From 849369d6c66d3054688672f97d31fceb8e8230fb Mon Sep 17 00:00:00 2001 From: root Date: Fri, 25 Dec 2015 04:40:36 +0000 Subject: initial_commit --- Documentation/input/multi-touch-protocol.txt | 333 +++++++++++++++++++++++++++ 1 file changed, 333 insertions(+) create mode 100644 Documentation/input/multi-touch-protocol.txt (limited to '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 00000000..71536e78 --- /dev/null +++ b/Documentation/input/multi-touch-protocol.txt @@ -0,0 +1,333 @@ +Multi-touch (MT) Protocol +------------------------- + Copyright (C) 2009-2010 Henrik Rydberg + + +Introduction +------------ + +In order to utilize the full power of the new multi-touch and multi-user +devices, a way to report detailed data from multiple contacts, i.e., +objects in direct contact with the device surface, is needed. This +document describes the multi-touch (MT) protocol which allows kernel +drivers to report details for an arbitrary number of contacts. + +The protocol is divided into two types, depending on the capabilities of the +hardware. For devices handling anonymous contacts (type A), the protocol +describes how to send the raw data for all contacts to the receiver. For +devices capable of tracking identifiable contacts (type B), the protocol +describes how to send updates for individual contacts via event slots. + + +Protocol Usage +-------------- + +Contact details are sent sequentially as separate packets of ABS_MT +events. Only the ABS_MT events are recognized as part of a contact +packet. Since these events are ignored by current single-touch (ST) +applications, the MT protocol can be implemented on top of the ST protocol +in an existing driver. + +Drivers for type A devices separate contact packets by calling +input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT +event, which instructs the receiver to accept the data for the current +contact and prepare to receive another. + +Drivers for type B devices separate contact packets by calling +input_mt_slot(), with a slot as argument, at the beginning of each packet. +This generates an ABS_MT_SLOT event, which instructs the receiver to +prepare for updates of the given slot. + +All drivers mark the end of a multi-touch transfer by calling the usual +input_sync() function. This instructs the receiver to act upon events +accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set +of events/packets. + +The main difference between the stateless type A protocol and the stateful +type B slot protocol lies in the usage of identifiable contacts to reduce +the amount of data sent to userspace. The slot protocol requires the use of +the ABS_MT_TRACKING_ID, either provided by the hardware or computed from +the raw data [5]. + +For type A devices, the kernel driver should generate an arbitrary +enumeration of the full set of anonymous contacts currently on the +surface. The order in which the packets appear in the event stream is not +important. Event filtering and finger tracking is left to user space [3]. + +For type B devices, the kernel driver should associate a slot with each +identified contact, and use that slot to propagate changes for the contact. +Creation, replacement and destruction of contacts is achieved by modifying +the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id +is interpreted as a contact, and the value -1 denotes an unused slot. A +tracking id not previously present is considered new, and a tracking id no +longer present is considered removed. Since only changes are propagated, +the full state of each initiated contact has to reside in the receiving +end. Upon receiving an MT event, one simply updates the appropriate +attribute of the current slot. + + +Protocol Example A +------------------ + +Here is what a minimal event sequence for a two-contact touch would look +like for a type A device: + + ABS_MT_POSITION_X x[0] + ABS_MT_POSITION_Y y[0] + SYN_MT_REPORT + ABS_MT_POSITION_X x[1] + ABS_MT_POSITION_Y y[1] + SYN_MT_REPORT + SYN_REPORT + +The sequence after moving one of the contacts looks exactly the same; the +raw data for all present contacts are sent between every synchronization +with SYN_REPORT. + +Here is the sequence after lifting the first contact: + + ABS_MT_POSITION_X x[1] + ABS_MT_POSITION_Y y[1] + SYN_MT_REPORT + SYN_REPORT + +And here is the sequence after lifting the second contact: + + SYN_MT_REPORT + SYN_REPORT + +If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the +ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the +last SYN_REPORT will be dropped by the input core, resulting in no +zero-contact event reaching userland. + + +Protocol Example B +------------------ + +Here is what a minimal event sequence for a two-contact touch would look +like for a type B device: + + ABS_MT_SLOT 0 + ABS_MT_TRACKING_ID 45 + ABS_MT_POSITION_X x[0] + ABS_MT_POSITION_Y y[0] + ABS_MT_SLOT 1 + ABS_MT_TRACKING_ID 46 + ABS_MT_POSITION_X x[1] + ABS_MT_POSITION_Y y[1] + SYN_REPORT + +Here is the sequence after moving contact 45 in the x direction: + + ABS_MT_SLOT 