Lifetime of a gesture¶
A gesture starts when the finger position and/or finger motion is unambiguous as to what gesture to trigger and continues until the first finger belonging to this gesture is lifted.
A single gesture cannot change the finger count. For example, if a user puts down a fourth finger during a three-finger swipe gesture, libinput will end the three-finger gesture and, if applicable, start a four-finger swipe gesture. A caller may however decide that those gestures are semantically identical and continue the two gestures as one single gesture.
Pinch gestures are executed when two or more fingers are located on the touchpad and are either changing the relative distance to each other (pinching) or are changing the relative angle (rotate). Pinch gestures may change both rotation and distance at the same time. For such gestures, libinput calculates a logical center for the gestures and provides the caller with the delta x/y coordinates of that center, the relative angle of the fingers compared to the previous event, and the absolute scale compared to the initial finger position.
The illustration above shows a basic pinch in the left image and a rotate in the right angle. Not shown is a movement of the logical center if the fingers move unevenly. Such a movement is supported by libinput, it is merely left out of the illustration.
Note that while position and angle is relative to the previous event, the scale is always absolute and a multiplier of the initial finger position’s scale.
Swipe gestures are executed when three or more fingers are moved synchronously in the same direction. libinput provides x and y coordinates in the gesture and thus allows swipe gestures in any direction, including the tracing of complex paths. It is up to the caller to interpret the gesture into an action or limit a gesture to specific directions only.
The illustration above shows a vertical three-finger swipe. The coordinates provided during the gesture are the movements of the logical center.
Touchscreen gestures are not interpreted by libinput. Rather, any touch point is passed to the caller and any interpretation of gestures is up to the caller or, eventually, the X or Wayland client.
Interpreting gestures on a touchscreen requires context that libinput does not have, such as the location of windows and other virtual objects on the screen as well as the context of those virtual objects:
In the above example, the finger movements are identical but in the left case both fingers are located within the same window, thus suggesting an attempt to zoom. In the right case both fingers are located on a window border, thus suggesting a window movement. libinput has no knowledge of the window coordinates and thus cannot differentiate the two.
Gestures on two-finger touchpads¶
As of kernel 4.2, many Partial multi-touch touchpads provide only two slots. This affects how gestures can be interpreted. Touchpads with only two slots can identify two touches by position but can usually tell that there is a third (or fourth) finger down on the touchpad - without providing positional information for that finger.
Touchpoints are assigned in sequential order and only the first two touch points are trackable. For libinput this produces an ambiguity where it is impossible to detect whether a gesture is a pinch gesture or a swipe gesture whenever a user puts the index and middle finger down first. Since the third finger does not have positional information, it’s location cannot be determined.
The image above illustrates this ambiguity. The index and middle finger are set down first, the data stream from both finger positions looks identical. In this case, libinput assumes the fingers are in a horizontal arrangement (the right image above) and use a swipe gesture.