Touchscreen cursor is running slower than pen at the touchscreen

[Jörg Zywek]

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. 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.
Best regards
Jörg