Apollo 150 - Manual (Page 169)

User Manual version 2007
APOLLO 120/150 III
9-159
Why Calibration is Necessary
Calibration is a necessary step for the touchscreen prior for
its operation. Unlike mouse or keyboard applications where
the cursor is part of the image, a touchscreen is a physical
overlay with an independent coordinate system. Only by
knowing the position of the image can the PC software
convert touchscreen coordinates into image coordinates.
Besides, the differences among touchscreens and controllers,
calibration also compensates for the variation in video image
among displays. The image is affected by horizontal and
vertical adjustments on the monitor and by the physical
mounting of the touchscreen.
Additional calibration complications include image blooming,
where bright-colored images expand, and the "pin cushion"
effect, which causes the corners of the display to be stretched.
Poor display linearity can cause similar-size boxes to be
larger at the edges of the screen than they are in the middle,
or vice-versa. The displayed image can also be tilted. Even
changing video modes can affect the screen size.
Perfect calibration cannot be achieved in all circumstances.
For example, the user can encounter parallax problems with
a change in position, or because the present user is not the
same stature as the person who calibrated the screen.
Even the most sophisticated calibration techniques can only
partially overcome such variations. Therefore, most
touchscreen software uses only a two or three-point
calibration sequence and relies instead on well-placed touch
zones and appropriate user feedback.
The three-point calibration sequence used by Elo
automatically corrects inverted touchscreen installations and
backward cable connections. X or Y-axis inversion is normal
for most touchscreens.