(where uncentered gradient is used). Trilinear interpolation is used to
 place each gradient in the appropriate spatial and orientation bin.

 For each resulting histogram (with nOrients bins), four different
 normalizations are computed using adjacent histograms, resulting in a
 nOrients*4 length feature vector for each region. To compute the
 normalizations, first for each block of adjacent 2x2 histograms we
 compute their L2 norm (over all 4*nOrient bins). Each histogram (except
 at boundaries) t