Based on kernel version 3.9. Page generated on 2013-05-02 23:09 EST.
1 N-Trig touchscreen Driver 2 ------------------------- 3 Copyright (c) 2008-2010 Rafi Rubin <firstname.lastname@example.org> 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.