This patchs adds documentation for the multi-touch protocol to Documentation/input/. Signed-off-by: Henrik Rydberg <rydberg@xxxxxxxxxxx> --- Documentation/input/multi-touch-protocol.txt | 140 ++++++++++++++++++++++++++ 1 files changed, 140 insertions(+), 0 deletions(-) create mode 100644 Documentation/input/multi-touch-protocol.txt diff --git a/Documentation/input/multi-touch-protocol.txt b/Documentation/input/multi-touch-protocol.txt new file mode 100644 index 0000000..2e99cd6 --- /dev/null +++ b/Documentation/input/multi-touch-protocol.txt @@ -0,0 +1,140 @@ +Multi-touch (MT) Protocol +------------------------- + Copyright (C) 2009 Henrik Rydberg <rydberg@xxxxxxxxxxx> + + +Introduction +------------ + +In order to utilize the full power of the new multi-touch devices, a way to +report detailed finger data to user space is needed. This document +describes the multi-touch (MT) protocol which allows kernel drivers to +report details for an arbitrary number of fingers. + + +Usage +----- + +Anonymous finger details are sent sequentially as separate packets of ABS +events. Only the ABS_MT events are recognized as part of a finger +packet. The end of a packet is marked by calling the input_mt_sync() +function, which generates a SYN_MT_REPORT event. The end of multi-touch +transfer is marked by calling the usual input_sync() function. + +A set of ABS_MT events with the desired properties are defined. The events +are divided into categories, to allow for partial implementation. The +minimum set consists of ABS_MT_TOUCH_MAJOR, ABS_MT_POSITION_X and +ABS_MT_POSITION_Y, which allows for multiple fingers to be tracked. If the +device supports it, the ABS_MT_WIDTH_MAJOR may be used to provide the size +of the approaching finger. Anisotropy and direction may be specified with +ABS_MT_TOUCH_MINOR, ABS_MT_WIDTH_MINOR and ABS_MT_ORIENTATION. Devices with +more granular information may specify general shapes as blobs, i.e., as a +sequence of rectangular shapes grouped together by a +ABS_MT_BLOB_ID. Finally, the ABS_MT_TOOL_TYPE may be used to specify +whether the touching tool is a finger or a pen or something else. + + +Event Semantics +--------------- + +The word "contact" is used to describe a tool which is in direct contact +with the surface. A finger, a pen or a rubber all classify as contacts. + +ABS_MT_TOUCH_MAJOR + +The length of the major axis of the contact. The length should be given in +surface units. If the surface has a X time Y resolution, the largest +possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diameter. + +ABS_MT_TOUCH_MINOR + +The length, in surface units, of the minor axis of the contact. If the +contact is circular, this event can be omitted. + +ABS_MT_WIDTH_MAJOR + +The length, in surface units, of the major axis of the approaching +tool. This should be understood as the size of the tool itself. The +orientation of the contact and the approaching tool are assumed to be the +same. + +ABS_MT_WIDTH_MINOR + +The length, in surface units, of the minor axis of the approaching +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 [1]. + +ABS_MT_ORIENTATION + +The orientation of the ellipse. The value should describe half a revolution +clockwise around the touch center. The scale of the value is arbitrary, but +zero should be returned for an ellipse aligned along the Y axis of the +surface. As an example, an index finger placed straight onto the axis could +return zero orientation, something negative when twisted to the left, and +something positive when twisted to the right. This value can be omitted if +the touching object is circular, or if the information is not available in +the kernel driver. + +ABS_MT_POSITION_X + +The surface X coordinate of the center of the touching ellipse. + +ABS_MT_POSITION_Y + +The surface Y coordinate of the center of the touching ellipse. + +ABS_MT_TOOL_TYPE + +The type of approaching tool. A lot of kernel drivers cannot distinguish +between different tool types, such as a finger or a pen. I such cases, the +event should be omitted. The protocol currently supports MT_TOOL_FINGER and +MT_TOOL_PEN [2]. + +ABS_MT_BLOB_ID + +The BLOB_ID groups several packets together into one arbitrarily shaped +contact. This is a low-level anonymous grouping, and should not be confused +with the high-level contactID, explained below. Most kernel drivers will +not have this capability, and can safely omit the event. + + +Finger Tracking +--------------- + +The kernel driver should generate an arbitrary enumeration of the set of +anonymous contacts currently on the surface. The order in which the packets +appear in the event stream is not important. + +The process of finger tracking, i.e., to assign a unique contactID to each +initiated contact on the surface, is left to user space; preferably the +multi-touch X driver [3]. In that driver, the contactID stays the same and +unique until the contact vanishes (when the finger leaves the surface). The +problem of assigning a set of anonymous fingers to a set of identified +fingers is a euclidian bipartite matching problem at each event update, and +relies on a sufficiently rapid update rate. + +Notes +----- + +In order to stay compatible with existing applications, the data +reported in a finger packet must not be recognized as single-touch +events. In addition, all finger data must bypass input filtering, +since subsequent events of the same type refer to different fingers. + +The first kernel driver to utilize the MT protocol is the bcm5974 driver, +where examples can be found. + +[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. +[2] The list can of course be extended. +[3] The multi-touch X driver is currently in the prototyping stage. At the +time of writing (April 2009), the MT protocol is not yet merged, and the +prototype implements finger matching, basic mouse support and two-finger +scrolling. The project aims at improving the quality of current multi-touch +functionality available in the synaptics X driver, and in addition +implement more advanced gestures. -- 1.5.6.3 -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
