libinput: Pointer acceleration

Pointer acceleration

libinput uses device-specific pointer acceleration methods, with the default being the Linear pointer acceleration.

The methods share common properties, such as Velocity calculation.

This page explains the high-level concepts used in the code. It aims to provide an overview for developers and is not necessarily useful for users.

# Pointer acceleration profiles

The profile decides the general method of pointer acceleration. libinput currently supports two profiles: "adaptive" and "flat". The aptive profile is the default profile for all devices and takes the current speed of the device into account when deciding on acceleration. The flat profile is simply a constant factor applied to all device deltas, regardless of the speed of motion (see The flat pointer acceleration profile). Most of this document describes the adaptive pointer acceleration.

# Velocity calculation

The device's speed of movement is measured across multiple input events through so-called "trackers". Each event prepends a the tracker item, each subsequent tracker contains the delta of that item to the current position, the timestamp of the event that created it and the cardinal direction of the movement at the time. If a device moves into the same direction, the velocity is calculated across multiple trackers. For example, if a device moves steadily for 10 events to the left, the velocity is calculated across all 10 events.

Whenever the movement changes direction or significantly changes speed, the velocity is calculated from the direction/speed change only. For example, if a device moves steadily for 8 events to the left and then 2 events to the right, the velocity is only that of the last 2 events.

An extra time limit prevents events that are too old to factor into the velocity calculation. For example, if a device moves steadily for 5 events to the left, then pauses, then moves again for 5 events to the left, only the last 5 events are used for velocity calculation.

The velocity is then used to calculate the acceleration factor

# Acceleration factor

The acceleration factor is the final outcome of the pointer acceleration calculations. It is a unitless factor that is applied to the current delta, a factor of 2 doubles the delta (i.e. speeds up the movement), a factor of less than 1 reduces the delta (i.e. slows the movement).

Any factor less than 1 requires the user to move the device further to move the visible pointer. This is called deceleration and enables high precision target selection through subpixel movements. libinput's current maximum deceleration factor is 0.3 (i.e. slow down to 30% of the pointer speed).

A factor higher than 1 moves the pointer further than the physical device moves. This is acceleration and allows a user to cross the screen quickly but effectively skips pixels. libinput's current maximum acceleration factor is 3.5.

# Linear pointer acceleration

The linear pointer acceleration method is the default for most pointer devices. It provides deceleration at very slow movements, a 1:1 mapping for regular movements and a linear increase to the maximum acceleration factor for fast movements.

Linear pointer acceleration applies to devices with above 1000dpi resolution and after Normalization of relative motion is applied.

Linear pointer acceleration

The image above shows the linear pointer acceleration settings at various speeds. The line for 0.0 is the default acceleration curve, speed settings above 0.0 accelerate sooner, faster and to a higher maximum acceleration. Speed settings below 0 delay when acceleration kicks in, how soon the maximum acceleration is reached and the maximum acceleration factor.

Extremely low speed settings provide no acceleration and additionally decelerate all movement by a constant factor.

# Pointer acceleration for low-dpi devices

Low-dpi devices are those with a physical resolution of less than 1000 dots per inch (dpi). The pointer acceleration is adjusted to provide roughly the same feel for all devices at normal to high speeds. At slow speeds, the pointer acceleration works on device-units rather than normalized coordinates (see Normalization of relative motion).

Pointer acceleration for low-dpi devices

The image above shows the default pointer acceleration curve for a speed of 0.0 at different DPI settings. A device with low DPI has the acceleration applied sooner and with a stronger acceleration factor.

# Pointer acceleration on touchpads

Touchpad pointer acceleration uses the Linear pointer acceleration profile, with a constant deceleration factor applied. The user expectation of how much a pointer should move in response to finger movement is different to that of a mouse device, hence the constant deceleration factor.

Pointer acceleration curve for touchpads

The image above shows the touchpad acceleration profile in comparison to the Linear pointer acceleration. The shape of the curve is identical but vertically squashed.

# Pointer acceleration on trackpoints

Trackpoint hardware is quite varied in how it reacts to user pressure and unlike other devices it cannot easily be normalized for physical properties. Measuring pressure objectively across a variety of hardware is nontrivial. libinput relies on some sytem-wide configured properties, specifically the Static device configuration via udev. The two properties that influence trackpoint acceleration `POINTINGSTICK_CONST_ACCEL` which is a constant factor applied to the trackpoint's motion data. For example, a factor of 3 will see all input data multipled by 3. This multiplication is done by libinput.

Additionally, some trackpoints provide the ability to adjust the sensitivity in hardware by modifying a sysfs file on the serio node. The udev property `POINTINGSTICK_SENSITIVITY` indicates the desired value, a udev builtin is expected to apply this to the device, i.e. libinput does not handle this property. Once applied, the sensitivity adjusts the deltas coming out of the hardware.

Trackpoint pointer acceleration uses the Pointer acceleration for low-dpi devices profile, with a constant acceleration factor taking the place of the DPI settings.

Pointer acceleration curves for trackpoints

The image above shows the trackpoint acceleration profile in comparison to the Linear pointer acceleration.

The constant acceleration factor, usually applied in Static device configuration via udev, shapes the acceleration profile and is pre-applied before any user-specific acceleration adjustments.

# The flat pointer acceleration profile

In a flat profile, the acceleration factor is constant regardless of the velocity of the pointer and each delta (dx, dy) results in an accelerated delta (dx * factor, dy * factor). This provides 1:1 movement between the device and the pointer on-screen.

# Pointer acceleration on tablets

Pointer acceleration for relative motion on tablet devices is a flat acceleration, with the speed seeting slowing down or speeding up the pointer motion by a constant factor. Tablets do not allow for switchable profiles.