1 .. include:: <isonum.txt> 2 3 ========================= 4 N-Trig touchscreen Driver 5 ========================= 6 7 :Copyright: |copy| 2008-2010 Rafi Rubin <rafi@seas.upenn.edu> 8 :Copyright: |copy| 2009-2010 Stephane Chatty 9 10 This driver provides support for N-Trig pen and multi-touch sensors. Single 11 and multi-touch events are translated to the appropriate protocols for 12 the hid and input systems. Pen events are sufficiently hid compliant and 13 are left to the hid core. The driver also provides additional filtering 14 and utility functions accessible with sysfs and module parameters. 15 16 This driver has been reported to work properly with multiple N-Trig devices 17 attached. 18 19 20 Parameters 21 ---------- 22 23 Note: values set at load time are global and will apply to all applicable 24 devices. Adjusting parameters with sysfs will override the load time values, 25 but only for that one device. 26 27 The following parameters are used to configure filters to reduce noise: 28 29 +-----------------------+-----------------------------------------------------+ 30 |activate_slack |number of fingers to ignore before processing events | 31 +-----------------------+-----------------------------------------------------+ 32 |activation_height, |size threshold to activate immediately | 33 |activation_width | | 34 +-----------------------+-----------------------------------------------------+ 35 |min_height, |size threshold below which fingers are ignored | 36 |min_width |both to decide activation and during activity | 37 +-----------------------+-----------------------------------------------------+ 38 |deactivate_slack |the number of "no contact" frames to ignore before | 39 | |propagating the end of activity events | 40 +-----------------------+-----------------------------------------------------+ 41 42 When the last finger is removed from the device, it sends a number of empty 43 frames. By holding off on deactivation for a few frames we can tolerate false 44 erroneous disconnects, where the sensor may mistakenly not detect a finger that 45 is still present. Thus deactivate_slack addresses problems where a users might 46 see breaks in lines during drawing, or drop an object during a long drag. 47 48 49 Additional sysfs items 50 ---------------------- 51 52 These nodes just provide easy access to the ranges reported by the device. 53 54 +-----------------------+-----------------------------------------------------+ 55 |sensor_logical_height, | the range for positions reported during activity | 56 |sensor_logical_width | | 57 +-----------------------+-----------------------------------------------------+ 58 |sensor_physical_height,| internal ranges not used for normal events but | 59 |sensor_physical_width | useful for tuning | 60 +-----------------------+-----------------------------------------------------+ 61 62 All N-Trig devices with product id of 1 report events in the ranges of 63 64 * X: 0-9600 65 * Y: 0-7200 66 67 However not all of these devices have the same physical dimensions. Most 68 seem to be 12" sensors (Dell Latitude XT and XT2 and the HP TX2), and 69 at least one model (Dell Studio 17) has a 17" sensor. The ratio of physical 70 to logical sizes is used to adjust the size based filter parameters. 71 72 73 Filtering 74 --------- 75 76 With the release of the early multi-touch firmwares it became increasingly 77 obvious that these sensors were prone to erroneous events. Users reported 78 seeing both inappropriately dropped contact and ghosts, contacts reported 79 where no finger was actually touching the screen. 80 81 Deactivation slack helps prevent dropped contact for single touch use, but does 82 not address the problem of dropping one of more contacts while other contacts 83 are still active. Drops in the multi-touch context require additional 84 processing and should be handled in tandem with tacking. 85 86 As observed ghost contacts are similar to actual use of the sensor, but they 87 seem to have different profiles. Ghost activity typically shows up as small 88 short lived touches. As such, I assume that the longer the continuous stream 89 of events the more likely those events are from a real contact, and that the 90 larger the size of each contact the more likely it is real. Balancing the 91 goals of preventing ghosts and accepting real events quickly (to minimize 92 user observable latency), the filter accumulates confidence for incoming 93 events until it hits thresholds and begins propagating. In the interest in 94 minimizing stored state as well as the cost of operations to make a decision, 95 I've kept that decision simple. 96 97 Time is measured in terms of the number of fingers reported, not frames since 98 the probability of multiple simultaneous ghosts is expected to drop off 99 dramatically with increasing numbers. Rather than accumulate weight as a 100 function of size, I just use it as a binary threshold. A sufficiently large 101 contact immediately overrides the waiting period and leads to activation. 102 103 Setting the activation size thresholds to large values will result in deciding 104 primarily on activation slack. If you see longer lived ghosts, turning up the 105 activation slack while reducing the size thresholds may suffice to eliminate 106 the ghosts while keeping the screen quite responsive to firm taps. 107 108 Contacts continue to be filtered with min_height and min_width even after 109 the initial activation filter is satisfied. The intent is to provide 110 a mechanism for filtering out ghosts in the form of an extra finger while 111 you actually are using the screen. In practice this sort of ghost has 112 been far less problematic or relatively rare and I've left the defaults 113 set to 0 for both parameters, effectively turning off that filter. 114 115 I don't know what the optimal values are for these filters. If the defaults 116 don't work for you, please play with the parameters. If you do find other 117 values more comfortable, I would appreciate feedback. 118 119 The calibration of these devices does drift over time. If ghosts or contact 120 dropping worsen and interfere with the normal usage of your device, try 121 recalibrating it. 122 123 124 Calibration 125 ----------- 126 127 The N-Trig windows tools provide calibration and testing routines. Also an 128 unofficial unsupported set of user space tools including a calibrator is 129 available at: 130 http://code.launchpad.net/~rafi-seas/+junk/ntrig_calib 131 132 133 Tracking 134 -------- 135 136 As of yet, all tested N-Trig firmwares do not track fingers. When multiple 137 contacts are active they seem to be sorted primarily by Y position.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.