Specifications

Sample SMARTSET Session 41
Besides the differences in 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 similarly-sized 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 instead relies on well-placed touch zones and
appropriate user feedback.
The three-point calibration sequence used by the SMARTSET program
automatically corrects inverted touchscreen installations and backwards cable
connections.
Type "T" and locate the corner where the X and Y values of the touchscreen are
lowest. This is the default origin of the touchscreen coordinate system. The X and
Y coordinates increase as you move to the diagonally opposite corner. Because the
coordinate values at the extremes of the touchscreen vary with every touchscreen
and controller combination, touchscreen coordinates are only useful if mapped to
the coordinate system of the image behind the touchscreen.
For example, your touchscreen may have its origin in the lower-left corner and
have a coordinate system ranging from 352,536 to 3715,3550. The active area of
the touchscreen will usually extend beyond the image, into the overscan area of
video displays. Your image may have its origin in the upper-left corner and have a
coordinate system from 1,1 to 80,25.
In Figure 4-5, Rx and Ry denote the raw coordinate system of the touchscreen
controller, and Sx and Sy denote the coordinate system for the screen image.
Rxlow, Rylow, Rxhigh, and Ryhigh are the calibration points for the position of
the image in raw coordinates. Given point Cx and Cy in raw coordinates, the X
and Y values must be determined in screen coordinates.