshold is usually high enough that it cannot be triggered by a finger movement.
On touchpads that support the ABS_MT_TOUCH_MAJOR axes, libinput can perform
thumb detection based on the size of the touch ellipse. This works similar to
the pressure-based palm detection in that a touch is labelled as palm when
it exceeds the (device-specific) touch size threshold.
Pressure- and size-based thumb detection depends on the location of the thumb and usually only applies within the Thumb detection areas.
For some information on how to detect pressure on a touch and debug the pressure ranges, see Touchpad pressure-based touch detection. Pressure- and size-based thumb detection require thresholds set in the Device quirks.
Thumb detection areas
Pressure and size readings are unreliable at the far bottom of the touchpad. 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 thumb. Immediately above that area is the area where libinput will label a thumb as such if the pressure or size thresholds are exceeded.
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 always labelled as thumb.
Moving outside the areas generally releases the thumb from being a thumb.
Thumb movement based on speed
Regular interactions with thumbs do not usually move the thumb. When fingers are moving across the touchpad and a thumb is dropped, this can cause erroneous scroll motion or similar issues. libinput observes the finger motion speed for all touches - where a finger has been moving a newly dropped finger is more likely to be labeled as thumb.
Thumb detection based on finger positions
The shape of the human hand and the interactions that usually involve a thumb imply that a thumb is situated in a specific position relative to other fingers (usually to the side and below). This is used by libinput to detect thumbs during some interactions that do not implicitly require a thumb (e.g. pinch-and-rotate).