Specifications

42 Chapter 4 - SmartSet Tutorial
Figure 4-5.Calibration Point Coordinates
We will now use the on-board calibration and scaling features of the SmartSet
controller so coordinates will be reported in the coordinate system of your image.
(If you do not wish to use this feature of the SmartSet controller, Appendix B
gives generalized calibration and scaling algorithms that a driver program can use.)
Go to the Calibration submenu. Note the default calibration points are 0-4095 for
each axis. Choose "Do Calibration", type "C" to calibrate in 80x25 text mode, and
touch the three points indicated.
When you complete the calibration sequence and return to the menu, the
calibration points have been changed. The new calibration points are the
coordinates of the upper-left and lower-right corners of the 80x25 image.
If the default orientation of your touchscreen had the origin in the lower-left
corner, as is typical, the calibration points will reflect a change in orientation by
having a low value greater than the high value in the Y axis. If your origin was in
the upper-right, the X values will be reversed. The process of calibration not only
defines the position of your image, it also aligns the origin of the touchscreen
coordinate system with that of the image. This is called hardware axis inversion.
In general, the first calibration point becomes the origin.
The third calibration point was used only to detect swapped axes. This can correct
inverted cabling or touchscreens rotated 90°. Normally after calibrating, the X/Y
Axis field indicates Normal, not Swapped.
You may have noticed that the calibration routine did not acquire its calibration
points in the corners of the video image. The points taken are offset from the
corners, then extrapolated to achieve an estimated value at the corners. This is
because the image on some monitors is not very linear, and usually least linear in
the corners, due to the "pin cushion" effect. By acquiring calibration points near
the corners instead of at the corners, more of the video will be closely calibrated
with the touchscreen.