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Input: MT - Include win8 support
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The newly released HID protocol for win8 multitouch devices is capable
of transmitting more information about each touch. In particular, it
includes details useful for touch alignment. This patch completes the
MT protocol with the ABS_MT_TOOL_X/Y events, and documents how to map
win8 devices.

Cc: Stephane Chatty <chatty@enac.fr>
Cc: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Cc: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Chase Douglas <chase.douglas@canonical.com>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
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Henrik Rydberg committed Jul 5, 2012
1 parent fdf8042 commit cab7fac
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118 changes: 92 additions & 26 deletions Documentation/input/multi-touch-protocol.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -162,26 +162,48 @@ are divided into categories, to allow for partial implementation. The
minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
allows for multiple contacts to be tracked. If the device supports it, the
ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
of the contact area and approaching contact, respectively.
of the contact area and approaching tool, respectively.

The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
looking through a window at someone gently holding a finger against the
glass. You will see two regions, one inner region consisting of the part
of the finger actually touching the glass, and one outer region formed by
the perimeter of the finger. The diameter of the inner region is the
ABS_MT_TOUCH_MAJOR, the diameter of the outer region is
ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger harder
against the glass. The inner region will increase, and in general, the
ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than
unity, is related to the contact pressure. For pressure-based devices,
the perimeter of the finger. The center of the touching region (a) is
ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
harder against the glass. The touch region will increase, and in general,
the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
than unity, is related to the contact pressure. For pressure-based devices,
ABS_MT_PRESSURE may be used to provide the pressure on the contact area
instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
indicate the distance between the contact and the surface.

In addition to the MAJOR parameters, the oval shape of the contact can be
described by adding the MINOR parameters, such that MAJOR and MINOR are the
major and minor axis of an ellipse. Finally, the orientation of the oval
shape can be describe with the ORIENTATION parameter.

Linux MT Win8
__________ _______________________
/ \ | |
/ \ | |
/ ____ \ | |
/ / \ \ | |
\ \ a \ \ | a |
\ \____/ \ | |
\ \ | |
\ b \ | b |
\ \ | |
\ \ | |
\ \ | |
\ / | |
\ / | |
\ / | |
\__________/ |_______________________|


In addition to the MAJOR parameters, the oval shape of the touch and finger
regions can be described by adding the MINOR parameters, such that MAJOR
and MINOR are the major and minor axis of an ellipse. The orientation of
the touch ellipse can be described with the ORIENTATION parameter, and the
direction of the finger ellipse is given by the vector (a - b).

For type A devices, further specification of the touch shape is possible
via ABS_MT_BLOB_ID.
Expand Down Expand Up @@ -224,7 +246,7 @@ tool. Omit if circular [4].
The above four values can be used to derive additional information about
the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
the notion of pressure. The fingers of the hand and the palm all have
different characteristic widths [1].
different characteristic widths.

ABS_MT_PRESSURE

Expand All @@ -240,17 +262,24 @@ the contact is hovering above the surface.

ABS_MT_ORIENTATION

The orientation of the ellipse. The value should describe a signed quarter
of a revolution clockwise around the touch center. The signed value range
is arbitrary, but zero should be returned for a finger aligned along the Y
axis of the surface, a negative value when finger is turned to the left, and
a positive value when finger turned to the right. When completely aligned with
the X axis, the range max should be returned. Orientation can be omitted
if the touching object is circular, or if the information is not available
in the kernel driver. Partial orientation support is possible if the device
can distinguish between the two axis, but not (uniquely) any values in
between. In such cases, the range of ABS_MT_ORIENTATION should be [0, 1]
[4].
The orientation of the touching ellipse. The value should describe a signed
quarter of a revolution clockwise around the touch center. The signed value
range is arbitrary, but zero should be returned for an ellipse aligned with
the Y axis of the surface, a negative value when the ellipse is turned to
the left, and a positive value when the ellipse is turned to the
right. When completely aligned with the X axis, the range max should be
returned.

Touch ellipsis are symmetrical by default. For devices capable of true 360
degree orientation, the reported orientation must exceed the range max to
indicate more than a quarter of a revolution. For an upside-down finger,
range max * 2 should be returned.

