Each block of the code corresponds to a part of the SIFT feature algorithm by the original paper : http://www.cs.ubc.ca/~lowe/papers/ijcv04.pdf
and the sketch note: http://people.csail.mit.edu/hasinoff/320/sift-notes.txt
I'm trying my best to do exactly the same as the original paper does, but the results are not perfect (or can be a few bugs or miscalculations there).
Please be free using my code, and I will be appreciated if bugs or miscalculations found and replied :)