How to convert xinput calibration to LibInput matrix

guide.md

Touch Coordinate Calibration with libinput

1. The Mathematical Framework

Modern libinput uses an Affine Transformation Matrix. To map a raw touch coordinate $(x, y)$ to a normalized screen coordinate $(x', y')$, we use the following equation:

$$ \begin{bmatrix} x' \ y' \ 1 \end{bmatrix} = \begin{bmatrix} a & b & c \ d & e & f \ 0 & 0 & 1 \end{bmatrix} \begin{bmatrix} x \ y \ 1 \end{bmatrix} $$

---

2. Input Data Required

To perform the conversion, you need the following values from xinput_calibrator:

• MinX, MaxX: The horizontal raw range.
• MinY, MaxY: The vertical raw range.
• Screen Resolution: Optional, but $65535$ is the standard normalized range for X11.

---

3. Step-by-Step Conversion Logic

Step A: Calculate the Spans

$$ \Delta X = MaxX - MinX $$ $$ \Delta Y = MaxY - MinY $$

Step B: Calculate Scale Factors ($a$ and $e$)

$$ a = \text{Target Range} / \Delta X $$ $$ e = \text{Target Range} / \Delta Y $$

Note: Use $1.0$ if the target is normalized, or $65535$ for raw X11.

Step C: Calculate Offsets ($c$ and $f$)

$$ c = -MinX / \Delta X $$ $$ f = -MinY / \Delta Y $$

Step D: Handle Rotations (The "Swap" Logic)

If the screen is rotated 90° or 270°, the $a$ and $e$ values move to the $b$ and $d$ positions.

---

4. Reference Conversion (Real World Example)

Based on the ADS7846 data:

• MinX: 3982, MaxX: 62782
• MinY: 62821, MaxY: 3430

Variable	Calculation	Result
$\Delta X$	$62782 - 3982$	58800
$\Delta Y$	$3430 - 62821$	-59391
Scale X ($a$)	$65535 / 58800$ (approx)	1.1145
Scale Y ($e$)	$65535 / -59391$ (approx)	-1.1034
Offset X ($c$)	$-3982 / 58800$	-0.0677
Offset Y ($f$)	$-62821 / -59391$	1.0577

Final libinput string

"1.1145 0 -0.0677 0 -1.1034 1.0577 0 0 1"

---

5. Troubleshooting & Configuration Notes

When generating these matrices, verify the following:

• Inversion Check: If $MinY > MaxY$, the $e$ value must be negative and the $f$ offset will usually be $> 1$.
• Precision: Provide at least 4 decimal places for accuracy on small (3.5") screens.
• Driver Specifics:
  • Remind the user that libinput configuration must be placed in /etc/X11/xorg.conf.d/.
  • The evdev driver must be uninstalled to prevent property overrides.