Multi-touch (MT) Protocol 小结
1， 两种多点触摸协议：
- A类： 处理无关联的接触： 用于直接发送原始数据；
- B类： 处理跟踪识别类的接触： 通过事件slot发送相关联的独立接触更新。
2， 触摸协议的使用：
A类协议：
A类协议在每发送完一个接触数据包后会调用 input_mt_sync() 声明 一次数据的结束； input_mt_sync() 会发出一个 SYN_MT_REPORT
B类协议：
与A类协议不同的是， B类在使用input_mt_slot()的时候会带有一个slot的参数，在每个数据包开始时 产生一个ABS_MT_SLOT事件，提示接收器更新数据。
最终A，B类协议均会调用 input_sync();
A类与B类协议不同的在于： B类协议 通过B类协议 slot 协议需要是用到ABS_MT_TRACKING_ID------ 可以从硬件上获取，或者从原始数据中计算。
3, B类协议：
ABS_MT_TRACKING_ID 表示一次接触； -1 代表一个不用的slot；
使用参考例子：
释放事件：
input_mt_slot(data->input_dev, i);
input_mt_report_slot_state(data->input_dev, MT_TOOL_FINGER, false);
点击事件：
input_mt_slot(data->input_dev, i);
input_mt_report_slot_state(data->input_dev, MT_TOOL_FINGER, true);
input_report_abs(data->input_dev, ABS_MT_TOUCH_MAJOR, 1);
input_report_abs(data->input_dev, ABS_MT_POSITION_X, current_events[i].x);
input_report_abs(data->input_dev, ABS_MT_POSITION_Y, current_events[i].y)
补充: 不同事件的标记： 备忘。
ABS_MT_TOUCH_MAJOR
The length of the major axis of the contact. The length should be given insurface units. If the surface has an X times Y resolution, the largestpossible 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 thecontact is circular, this event can be omitted [4].
ABS_MT_WIDTH_MAJOR
The length, in surface units, of the major axis of the approachingtool. This should be understood as the size of the tool itself. Theorientation of the contact and the approaching tool are assumed to be thesame [4].
ABS_MT_WIDTH_MINOR
The length, in surface units, of the minor axis of the approachingtool. Omit if circular [4].The above four values can be used to derive additional information aboutthe contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximatesthe notion of pressure. The fingers of the hand and the palm all havedifferent characteristic widths [1].
ABS_MT_PRESSURE
The pressure, in arbitrary units, on the contact area. May be used insteadof TOUCH and WIDTH for pressure-based devices or any device with a spatialsignal intensity distribution.
ABS_MT_DISTANCE
The distance, in surface units, between the contact and the surface. Zerodistance means the contact is touching the surface. A positive number meansthe contact is hovering above the surface.
ABS_MT_ORIENTATION
The orientation of the ellipse. The value should describe a signed quarterof a revolution clockwise around the touch center. The signed value rangeis arbitrary, but zero should be returned for a finger aligned along the Yaxis of the surface, a negative value when finger is turned to the left, anda positive value when finger turned to the right. When completely aligned withthe X axis, the range max should be returned. Orientation can be omittedif the touching object is circular, or if the information is not availablein the kernel driver. Partial orientation support is possible if the devicecan distinguish between the two axis, but not (uniquely) any values inbetween. 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_YThe 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 distinguishbetween different tool types, such as a finger or a pen. In such cases, theevent should be omitted. The protocol currently supports MT_TOOL_FINGER andMT_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 shapedcontact. The sequence of points forms a polygon which defines the shape ofthe contact. This is a low-level anonymous grouping for type A devices, andshould not be confused with the high-level trackingID [5]. Most type Adevices 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 ensureunique identification of a contact maintained over an extended period oftime. For type B devices, this event is handled by input core; driversshould instead use input_mt_report_slot_state().