Orientation can be omitted if the touch area is circular, or if the
information is not available in the kernel driver. Partial orientation
support is possible if the device can distinguish between the two axis, but
not (uniquely) any values in between. In such cases, the range of
ABS_MT_ORIENTATION should be [0, 1] [4].

ABS_MT_POSITION_X

Expand All @@ -260,6 +289,23 @@ ABS_MT_POSITION_Y

The surface Y coordinate of the center of the touching ellipse.

ABS_MT_TOOL_X

The surface X coordinate of the center of the approaching tool. Omit if
the device cannot distinguish between the intended touch point and the
tool itself.

ABS_MT_TOOL_Y

The surface Y coordinate of the center of the approaching tool. Omit if the
device cannot distinguish between the intended touch point and the tool
itself.

The four position values can be used to separate the position of the touch
from the position of the tool. If both positions are present, the major
tool axis points towards the touch point [1]. Otherwise, the tool axes are
aligned with the touch axes.

ABS_MT_TOOL_TYPE

The type of approaching tool. A lot of kernel drivers cannot distinguish
Expand Down Expand Up @@ -305,6 +351,28 @@ The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
the device can distinguish between a finger along the Y axis (0) and a
finger along the X axis (1).

For win8 devices with both T and C coordinates, the position mapping is

ABS_MT_POSITION_X := T_X
ABS_MT_POSITION_Y := T_Y
ABS_MT_TOOL_X := C_X
ABS_MT_TOOL_X := C_Y

Unfortunately, there is not enough information to specify both the touching
ellipse and the tool ellipse, so one has to resort to approximations. One
simple scheme, which is compatible with earlier usage, is:

ABS_MT_TOUCH_MAJOR := min(X, Y)
ABS_MT_TOUCH_MINOR := <not used>
ABS_MT_ORIENTATION := <not used>
ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
ABS_MT_WIDTH_MINOR := min(X, Y)

Rationale: We have no information about the orientation of the touching
ellipse, so approximate it with an inscribed circle instead. The tool
ellipse should align with the the vector (T - C), so the diameter must
increase with distance(T, C). Finally, assume that the touch diameter is
equal to the tool thickness, and we arrive at the formulas above.

Finger Tracking
---------------
Expand Down Expand Up @@ -338,9 +406,7 @@ subsequent events of the same type refer to different fingers.
For example usage of the type A protocol, see the bcm5974 driver. For
example usage of the type B protocol, see the hid-egalax driver.

[1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the
difference between the contact position and the approaching tool position
could be used to derive tilt.
[1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
[2] The list can of course be extended.
[3] The mtdev project: http://bitmath.org/code/mtdev/.
[4] See the section on event computation.
Expand Down
8 changes: 5 additions & 3 deletions include/linux/input.h
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Original file line number Diff line number Diff line change
Expand Up @@ -806,18 +806,20 @@ struct input_keymap_entry {
#define ABS_MT_WIDTH_MAJOR 0x32 /* Major axis of approaching ellipse */
#define ABS_MT_WIDTH_MINOR 0x33 /* Minor axis (omit if circular) */
#define ABS_MT_ORIENTATION 0x34 /* Ellipse orientation */
#define ABS_MT_POSITION_X 0x35 /* Center X ellipse position */
#define ABS_MT_POSITION_Y 0x36 /* Center Y ellipse position */
#define ABS_MT_POSITION_X 0x35 /* Center X touch position */
#define ABS_MT_POSITION_Y 0x36 /* Center Y touch position */
#define ABS_MT_TOOL_TYPE 0x37 /* Type of touching device */
#define ABS_MT_BLOB_ID 0x38 /* Group a set of packets as a blob */
#define ABS_MT_TRACKING_ID 0x39 /* Unique ID of initiated contact */
#define ABS_MT_PRESSURE 0x3a /* Pressure on contact area */
#define ABS_MT_DISTANCE 0x3b /* Contact hover distance */
#define ABS_MT_TOOL_X 0x3c /* Center X tool position */
#define ABS_MT_TOOL_Y 0x3d /* Center Y tool position */

#ifdef __KERNEL__
/* Implementation details, userspace should not care about these */
#define ABS_MT_FIRST ABS_MT_TOUCH_MAJOR
#define ABS_MT_LAST ABS_MT_DISTANCE
#define ABS_MT_LAST ABS_MT_TOOL_Y
#endif

#define ABS_MAX 0x3f
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