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Diffstat (limited to 'kernel/Documentation/input/ntrig.txt')
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diff --git a/kernel/Documentation/input/ntrig.txt b/kernel/Documentation/input/ntrig.txt new file mode 100644 index 000000000..be1fd981f --- /dev/null +++ b/kernel/Documentation/input/ntrig.txt @@ -0,0 +1,126 @@ +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. |