Touchscreen cursor is running slower than pen at the touchscreen

[Peter Hutterer]

On Sun, Jul 02, 2017 at 12:00:39PM +0200, Jörg Zywek wrote:
> Hi,
> I'm using Raspberry Pi 2B, actual Raspian Jessie (Jun 2017) and a eGalax 8"
> touchscreen. In general I've got the touchscreen working by modifying the
> TransmissionMatrix to the following values:
> 
> Option "TransformationMatrix" "0 1 -0.001 -1 0 0.995 0 0 1"
> (The display is rotated to 180 degrees by rotate instruction in the
> /boot/config.txt file.)
> 
> The position of the shown cursor is quite exactly, if I point at the middle
> of the screen. But when I move to right or to left the cursor won't follow
> the pen exactly: the more I reach the right (or left) margin of the display
> the cursor follows less. When I touch the right border of the display, the
> cursor stands about 30 pixels left from the physical position of the pen. At
> the left side it's just the opposite behaviour. Moving up an down, the
> cursor "loses" about 50 pixels from the physical position at the top or
> bottom of the display.
> 
> The strange values above (-0.0001 or 0.995) are incremental tries to
> approach the correct behaviour. But I'm not sure, that it is the right und
> promising path. 

yes, that's the right path. You need to find a matrix that provides the
correct mapping, don't worry about screen-edge clipping because the server
will handle that anyway. Unless the device has non-linear calibration, there
will be a matrix that will make the device pixel-precise or almost so.

> And I can't find any reference manual what the values of the
> TransformationMatrix are standing for. Can you help me or give me a hint.

The first two links that come up for me are:
https://wiki.ubuntu.com/X/InputCoordinateTransformation
https://wiki.archlinux.org/index.php/Calibrating_Touchscreen

but it's just a standard coordinate transformation matrix so you can work
the various angles and offsets in as well. Wikipedia has heaps of
information about those (maybe too much :)

Cheers,
   Peter