Elo TouchSystems DOS and Windows Driver Disk, Руководство пользователя

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ELOCALIB 39
Point Extrapolation
You may have noticed ELOCALIB does not acquire its calibration points in the
corners of the video image. The points are taken a little in from the edges, then
extrapolated to achieve an estimated value at the edges. This is because the image
on many monitors is not very linear, and usually least linear in the corners, due to
the pin cushion effect. By acquiring calibration points near the edges instead of at
the edges, more of the video will be closely calibrated with the touchscreen.
Third Point
The third calibration point is used strictly for detecting swapped axes. Your touch
does not have to be as precise for this point.