Touch Input Mapping

Xorg has very logical but cumbersome way of configuring your touch input: xinput.

Get your touch screens ID

First we need to find the name that identifies out touch screen. You can get a list of all attached xinput devices like this:

xinput list --name-only

Output:

Virtual core pointer
Virtual core XTEST pointer
ILITEK       ILITEK-TOUCH                                                    
Microsoft Microsoft® Nano Transceiver v2.0 Mouse
Microsoft Microsoft® Nano Transceiver v2.0 Consumer Control
Microsoft Microsoft® Nano Transceiver v2.0 Consumer Control
Virtual core keyboard
Virtual core XTEST keyboard
Power Button
Power Button
Sleep Button
Microsoft Microsoft® Nano Transceiver v2.0
Microsoft Microsoft® Nano Transceiver v2.0 System Control
Microsoft Microsoft® Nano Transceiver v2.0 Consumer Control
Microsoft Microsoft® Nano Transceiver v2.0 Consumer Control

Test your ID like this:

xinput list-props "ILITEK       ILITEK-TOUCH                                                    "

Input Mapping

xinput expects a transformation matrix to map the input to the screen area. Here is what the X.org documentation sais about it:

Specifies the 3x3 transformation matrix for absolute input devices. The input device will be bound to the area given in the matrix. In most configurations, "a" and "e" specify the width and height of the area the device is bound to, and "c" and "f" specify the x and y offset of the area. The value range is 0 to 1, where 1 represents the width or height of all root windows together, 0.5 represents half the area, etc. The values represent a 3x3 matrix, with the first, second and third group of three values representing the first, second and third row of the matrix, respectively. The identity matrix is "1 0 0 0 1 0 0 0 1".

What the matrix actually describes is an Affine Transformation, e. g. a series of transformations (rotation, scaling, etc.) combined.

Here is, how we can calculate ours:

Rotation

All rotations are clockwise.

The 3×3 matrix for a rotation:

    ⎡ cos(⍺) -sin(⍺) 0 ⎤
r = ⎜ sin(⍺)  cos(⍺) 1 ⎥ 
    ⎣   0       0    1 ⎦

For a display oriented bottom right (⍺ = 270°), that results in

    ⎡  0 1 0 ⎤
r = ⎜ -1 0 1 ⎥ 
    ⎣  0 0 1 ⎦

If all we want is to rotate touch input, we’re done and can use that matrix (see Applying the matrix).

Scale

The 3×3 matrix for scaling:

    ⎡ w 0 0 ⎤
s = ⎜ 0 h 0 ⎥
    ⎣ 0 0 1 ⎦

w and h are precentages of the total screen area.

w = touchscreen width / total screen width  
h = touchscreen height / total screen height

Example

┌────────────┬─────────────────────────────┐
│            │                             │
│            │                             │
│ 768 × 1024 │                             │
│ Touchscreen│        1920 × 1080          │
│            │                             │
└────────────┤                             │
             │                             │
             └─────────────────────────────┘

w = 768 / (768+1920) = 0.286  
h = 1024 / 1080 = 0.948

    ⎡ 0.286 0     0 ⎤
s = ⎜ 0     0.948 0 ⎥
    ⎣ 0     0     1 ⎦

Combining scale and rotation

m = s · r

You can use an online calculator.

In the example above the resulting matrix would look like this:

    ⎡ 0.286 0     0 ⎤   ⎡  0 1 0 ⎤   ⎡  0     0.286 0     ⎤
m = ⎜ 0     0.948 0 ⎥ · ⎜ -1 0 1 ⎥ = ⎜ -0.948 0     0.948 ⎥
    ⎣ 0     0     1 ⎦   ⎣  0 0 1 ⎦   ⎣  0     0     1     ⎦

Applying the matrix

Now we can use the ID and the matrix and apply them to xinput device.

xinput set-prop <DEVICE_ID> 'Coordinate Transformation Matrix' <MATRIX>

# example from above (including weird ID)

xinput set-prop "ILITEK     ILITEK-TOUCH                                                    " 'Coordinate Transformation Matrix' 0 0.286 0 -0.948 0 0.948 0 0 1 &

To make this mapping permanent, copy that line to
/home/tooloop/.config/openbox/autostart/