0 + ABS_MT_POSITION_X x[0] + SYN_REPORT + +Here is the sequence after lifting the contact in slot 0: + + ABS_MT_TRACKING_ID -1 + SYN_REPORT + +The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The +message removes the association of slot 0 with contact 45, thereby +destroying contact 45 and freeing slot 0 to be reused for another contact. + +Finally, here is the sequence after lifting the second contact: + + ABS_MT_SLOT 1 + ABS_MT_TRACKING_ID -1 + SYN_REPORT + + +Event Usage +----------- + +A set of ABS_MT events with the desired properties is defined. The events +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. + +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, +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. + +For type A devices, further specification of the touch shape is possible +via ABS_MT_BLOB_ID. + +The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a +finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event +may be used to track identified contacts over time [5]. + +In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are +implicitly handled by input core; drivers should instead call +input_mt_report_slot_state(). + + +Event Semantics +--------------- + +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 an X times Y resolution, the largest +possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4]. + +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 [4]. + +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 [4]. + +ABS_MT_WIDTH_MINOR + +The length, in surface units, of the minor axis of the approaching +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]. + +ABS_MT_PRESSURE + +The pressure, in arbitrary units, on the contact area. May be used instead +of TOUCH and WIDTH for pressure-based devices or any device with a spatial +signal intensity distribution. + +ABS_MT_DISTANCE + +The distance, in surface units, between the contact and the surface. Zero +distance means the contact is touching the surface. A positive number means +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]. + +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. In such cases, the +event should be omitted. The protocol currently supports MT_TOOL_FINGER and +MT_TOOL_PEN [2]. For type B devices, this event is handled by input core; +drivers should instead use input_mt_report_slot_state(). + +ABS_MT_BLOB_ID + +The BLOB_ID groups several packets together into one arbitrarily shaped +contact. The sequence of points forms a polygon which defines the shape of +the contact. This is a low-level anonymous grouping for type A devices, and +should not be confused with the high-level trackingID [5]. Most type A +devices do not have blob capability, so drivers can safely omit this event. + +ABS_MT_TRACKING_ID + +The TRACKING_ID identifies an initiated contact throughout its life cycle +[5]. The value range of the TRACKING_ID should be large enough to ensure +unique identification of a contact maintained over an extended period of +time. For type B devices, this event is handled by input core; drivers +should instead use input_mt_report_slot_state(). + + +Event Computation +----------------- + +The flora of different hardware unavoidably leads to some devices fitting +better to the MT protocol than others. To simplify and unify the mapping, +this section gives recipes for how to compute certain events. + +For devices reporting contacts as rectangular shapes, signed orientation +cannot be obtained. Assuming X and Y are the lengths of the sides of the +touching rectangle, here is a simple formula that retains the most +information possible: + + ABS_MT_TOUCH_MAJOR := max(X, Y) + ABS_MT_TOUCH_MINOR := min(X, Y) + ABS_MT_ORIENTATION := bool(X > Y) + +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). + + +Finger Tracking +--------------- + +The process of finger tracking, i.e., to assign a unique trackingID to each +initiated contact on the surface, is a Euclidian Bipartite Matching +problem. At each event synchronization, the set of actual contacts is +matched to the set of contacts from the previous synchronization. A full +implementation can be found in [3]. + + +Gestures +-------- + +In the specific application of creating gesture events, the TOUCH and WIDTH +parameters can be used to, e.g., approximate finger pressure or distinguish +between index finger and thumb. With the addition of the MINOR parameters, +one can also distinguish between a sweeping finger and a pointing finger, +and with ORIENTATION, one can detect twisting of fingers. + + +Notes +----- + +In order to stay compatible with existing applications, the data reported +in a finger packet must not be recognized as single-touch events. + +For type A devices, all finger data bypasses input filtering, since +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. +[2] The list can of course be extended. +[3] The mtdev project: http://bitmath.org/code/mtdev/. +[4] See the section on event computation. +[5] See the section on finger tracking. -- cgit v1.2.3