HID: ntrig: add documention

N-Trig touchscreen Driver

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	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

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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

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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

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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

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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

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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.