What do I multiply in this matrix calculation to configure my touchscreen?

Question

I use 2 displays (1920x1080 touchscreen on the left, 1440x900 on the right). If I ever use

xinput set-prop "Acer T230H" "Coordinate Transformation Matrix" 0.566 0 0 0 .71111 0 0 0 1

(as matrix value)

0.566    0       0
0        0.71111 0
0        0       1

the x-coordinate is caluclated correctly, but the y-coordinate is must be multiplyed by ~1.2 to be correct.

Where do i have to place my 1.2?

Answer 1

If you want to scale the y coordinate, you should multiply 0.71111 by your scaling factor 1.2. The general formula is:

if the matrix is

a b c
d e f
g h i

then the formula is

new_x = (a * old_x + b * old_y + c) / (g * old_x + h * old_y + i)
new_y = (d * old_x + e * old_y + f) / (g * old_x + h * old_y + i)

This is a projective transformation. In almost all cases (meaning, all cases unless your display is a beamer that projects skewly on a wall), you just need a linear transformation, so you set g h i to 0 0 1.

If you're ever going to do more advanced stuff, then you should know that for the porpose of this calculation, old_x/new_x etc are in the range between 0 and 1.