Glad it worked...
There is a slight difference in how the picture is stretched. In non-linear stretching, the edges are stretched more than the middle. The notion is that the most important elements are in the center of the image frame, so less stretching is applied there, whereas the least important elements are at the edges, so the most stretching is applied there. In linear stretching, the stretching is applied evenly across the image (i.e, same amount of stretching in the center as at the edges).