This page provides an outline of touchpad devices. Touchpads aren’t simply categorised into a single type, instead they have a set of properties, a combination of number of physical buttons, multitouch support abilities and other properties.
Virtually all touchpads available now can detect multiple fingers on the touchpad, i.e. provide information on how many fingers are on the touchpad. The touch capabilities described here specify how many fingers a device can track, i.e. provide reliable positional information for. In the kernel each finger is tracked in a so-called “slot”, the number of slots thus equals the number of simultaneous touches a device can track.
Single-finger touchpads can track a single touchpoint. Most single-touch touchpads can also detect three fingers on the touchpad, but no positional information is provided for those. In libinput, these touches are termed “fake touches”. The kernel sends BTN_TOOL_DOUBLETAP, BTN_TOOL_TRIPLETAP, BTN_TOOL_QUADTAP and BTN_TOOL_QUINTTAP events when multiple fingers are detected.
Pure multi-touch touchpads
Pure multi-touch touchpads are those that can track, i.e. identify the location of all fingers on the touchpad. Apple’s touchpads support 16 touches, others support 5 touches like the Synaptics touchpads when using SMBus.
These touchpads usually also provide extra information. Apple touchpads provide an ellipse and the orientation of the ellipse for each touch point. Other touchpads provide a pressure value for each touch point (see Touchpads pressure handling).
Note that the kernel sends BTN_TOOL_DOUBLETAP, BTN_TOOL_TRIPLETAP, BTN_TOOL_QUADTAP and BTN_TOOL_QUINTTAP events for all touches for backwards compatibility. libinput ignores these events if the touchpad can track touches correctly.
Partial multi-touch touchpads
The vast majority of touchpads fall into this category, the half-way point between single-touch and pure multi-touch. These devices can track N fingers, but detect more than N. For example, when using the serial protocol, Synaptics touchpads can track two fingers but may detect up to five.
The number of slots may limit which features are available in libinput. Any device with two slots can support two-finger scrolling, but Thumb detection or Palm detection may be limited if only two slots are available.
A sub-class of partial multi-touch touchpads. These touchpads can technically detect two fingers but the location of both is limited to the bounding box, i.e. the first touch is always the top-left one and the second touch is the bottom-right one. Coordinates jump around as fingers move past each other.
Many semi-mt touchpads also have a lower resolution for the second touch, or both touches. This may limit some features such as Gestures or Scrolling.
Semi-mt are labelled by the kernel with the INPUT_PROP_SEMI_MT input property.
Other touchpad properties
External touchpads are USB or Bluetooth touchpads not in a laptop chassis, e.g. Apple Magic Trackpad or the Logitech T650. These are usually Clickpads the biggest difference is that they can be removed or added at runtime.
One interaction method that is only possible on external touchpads is a thumb resting on the very edge/immediately next to the touchpad. On the far edge, touchpads don’t always detect the finger location so clicking with a thumb barely touching the edge makes it hard or impossible to figure out which software button area the finger is on.
These touchpads also don’t need Palm detection - since they’re not located underneath the keyboard, accidental palm touches are a non-issue.
Touchpads pressure handling
Pressure is usually directly related to contact area. Human fingers flatten out as the pressure on the pad increases, resulting in a bigger contact area and the firmware then calculates that back into a pressure reading.
libinput uses pressure to detect accidental palm contact and thumbs, though pressure data is often device-specific and unreliable.
Only listed for completeness, circular touchpads have not been used in laptops for a number of years. These touchpad shaped in an ellipse or straight.
Touch-capable graphics tablets are effectively external touchpads, with two differentiators: they are larger than normal touchpads and they have no regular touchpad buttons. They either work like a Forcepads Forcepad, or rely on interaction methods that don’t require buttons (like Tap-to-click behaviour). Since the physical device is shared with the pen input, some touch arbitration is required to avoid touch input interfering when the pen is in use.
Dedicated edge scroll area
Before Two-finger scrolling became the default scroll method, some touchpads provided a marking on the touch area that designates the edge to be used for scrolling. A finger movement in that edge zone should trigger vertical motions. Some touchpads had markers for a horizontal scroll area too at the bottom of the touchpad.