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Diffstat (limited to 'Documentation/input/multi-touch-protocol.txt')
| -rw-r--r-- | Documentation/input/multi-touch-protocol.txt | 118 |
1 files changed, 92 insertions, 26 deletions
diff --git a/Documentation/input/multi-touch-protocol.txt b/Documentation/input/multi-touch-protocol.txt index 543101c5bf26..2c179613f81b 100644 --- a/Documentation/input/multi-touch-protocol.txt +++ b/Documentation/input/multi-touch-protocol.txt @@ -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. @@ -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 @@ -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 @@ -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 @@ -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 --------------- @@ -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. |