ure.cpp by Deva Ramanan and massive additional gains from using SSE.

 If I has dimensions [hxw], the size of the computed feature vector H is
 floor([h/binSize w/binSize nOrients*4]). For each binSize x binSize
 region, computes a histogram of gradients, with each gradient quantized
 by its angle and weighed by its magnitude. For color images, the gradient
 is computed separately for each color channel and the one with maximum
 magnitude is used. The centered gradient is used except at boundaries
 