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I am trying to calibrate a 3.5" pitft touchscreen on a Raspberry Pi 3 using xinput.

I have the display rotated so it uses a landscape orientation, with the power connector facing "up" so it can rest on my desk - this means a 270 degree rotation from default. The first challenge was rotating the input to match the screen orientation, which I acheived with:

xinput --set-prop 'stmpe-ts' 'Coordinate Transformation Matrix' 0 -1 1 1 0 0 0 0 1

Now I need to match my touch area to the screen area - currently it uses only the inner 80% or so, so as touches move out from center they become increasingly inaccurate, and things like a scrollbar on the screen edge are completely unreachable.

How can I adjust the command above to calibrate touch area size and offset, without losing my rotation or introducing shear, and without going back to school for a linear algebra degree?

update

I found this:

https://wiki.archlinux.org/index.php/Talk:Calibrating_Touchscreen

which talks about using xinput_calibrator to generate a transformation matrix. Unfortunately none of the scripts referenced there actually work - they don't appear to take rotation into consideration. The calibrator tool on it's own generates something like this:

Section "InputClass"
    Identifier  "calibration"
    MatchProduct    "stmpe-ts"
    Option  "MinX"  "24507"
    Option  "MaxX"  "24507"
    Option  "MinY"  "43599"
    Option  "MaxY"  "43599"
    Option  "SwapXY"    "1" # unless it was already set to 1
    Option  "InvertX"   "0"  # unless it was already set
    Option  "InvertY"   "0"  # unless it was already set
EndSection

The scripts on that wiki are able to get me a matrix like this:

-144.03296703296703296703 0 52.63736263736263736263 
0 -49.08988764044943820224 33.53333333333333333333 
0 0 1

But that seems to still be a bit off, and comparing to the "simple" rotation above seems to be conspicuously missing some values.

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