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