Diffstat (limited to 'Documentation/input/multi-touch-protocol.txt')
1 files changed, 92 insertions, 26 deletions
diff --git a/Documentation/input/multi-touch-protocol.txt b/Documentation/input/multi-touch-protocol.txt
index 543101c..2c17961 100644
@@ -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
@@ -224,7 +246,7 @@ tool. Omit if circular .
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 .
+different characteristic widths.
@@ -240,17 +262,24 @@ the contact is hovering above the surface.
-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]
+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
+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] .
@@ -260,6 +289,23 @@ ABS_MT_POSITION_Y
The surface Y coordinate of the center of the touching ellipse.
+The surface X coordinate of the center of the approaching tool. Omit if
+the device cannot distinguish between the intended touch point and the
+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
+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 . Otherwise, the tool axes are
+aligned with the touch axes.
The type of approaching tool. A lot of kernel drivers cannot distinguish
@@ -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.
@@ -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.
- 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.
+ Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
 The list can of course be extended.
 The mtdev project: http://bitmath.org/code/mtdev/.
 See the section on event computation.