Can disparity range from negative to positive values for parallel cameras?
It was the way I was setting principal points during rectification, to center the images back for the warping. If I have same principal points in both cameras I only get positive disparities, as expected. If cameras are parallel, it is not possible to have both negative and positive disparities. That only happens with converging stereo cameras (negative disparities for objects in front of point of convergence and positive for objects in the back).