127 lines
5.4 KiB
Plaintext
127 lines
5.4 KiB
Plaintext
N-Trig touchscreen Driver
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-------------------------
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Copyright (c) 2008-2010 Rafi Rubin <rafi@seas.upenn.edu>
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Copyright (c) 2009-2010 Stephane Chatty
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This driver provides support for N-Trig pen and multi-touch sensors. Single
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and multi-touch events are translated to the appropriate protocols for
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the hid and input systems. Pen events are sufficiently hid compliant and
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are left to the hid core. The driver also provides additional filtering
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and utility functions accessible with sysfs and module parameters.
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This driver has been reported to work properly with multiple N-Trig devices
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attached.
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Parameters
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----------
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Note: values set at load time are global and will apply to all applicable
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devices. Adjusting parameters with sysfs will override the load time values,
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but only for that one device.
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The following parameters are used to configure filters to reduce noise:
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activate_slack number of fingers to ignore before processing events
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activation_height size threshold to activate immediately
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activation_width
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min_height size threshold bellow which fingers are ignored
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min_width both to decide activation and during activity
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deactivate_slack the number of "no contact" frames to ignore before
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propagating the end of activity events
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When the last finger is removed from the device, it sends a number of empty
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frames. By holding off on deactivation for a few frames we can tolerate false
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erroneous disconnects, where the sensor may mistakenly not detect a finger that
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is still present. Thus deactivate_slack addresses problems where a users might
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see breaks in lines during drawing, or drop an object during a long drag.
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Additional sysfs items
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----------------------
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These nodes just provide easy access to the ranges reported by the device.
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sensor_logical_height the range for positions reported during activity
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sensor_logical_width
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sensor_physical_height internal ranges not used for normal events but
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sensor_physical_width useful for tuning
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All N-Trig devices with product id of 1 report events in the ranges of
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X: 0-9600
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Y: 0-7200
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However not all of these devices have the same physical dimensions. Most
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seem to be 12" sensors (Dell Latitude XT and XT2 and the HP TX2), and
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at least one model (Dell Studio 17) has a 17" sensor. The ratio of physical
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to logical sizes is used to adjust the size based filter parameters.
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Filtering
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---------
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With the release of the early multi-touch firmwares it became increasingly
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obvious that these sensors were prone to erroneous events. Users reported
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seeing both inappropriately dropped contact and ghosts, contacts reported
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where no finger was actually touching the screen.
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Deactivation slack helps prevent dropped contact for single touch use, but does
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not address the problem of dropping one of more contacts while other contacts
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are still active. Drops in the multi-touch context require additional
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processing and should be handled in tandem with tacking.
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As observed ghost contacts are similar to actual use of the sensor, but they
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seem to have different profiles. Ghost activity typically shows up as small
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short lived touches. As such, I assume that the longer the continuous stream
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of events the more likely those events are from a real contact, and that the
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larger the size of each contact the more likely it is real. Balancing the
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goals of preventing ghosts and accepting real events quickly (to minimize
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user observable latency), the filter accumulates confidence for incoming
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events until it hits thresholds and begins propagating. In the interest in
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minimizing stored state as well as the cost of operations to make a decision,
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I've kept that decision simple.
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Time is measured in terms of the number of fingers reported, not frames since
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the probability of multiple simultaneous ghosts is expected to drop off
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dramatically with increasing numbers. Rather than accumulate weight as a
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function of size, I just use it as a binary threshold. A sufficiently large
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contact immediately overrides the waiting period and leads to activation.
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Setting the activation size thresholds to large values will result in deciding
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primarily on activation slack. If you see longer lived ghosts, turning up the
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activation slack while reducing the size thresholds may suffice to eliminate
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the ghosts while keeping the screen quite responsive to firm taps.
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Contacts continue to be filtered with min_height and min_width even after
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the initial activation filter is satisfied. The intent is to provide
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a mechanism for filtering out ghosts in the form of an extra finger while
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you actually are using the screen. In practice this sort of ghost has
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been far less problematic or relatively rare and I've left the defaults
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set to 0 for both parameters, effectively turning off that filter.
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I don't know what the optimal values are for these filters. If the defaults
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don't work for you, please play with the parameters. If you do find other
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values more comfortable, I would appreciate feedback.
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The calibration of these devices does drift over time. If ghosts or contact
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dropping worsen and interfere with the normal usage of your device, try
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recalibrating it.
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Calibration
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-----------
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The N-Trig windows tools provide calibration and testing routines. Also an
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unofficial unsupported set of user space tools including a calibrator is
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available at:
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http://code.launchpad.net/~rafi-seas/+junk/ntrig_calib
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Tracking
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--------
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As of yet, all tested N-Trig firmwares do not track fingers. When multiple
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contacts are active they seem to be sorted primarily by Y position.
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