libinput/doc/tablet-support.dox

/**
@page tablet-support Tablet support

This page provides details about the graphics tablet
support in libinput. Note that the term "tablet" in libinput refers to
graphics tablets only (e.g. Wacom Intuos), not to tablet devices like the
Apple iPad.

@image html tablet.svg "Illustration of a graphics tablet"

@section tablet-tools Tablet buttons vs. tablet tools

Most tablets provide two types of devices. The pysical tablet often provides
a number of buttons and a touch ring or strip. Interaction on the drawing
surface of the tablet requires a tool, usually in the shape of a stylus.
The libinput interface exposed by devices with the @ref
LIBINPUT_DEVICE_CAP_TABLET_TOOL applies only to events generated by tools.

Touch events on the tablet integrated into a screen itself are exposed
through the @ref LIBINPUT_DEVICE_CAP_TOUCH capability. Touch events on a
standalone tablet are exposed through the @ref LIBINPUT_DEVICE_CAP_POINTER
capability.  In both cases, the kernel usually provides a separate event
node for the touch device, resulting in a separate libinput device.
See libinput_device_get_device_group() for information on how to associate
the touch part with other devices exposed by the same physical hardware.

@section tablet-tip Tool tip events vs. button events

The primary use of a tablet tool is to draw on the surface of the tablet.
When the tool tip comes into contact with the surface, libinput sends an
event of type @ref LIBINPUT_EVENT_TABLET_TOOL_TIP, and again when the tip
ceases contact with the surface.

Tablet tools may send button events; these are exclusively for extra buttons
unrelated to the tip. A button event is independent of the tip and can while
the tip is down or up.

@section tablet-axes Special axes on tablet tools

A tablet tool usually provides additional information beyond x/y positional
information and the tip state. A tool may provide the distance to the tablet
surface and the pressure exerted on the tip when in contact. Some tablets
additionally provide tilt information along the x and y axis.

@image html tablet-axes.svg "Illustration of the distance, pressure and tilt axes"

The granularity and precision of these axes varies between tablet devices
and cannot usually be mapped into a physical unit.
libinput normalizes distance and pressure into the [0, 1] range and the tilt
axes into the [-1, 1] range with 0 as the neutral point.

While the normalization range is identical for these axes, a caller should
not interpret identical values as identical across axes, i.e. a value v1 on
the distance axis has no relation to the same value v1 on the pressure axis.

@section tablet-fake-proximity Handling of proximity events

libinput's @ref LIBINPUT_EVENT_TABLET_TOOL_PROXIMITY events notify a caller
when a tool comes into sensor range or leaves the sensor range. On some
tools this range does not represent the physical range but a reduced
tool-specific logical range. If the range is reduced, this is done
transparent to the caller.

For example, the Wacom mouse and lens cursor tools are usually
used in relative mode, lying flat on the tablet. Movement typically follows
the interaction normal mouse movements have, i.e. slightly lift the tool and
place it in a separate location. The proximity detection on Wacom
tablets however extends further than the user may lift the mouse, i.e. the
tool may not be lifted out of physical proximity. For such tools, libinput 
provides software-emulated proximity. 

@section tablet-pressure-offset Pressure offset on worn-out tools

When a tool is used for an extended period it can wear down physically. A
worn-down tool may never return a zero pressure value. Even when hovering
above the surface, the pressure value returned by the tool is nonzero,
creating a fake surface touch and making interaction with the tablet less
predictable.

libinput automatically detects pressure offsets and rescales the remaining
pressure range into the available range, making pressure-offsets transparent
to the caller. A tool with a pressure offset will thus send a 0 pressure
value for the detected offset and nonzero pressure values for values higher
than that offset.

Some limitations apply to avoid misdetection of pressure offsets,
specifically:
- pressure offset is only detected on proximity in, and if a device is
  capable of detection distances,
- pressure offset is only detected if the distance between the tool and the
  tablet is high enough,
- pressure offset is only used if it is 20% or less of the pressure range
  available to the tool. A pressure offset higher than 20% indicates either
  a misdetection or a tool that should be replaced, and
- if a pressure value less than the current pressure offset is seen, the
  offset resets to that value.

Pressure offsets are not detected on @ref LIBINPUT_TABLET_TOOL_TYPE_MOUSE
and @ref LIBINPUT_TABLET_TOOL_TYPE_LENS tools.

@section tablet-serial-numbers Tracking unique tools

Some tools provide hardware information that enables libinput to uniquely
identify the physical device. For example, tools compatible with the Wacom
Intuos 4, Intuos 5, Intuos Pro and Cintiq series are uniquely identifiable
through a serial number. libinput does not specify how a tool can be
identified uniquely, a caller should use libinput_tablet_tool_is_unique() to
check if the tool is unique.

libinput creates a struct libinput_tablet_tool on the first proximity in of
this tool. By default, this struct is destroyed on proximity out and
re-initialized on the next proximity in. If a caller keeps a reference to
the tool by using libinput_tablet_tool_ref() libinput re-uses this struct
whenever that same physical tool comes into proximity on any tablet
recognized by libinput. It is possible to attach tool-specific virtual state
to the tool. For example, a graphics program such as the GIMP may assign a
specific color to each tool, allowing the artist to use the tools like
physical pens of different color. In multi-tablet setups it is also
possible to track the tool across devices.

If the tool does not have a unique identifier, libinput creates a single
struct libinput_tablet_tool per tool type on each tablet the tool is used
on.

@section tablet-tool-types Vendor-specific tablet tool types

libinput supports a number of high-level tool types that describe the
general interaction expected with the tool. For example, a user would expect
a tool of type @ref LIBINPUT_TABLET_TOOL_TYPE_PEN to interact with a
graphics application taking pressure and tilt into account. The default
virtual tool assigned should be a drawing tool, e.g. a virtual pen or brush.
A tool of type @ref LIBINPUT_TABLET_TOOL_TYPE_ERASER would normally be
mapped to an eraser-like virtual tool. See @ref libinput_tablet_tool_type
for the list of all available tools.

Vendors may provide more fine-grained information about the tool in use by
adding a hardware-specific tool ID. libinput provides this ID to the caller
with libinput_tablet_tool_get_tool_id() but makes no promises about the
content or format of the ID.

libinput currently supports Wacom-style tool IDs as provided on the Wacom
Intuos 3, 4, 5, Wacon Cintiq and Wacom Intuos Pro series. The tool ID can
be used to distinguish between e.g. a Wacom Classic Pen or a Wacom Pro Pen.
It is  the caller's responsibility to interpret the tool ID.

*/