Abstract: The multi touch system using four cameras solves the occlusion by fitting the four directing lines of each toueh point. This paper presented three methods to build the directing line, including look-up table method, vanishing point method and stereo calibration method. The look-up table method stations a set of reference points along the rectangle frame,finds the closest reference point by interpolation, and builds the line according to the projection center and the reference point; the vanishing point method builds the directing line according to the vanishing point of the touch point and the projection center; stereo calibration method builds the directing line according to epipolar geometry back projection line. Results show that look up table is applicable to small platforms and vanishing point method fits large size platforms while stereo calibration method needs to improve the precision.

Key words: Human-computer interaction, Multi-touch, Look-up table, Vanishing point, Stereo calibration