libinput: Normalization of relative motion

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Normalization of relative motion

Most relative input devices generate input in so-called "mickeys".

A mickey is in device-specific units that depend on the resolution of the sensor. Most optical mice use sensors with 1000dpi resolution, but some devices range from 100dpi to well above 8000dpi.

Without a physical reference point, a relative coordinate cannot be interpreted correctly. A delta of 10 mickeys may be a millimeter of physical movement or 10 millimeters, depending on the sensor. This affects pointer acceleration in libinput and interpretation of relative coordinates in callers.

libinput does partial normalization of relative input. For devices with a resolution of 1000dpi and higher, motion events are normalized to a default of 1000dpi before pointer acceleration is applied. As a result, devices with 1000dpi and above feel the same.

Devices below 1000dpi are not normalized (normalization of a 1-device unit movement on a 400dpi mouse would cause a 2.5 pixel movement). Instead, libinput applies a dpi-dependent acceleration function. At low speeds, a 1-device unit movement usually translates into a 1-pixel movements. As the movement speed increases, acceleration is applied - at high speeds a low-dpi device will roughly feel the same as a higher-dpi mouse.

This normalization only applies to accelerated coordinates, unaccelerated coordinates are left in device-units. It is up to the caller to interpret those coordinates correctly.

# Normalization of touchpad coordinates

Touchpads may have a different resolution for the horizontal and vertical axis. Interpreting coordinates from the touchpad without taking resolution into account results in uneven motion.

libinput scales unaccelerated touchpad motion to the resolution of the touchpad's x axis, i.e. the unaccelerated value for the y axis is: y = (x / resolution_x) * resolution_y

# Normalization of tablet coordinates

See Relative motion for tablet tools

# Setting custom DPI settings

Devices usually do not advertise their resolution and libinput relies on the udev property MOUSE_DPI for this information. This property is usually set via the udev hwdb. The "mouse-dpi-tool" utility provided by libevdev should be used to measure a device's resolution.

The format of the property for single-resolution mice is:

MOUSE_DPI=resolution@frequency

The resolution is in dots per inch, the frequency in Hz. The format of the property for multi-resolution mice may list multiple resolutions and frequencies:

MOUSE_DPI=r1@f1 *r2@f2 r3@f3

The default frequency must be pre-fixed with an asterisk.

For example, these two properties are valid:

MOUSE_DPI=800@125
MOUSE_DPI=400@125 800@125 *1000@500 5500@500

The behavior for a malformed property is undefined.

If the property is unset, libinput assumes the resolution is 1000dpi.

Note that HW does not usually provide information about run-time resolution changes, libinput will thus not detect when a resolution changes to the non-default value.