libinput/doc/normalization-of-relative-motion.dox

/**
@page motion_normalization 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 normalizes all relative input to a physical resolution of
1000dpi, the same delta from two different devices thus represents the
same physical movement of those two devices (within sensor error
margins).

Devices usually do not advertise their resolution and libinput relies on
the udev property MOUSE_DPI for this information.

The format of the property for single-resolution mice is:
@code
     MOUSE_DPI=resolution@frequency
@endcode

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:
@code
     MOUSE_DPI=r1@f1 *r2@f2 r3@f3
@endcode

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

For example, these two properties are valid:
@code
     MOUSE_DPI=800@125
     MOUSE_DPI=400@125 800@125 *1000@500 5500@500
@endcode

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.

*/