A control device reads a slice image, positions a boundary line in an overlapped area in the slice image, extracts first and second image segments from the slice image, converts luminance values in the boundary line and an area in the first image segment to zero, converts luminance values in the boundary line and an area in the second image segment to zero, and outputs picture signals according to the image segments to projectors, respectively. The control device displaces the boundary line every time when it positions the boundary line in repeating such operations.