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Documentation / input / ntrig.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

1	N-Trig touchscreen Driver
2	-------------------------
3		Copyright (c) 2008-2010 Rafi Rubin <rafi@seas.upenn.edu>
4		Copyright (c) 2009-2010 Stephane Chatty
5	
6	This driver provides support for N-Trig pen and multi-touch sensors.  Single
7	and multi-touch events are translated to the appropriate protocols for
8	the hid and input systems.  Pen events are sufficiently hid compliant and
9	are left to the hid core.  The driver also provides additional filtering
10	and utility functions accessible with sysfs and module parameters.
11	
12	This driver has been reported to work properly with multiple N-Trig devices
13	attached.
14	
15	
16	Parameters
17	----------
18	
19	Note: values set at load time are global and will apply to all applicable
20	devices.  Adjusting parameters with sysfs will override the load time values,
21	but only for that one device.
22	
23	The following parameters are used to configure filters to reduce noise:
24	
25	activate_slack		number of fingers to ignore before processing events
26	
27	activation_height	size threshold to activate immediately
28	activation_width
29	
30	min_height		size threshold bellow which fingers are ignored
31	min_width		both to decide activation and during activity
32	
33	deactivate_slack	the number of "no contact" frames to ignore before
34				propagating the end of activity events
35	
36	When the last finger is removed from the device, it sends a number of empty
37	frames.  By holding off on deactivation for a few frames we can tolerate false
38	erroneous disconnects, where the sensor may mistakenly not detect a finger that
39	is still present.  Thus deactivate_slack addresses problems where a users might
40	see breaks in lines during drawing, or drop an object during a long drag.
41	
42	
43	Additional sysfs items
44	----------------------
45	
46	These nodes just provide easy access to the ranges reported by the device.
47	sensor_logical_height	the range for positions reported during activity
48	sensor_logical_width
49	
50	sensor_physical_height	internal ranges not used for normal events but
51	sensor_physical_width	useful for tuning
52	
53	All N-Trig devices with product id of 1 report events in the ranges of
54	X: 0-9600
55	Y: 0-7200
56	However not all of these devices have the same physical dimensions.  Most
57	seem to be 12" sensors (Dell Latitude XT and XT2 and the HP TX2), and
58	at least one model (Dell Studio 17) has a 17" sensor.  The ratio of physical
59	to logical sizes is used to adjust the size based filter parameters.
60	
61	
62	Filtering
63	---------
64	
65	With the release of the early multi-touch firmwares it became increasingly
66	obvious that these sensors were prone to erroneous events.  Users reported
67	seeing both inappropriately dropped contact and ghosts, contacts reported
68	where no finger was actually touching the screen.
69	
70	Deactivation slack helps prevent dropped contact for single touch use, but does
71	not address the problem of dropping one of more contacts while other contacts
72	are still active.  Drops in the multi-touch context require additional
73	processing and should be handled in tandem with tacking.
74	
75	As observed ghost contacts are similar to actual use of the sensor, but they
76	seem to have different profiles.  Ghost activity typically shows up as small
77	short lived touches.  As such, I assume that the longer the continuous stream
78	of events the more likely those events are from a real contact, and that the
79	larger the size of each contact the more likely it is real.  Balancing the
80	goals of preventing ghosts and accepting real events quickly (to minimize
81	user observable latency), the filter accumulates confidence for incoming
82	events until it hits thresholds and begins propagating.  In the interest in
83	minimizing stored state as well as the cost of operations to make a decision,
84	I've kept that decision simple.
85	
86	Time is measured in terms of the number of fingers reported, not frames since
87	the probability of multiple simultaneous ghosts is expected to drop off
88	dramatically with increasing numbers.  Rather than accumulate weight as a
89	function of size, I just use it as a binary threshold.  A sufficiently large
90	contact immediately overrides the waiting period and leads to activation.
91	
92	Setting the activation size thresholds to large values will result in deciding
93	primarily on activation slack.  If you see longer lived ghosts, turning up the
94	activation slack while reducing the size thresholds may suffice to eliminate
95	the ghosts while keeping the screen quite responsive to firm taps.
96	
97	Contacts continue to be filtered with min_height and min_width even after
98	the initial activation filter is satisfied.  The intent is to provide
99	a mechanism for filtering out ghosts in the form of an extra finger while
100	you actually are using the screen.  In practice this sort of ghost has
101	been far less problematic or relatively rare and I've left the defaults
102	set to 0 for both parameters, effectively turning off that filter.
103	
104	I don't know what the optimal values are for these filters.  If the defaults
105	don't work for you, please play with the parameters.  If you do find other
106	values more comfortable, I would appreciate feedback.
107	
108	The calibration of these devices does drift over time.  If ghosts or contact
109	dropping worsen and interfere with the normal usage of your device, try
110	recalibrating it.
111	
112	
113	Calibration
114	-----------
115	
116	The N-Trig windows tools provide calibration and testing routines.  Also an
117	unofficial unsupported set of user space tools including a calibrator is
118	available at:
119	http://code.launchpad.net/~rafi-seas/+junk/ntrig_calib
120	
121	
122	Tracking
123	--------
124	
125	As of yet, all tested N-Trig firmwares do not track fingers.  When multiple
126	contacts are active they seem to be sorted primarily by Y position.
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