|N-Trig touchscreen Driver
| Copyright (c) 2008-2010 Rafi Rubin <firstname.lastname@example.org>
| Copyright (c) 2009-2010 Stephane Chatty
|This driver provides support for N-Trig pen and multi-touch sensors. Single
|and multi-touch events are translated to the appropriate protocols for
|the hid and input systems. Pen events are sufficiently hid compliant and
|are left to the hid core. The driver also provides additional filtering
|and utility functions accessible with sysfs and module parameters.
|This driver has been reported to work properly with multiple N-Trig devices
|Note: values set at load time are global and will apply to all applicable
|devices. Adjusting parameters with sysfs will override the load time values,
|but only for that one device.
|The following parameters are used to configure filters to reduce noise:
|activate_slack number of fingers to ignore before processing events
|activation_height size threshold to activate immediately
|min_height size threshold bellow which fingers are ignored
|min_width both to decide activation and during activity
|deactivate_slack the number of "no contact" frames to ignore before
| propagating the end of activity events
|When the last finger is removed from the device, it sends a number of empty
|frames. By holding off on deactivation for a few frames we can tolerate false
|erroneous disconnects, where the sensor may mistakenly not detect a finger that
|is still present. Thus deactivate_slack addresses problems where a users might
|see breaks in lines during drawing, or drop an object during a long drag.
|Additional sysfs items
|These nodes just provide easy access to the ranges reported by the device.
|sensor_logical_height the range for positions reported during activity
|sensor_physical_height internal ranges not used for normal events but
|sensor_physical_width useful for tuning
|All N-Trig devices with product id of 1 report events in the ranges of
|However not all of these devices have the same physical dimensions. Most
|seem to be 12" sensors (Dell Latitude XT and XT2 and the HP TX2), and
|at least one model (Dell Studio 17) has a 17" sensor. The ratio of physical
|to logical sizes is used to adjust the size based filter parameters.
|With the release of the early multi-touch firmwares it became increasingly
|obvious that these sensors were prone to erroneous events. Users reported
|seeing both inappropriately dropped contact and ghosts, contacts reported
|where no finger was actually touching the screen.
|Deactivation slack helps prevent dropped contact for single touch use, but does
|not address the problem of dropping one of more contacts while other contacts
|are still active. Drops in the multi-touch context require additional
|processing and should be handled in tandem with tacking.
|As observed ghost contacts are similar to actual use of the sensor, but they
|seem to have different profiles. Ghost activity typically shows up as small
|short lived touches. As such, I assume that the longer the continuous stream
|of events the more likely those events are from a real contact, and that the
|larger the size of each contact the more likely it is real. Balancing the
|goals of preventing ghosts and accepting real events quickly (to minimize
|user observable latency), the filter accumulates confidence for incoming
|events until it hits thresholds and begins propagating. In the interest in
|minimizing stored state as well as the cost of operations to make a decision,
|I've kept that decision simple.
|Time is measured in terms of the number of fingers reported, not frames since
|the probability of multiple simultaneous ghosts is expected to drop off
|dramatically with increasing numbers. Rather than accumulate weight as a
|function of size, I just use it as a binary threshold. A sufficiently large
|contact immediately overrides the waiting period and leads to activation.
|Setting the activation size thresholds to large values will result in deciding
|primarily on activation slack. If you see longer lived ghosts, turning up the
|activation slack while reducing the size thresholds may suffice to eliminate
|the ghosts while keeping the screen quite responsive to firm taps.
|Contacts continue to be filtered with min_height and min_width even after
|the initial activation filter is satisfied. The intent is to provide
|a mechanism for filtering out ghosts in the form of an extra finger while
|you actually are using the screen. In practice this sort of ghost has
|been far less problematic or relatively rare and I've left the defaults
|set to 0 for both parameters, effectively turning off that filter.
|I don't know what the optimal values are for these filters. If the defaults
|don't work for you, please play with the parameters. If you do find other
|values more comfortable, I would appreciate feedback.
|The calibration of these devices does drift over time. If ghosts or contact
|dropping worsen and interfere with the normal usage of your device, try
|The N-Trig windows tools provide calibration and testing routines. Also an
|unofficial unsupported set of user space tools including a calibrator is
|As of yet, all tested N-Trig firmwares do not track fingers. When multiple
|contacts are active they seem to be sorted primarily by Y position.