Calibrating touch in Linux

One of the primary problem seen when integrating touch controller in Linux is calibration. I would like to introduce two different methods of calibration. Please use the table below to choose the correct tools

Tool name	Application name	Requirements	Remarks
tslib	ts_calibrate	The touch device need to be exposed as /dev/input/eventX input device and a video framebuffer support like /dev/fbY is mandatory	Standard for touchscreens on embedded ARM systems
xinput_calibrator	xinput_calibrator	Needs no special driver	Used on X based devices with no frame buffer support

Calibrating touch using tslib in Linux

Calibrating touch using xinput_calibrator in Linux

Calibrating touch using xinput calibrator in Linux

Xinput_calibrator needs no special driver and is ideal for X based graphical interfaces.

Installing the tool

The tool can be installed form the standard repository of Ubuntu, Debian, Suse and RedHat flavours. For Debian based system use the command:

sudo apt-get install xinput-calibrator

The tools can be compiled from the sources, follow the steps described:

1. Clone the repository using the command.

git clone https://github.com/tias/xinput_calibrator

2. As mentioned in the readme file https://github.com/tias/xinput_calibrator/blob/master/README, configure the build for the GUI library used in the system using the command

./autogen.sh –with-gui=x11

3. Run the make command. The program will be built in the src folder.

Calibrating with single display

For calibrating the touch using the tool follow the steps described:

1. Run the command xinput_calibarator in the command prompt.

2. Graphics with a cross hair will be shown

3. Touch the cross hair until it turns from red to white.

4. Once the calibration has been completed for the 4 cross hairs, tool will exit automatically and the calibration information will be shown in the terminal. As mentioned copy the section in the file specified. An example of the same is shown below

As explained in the log store the portion marked in brown in the configuration file. This will be picked by the X org automatically.

Calibrating with multiple display

The process of calibrating with multiple displays involves mapping the display and the touch controller. Follow additional steps other than described above is needed:

1. Run xinput in terminal to get the id of input device

2. Run xrand in terminal to get the id of display

test@test-box:~$ xrand
Screen 0: minimum 8 x 8, current 3840 x 1080, maximum 32767 x 32767
eDP1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 382mm x 215mm
   1920x1080      60.0*+   59.9     40.0
DMI1 connected primary 1920x1080+1920+0 (normal left inverted right x axis y axis) 600mm x 340mm
   1920x1080      60.0*+   50.0     59.9
   1680x1050      59.9

3. In this case the display is eDP1 and the touch id is 12. Map the display to input using the command

xinput map-to-output 12 eDP1

4. Follow the steps in the section Calibrating with single display to complete the calibration

Calibrating touch using tslib in Linux

Tslib is a user-space based tool, written in C, used for calibrating touch in ARM based Linux system. Tslib acts as a thin layer between the drivers and the GUI, provides standard filters and provisions for adding new filters as plugins. .

Prerequisites

• Tslib needs the touch to be exposed as a device in the dev folder.
• Calibration can only be performed on devices with frame buffer support.
• Framebuffer needs write access.

Installing the tool

The tool can be installed form the standard repository of Ubuntu and Debian flavours, using the command:

sudo apt-get install libts-bin

or the tool can be compiled form sources. The steps described:

1. Clone the repository using the command.

git clone https://github.com/kergoth/tslib.git

2. Change to the tslib directory where the code was cloned. And run the command

./autogen.sh

3. Since tslib is generally used for cross compiled platforms like ARM the step here is described for the same. Run the command

./configure --prefix=/home/test-box/Project/tslib/output --host=arm-linux-gnueabi

Where the prefix denote the folder location of the build output will me stored.

4. Run the make and make install. This will copy the executables and the libraries in the prefix folder mentioned above.

Calibrating the touch

To calibrate the touch, follow the steps below:
1. Set the display (frame buffer), input device and config file to be used with the commands

export TSLIB_FBDEVICE=/dev/fbX
export TSLIB_TSDEVICE=/dev/input/eventY
export TSLIB_CONFFILE=/etc/ts.conf

X should be the number of the framebuffer mapped.
Y should be the event number.

2. Run the command ts_calibrate. A dark screen with cross hairs as shown is displayed. Touch at the 5 cross hairs to complete the calibration process.

3. The calibration can be tested using the ts_test utiliy.

4. The calibration is stored in the configuration file. Filters and plugins can be enabled using the ts.conf file.

module pthres pmin=1
module variance delta=30
module dejitter delta=100
module linear

5. The X – Window system can be configure to use the touchscreen driver by adding a InputDevice section to /etc/x11/xorg.conf

Section "InputDevice"
    Identifier "tslib"
    Driver "tslib"
    Option "ScreenNumber"   "0"
    Option "Width"      "800"
    Option "Height"     "480"
    Option "Rotate"     "NONE"
    Option "TslibDevice"    "/dev/input/event0"
    Option "CorePointer"
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