ROI Assertion Error

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Three webcams are used to capture three images which overlap as shown here: http://i1276.photobucket.com/albums/y475/sesj13/ROI_zpsffce8c7d.png

I am trying to reduce the ROIs to just the overlapped regions (1/4 of the width of each image), but having problems. My code and the associated error is shown here: http://i1276.photobucket.com/albums/y475/sesj13/ROIError_zpsede8e7d3.jpg

I am trying this (which I believe is correct), but to no avail:

img_roi0 = img_grey0(Rect((WIDTH-(WIDTH/4)), 0, WIDTH, HEIGHT));

img_roi1 = img_grey1(Rect(0, 0, (WIDTH/4), HEIGHT));

img_roi2 = img_grey1(Rect((WIDTH-(WIDTH/4)), 0, WIDTH, HEIGHT));

img_roi3 = img_grey2(Rect(0, 0, (WIDTH/4), HEIGHT));

Any input would be appreciated!