path: root/Documentation/input
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authorRafi Rubin <rafi@seas.upenn.edu>2010-08-26 00:54:54 -0400
committerJiri Kosina <jkosina@suse.cz>2010-08-30 15:25:18 +0200
commit29cf28ae8dc244f89e213dc198f2286659d521b5 (patch)
tree0847ad5ba46f3e47103da044433ea93623837854 /Documentation/input
parent763008c4357b73c8d18396dfd8d79dc58fa3f99d (diff)
HID: ntrig: add documention
The doctumentation includes a brief introduction to the driver and explanations of the filtering parameters as well as a discussion of the need for and working of the filters. Signed-off-by: Rafi Rubin <rafi@seas.upenn.edu> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
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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
+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
+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.
+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.
+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:
+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.