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-N-Trig touchscreen Driver
--------------------------
- Copyright (c) 2008-2010 Rafi Rubin <rafi@seas.upenn.edu>
- 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
-attached.
-
-
-Parameters
-----------
-
-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
-activation_width
-
-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_logical_width
-
-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
-X: 0-9600
-Y: 0-7200
-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.
-
-
-Filtering
----------
-
-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
-recalibrating it.
-
-
-Calibration
------------
-
-The N-Trig windows tools provide calibration and testing routines. Also an
-unofficial unsupported set of user space tools including a calibrator is
-available at:
-http://code.launchpad.net/~rafi-seas/+junk/ntrig_calib
-
-
-Tracking
---------
-
-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.