### The `evdev` backend

This backend allows using Linux `evdev` devices such as mouses, keyboards, gamepads and joysticks
as input and output devices. All buttons and axes available to the Linux system are mappable.
Output is provided by the `uinput` kernel module, which allows creation of virtual input devices.
This functionality may require elevated privileges (such as special group membership or root access).

#### Global configuration

This backend does not take any global configuration.

#### Instance configuration

| Option	| Example value		| Default value | Description						|
|---------------|-----------------------|---------------|-------------------------------------------------------|
| `device`	| `/dev/input/event1`	| none		| `evdev` device to use as input device			|
| `input`	| `Xbox Wireless`	| none		| Presentation name of evdev device to use as input (prefix-matched) |
| `output`	| `My Input Device`	| none		| Output device presentation name. Setting this option enables the instance for output	|
| `exclusive`	| `1`			| `0`		| Prevent other processes from using the device		|
| `id`		| `0x1 0x2 0x3`		| none		| Set output device bus identification (Vendor, Product and Version), optional |
| `axis.AXISNAME`| `34300 0 65536 255 4095` | none	| Specify absolute axis details (see below) for output. This is required for any absolute axis to be output.

The absolute axis details configuration (e.g. `axis.ABS_X`) is required for any absolute axis on output-enabled
instances. The configuration value contains, space-separated, the following values:

* `value`: The value to assume for the axis until an event is received
* `minimum`: The axis minimum value
* `maximum`: The axis maximum value
* `fuzz`: A value used for filtering the input stream
* `flat`: An offset, below which all deviations will be ignored
* `resolution`: Axis resolution in units per millimeter (or units per radian for rotational axes)

For real devices, all of these parameters for every axis can be found by running `evtest` on the device.

#### Channel specification

A channel is specified by its event type and event code, separated by `.`. For a complete list of event types and codes
see the [kernel documentation](https://www.kernel.org/doc/html/v4.12/input/event-codes.html). The most interesting event types are

* `EV_KEY` for keys and buttons
* `EV_ABS` for absolute axes (such as Joysticks)
* `EV_REL` for relative axes (such as Mouses)

The `evtest` tool is useful to gather information on devices active on the local system, including names, types, codes
and configuration supported by these devices.

Example mapping:
```
ev1.EV_KEY.KEY_A > ev1.EV_ABS.ABS_X
```

Note that to map an absolute axis on an output-enabled instance, additional information such as the axis minimum
and maximum are required. These must be specified in the instance configuration. When only mapping the instance
as a channel input, this is not required.

#### Known bugs / problems

Creating an `evdev` output device requires elevated privileges, namely, write access to the system's
`/dev/uinput`. Usually, this is granted for users in the `input` group and the `root` user.

Input devices may synchronize logically connected event types (for example, X and Y axes) via `EV_SYN`-type
events. The MIDIMonster also generates these events after processing channel events, but may not keep the original
event grouping.

Relative axes (`EV_REL`-type events), such as generated by mouses, are currently handled in a very basic fashion,
generating only the normalized channel values of `0`, `0.5` and `1` for any input less than, equal to and greater
than `0`, respectively. As for output, only the values `-1`, `0` and `1` are generated for the same interval.

`EV_KEY` key-down events are sent for normalized channel values over `0.9`.

Extended event type values such as `EV_LED`, `EV_SND`, etc are recognized in the MIDIMonster configuration file
but may or may not work with the internal channel mapping and normalization code.