Touchscreen access library
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tslib

Touchscreen access library

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General

The idea of tslib is to have a core library that provides standardised services, and a set of plugins to manage the conversion and filtering as needed.

The plugins for a particular touchscreen are loaded automatically by the library under the control of a static configuration file, ts.conf. ts.conf gives the library basic configuration information. Each line specifies one module, and the parameters for that module. The modules are loaded in order, with the first one processing the touchscreen data first. For example:

  module_raw input
  module variance delta=30
  module dejitter delta=100
  module linear

These parameters are described below.

With this configuration file, we end up with the following data flow through the library:

  raw read --> variance --> dejitter --> linear --> application
  module       module       module       module

You can re-order these modules as you wish, add more modules, or remove them all together. When you call ts_read(), the values you read are values that have passed through the chain of filters and scaling conversions. Another call is provided, ts_read_raw() which bypasses all the modules and reads the raw data directly from the device.

There are a couple of programs in the tslib/tests directory which give example usages. They are by no means exhaustive, nor probably even good examples. They are basically the programs used to test this library.

Use tslib via a normal input event device

Instead of using tslib's API calls, you can use tslib/tools/ts_uinput which creates (via uinput) a new standard input event device you can use in your environment. The new device provides the filtered and calibrated values and should work with single- and multitouch devices.

Multitouch

Similar to the mentioned ts_read(), ts_read_mt() reads one struct ts_sample_mt per slot (number of possible contacts) and desired number of samples. You have to provide slots*nr of them to hold the resulting values, see the multitouch programs in tslib/tests for examples; there is, of course, ts_read_raw_mt() too.

ts_read_mt() aims to be a drop-in replacement for ts_read(), so you can use it for any single touch device too, providing space for one slot.

Environment Variables

TSLIB_TSDEVICE          TS device file name.
                Default (non inputapi): /dev/touchscreen/ucb1x00
                Default (inputapi): /dev/input/event0
TSLIB_CALIBFILE         Calibration file.
                Default: ${sysconfdir}/pointercal
TSLIB_CONFFILE          Config file.
                Default: ${sysconfdir}/ts.conf
TSLIB_PLUGINDIR         Plugin directory.
                Default: ${datadir}/plugins
TSLIB_CONSOLEDEVICE     Console device.
                Default: /dev/tty
TSLIB_FBDEVICE          Framebuffer device.
                Default: /dev/fb0

Module Parameters

module: variance

Description:

Variance filter. Tries to do it's best in order to filter out random noise coming from touchscreen ADC's. This is achieved by limiting the sample movement speed to some value (e.g. the pen is not supposed to move quicker than some threshold).

This is a 'greedy' filter, e.g. it gives less samples on output than receives on input. It can cause problems on capacitive touchscreens that already apply such a filter.

There is no multitouch support for this filter (yet). ts_read_mt() will only read one slot, when this filter is used.

Parameters:

  • delta

    Set the squared distance in touchscreen units between previous and current pen position (e.g. (X2-X1)^2 + (Y2-Y1)^2). This defines the criteria for determining whenever two samples are 'near' or 'far' to each other.

    Now if the distance between previous and current sample is 'far', the sample is marked as 'potential noise'. This doesn't mean yet that it will be discarded; if the next reading will be close to it, this will be considered just a regular 'quick motion' event, and it will sneak to the next layer. Also, if the sample after the 'potential noise' is 'far' from both previously discussed samples, this is also considered a 'quick motion' event and the sample sneaks into the output stream.

module: dejitter

Description:

Removes jitter on the X and Y co-ordinates. This is achieved by applying a weighted smoothing filter. The latest samples have most weight; earlier samples have less weight. This allows to achieve 1:1 input->output rate.

Parameters:

  • delta

    Squared distance between two samples ((X2-X1)^2 + (Y2-Y1)^2) that defines the 'quick motion' threshold. If the pen moves quick, it is not feasible to smooth pen motion, besides quick motion is not precise anyway; so if quick motion is detected the module just discards the backlog and simply copies input to output.

module: linear

Description:

Linear scaling module, primerily used for conversion of touch screen co-ordinates to screen co-ordinates.

Parameters:

  • xyswap

    interchange the X and Y co-ordinates -- no longer used or needed if the new linear calibration utility ts_calibrate is used.

  • pressure_offset

    offset applied to the pressure value

  • pressure_mul

    factor to multiply the pressure value with

  • pressure_div

    value to divide the pressure value by

module: pthres

Description:

Pressure threshold filter. Given a release is always pressure 0 and a press is always >= 1, this discards samples below / above the specified pressure threshold.

Parameters:

  • pmin

    Minimum pressure value for a sample to be valid.

  • pmax

    Maximum pressure value for a sample to be valid.

module: debounce

Description:

Simple debounce mechanism that drops input events for the specified time after a touch gesture stopped.

Parameters:

  • drop_threshold

    drop events up to this number of milliseconds after the last release event.

module: skip

Description:

Skip nhead samples after press and ntail samples before release. This should help if for the device the first or last samples are unreliable.

Parameters:

  • nhead

    Number of events to drop after pressure

  • ntail

    Number of events to drop before release

module: median

Description:

Similar to what the variance filter does, the median filter suppresses spikes in the gesture. For some theory, see https://en.wikipedia.org/wiki/Median_filter

Parameters:

  • depth

    Number of samples to apply the median filter to

Module Creation Notes

For those creating tslib modules, it is important to note a couple things with regard to handling of the ability for a user to request more than one ts event at a time. The first thing to note is that the lower layers may send up less events than the user requested, because some events may be filtered out by intermediate layers. Next, your module should send up just as many events as the user requested in nr. If your module is one that consumes events, such as variance, then you loop on the read from the lower layers, and only send the events up when 1. you have the number of events requested by the user, or 2. one of the events from the lower layers was a pen release.