SURF GPU Small Image Size

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Hello all,

I have seen a massive increase in speed using the GPU SURF algorithm compared to running it purely on the CPU. I have three images which overlap as illustrated here:

http://i1276.photobucket.com/albums/y475/sesj13/ROI_zpsffce8c7d.png

Where, Width = 640 and Height = 480

I wish to find SURF descriptors on each of the four sub-images which are of dimensions 160*480, however when I try to feed the GPU SURF algorithm I get the error:

http://i1276.photobucket.com/albums/y475/sesj13/SURFError_zpsa71a9022.png

I believe I read somewhere that this is to do with the size of the image being passed into the GPU SURF function being too small, is there anyway around this?

Cheers!