Hi,..
I have 2 SIFT code implementations, the first is in matlab(siftDemoV4) and use from an exe file to detect keypoints.
second(siftpp) is in C and.
i wonderd why these don't detect same keypoints and why siftpp keypoint value has two many zero while siftDemoV4 dosn't have?!
is it possible?!

for example one of the keypoint siftDemoV4 detects is :

133.92 135.88 14.38 -2.732
3 12 23 38 10 15 78 20 39 67 42 8 12 8 39 35 118 43 17 0
0 1 12 109 9 2 6 0 0 21 46 22 14 18 51 19 5 9 41 52
65 30 3 21 55 49 26 30 118 118 25 12 8 3 2 60 53 56 72 20
7 10 16 7 88 23 13 15 12 11 11 71 45 7 4 49 82 38 38 91
118 15 2 16 33 3 5 118 98 38 6 19 36 1 0 15 64 22 1 2
6 11 18 61 31 3 0 6 15 23 118 118 13 0 0 35 38 18 40 96
24 1 0 13 17 3 24 98

while one of the keypoint siftpp detects is :

318.86 7.48 1.12 1.671
0 0 0 0 0 1 0 0 0 0 1 0 0 0 16 11 0 0 1 0
0 0 63 8 0 0 2 0 0 0 69 13 14 2 0 2 2 2 0 5
128 6 0 0 0 0 104 174 17 0 0 0 0 2 174 99 0 0 0 0
0 1 174 53 26 5 0 1 4 3 0 1 174 80 0 0 0 1 9 39
115 23 0 0 7 8 62 24 0 0 5 14 9 6 40 4 9 0 0 0
3 3 0 0 174 43 0 0 0 0 0 0 156 20 0 0 3 3 3 2
2 0 3 16 56 9 2 3

It's not really surprising that two different implementations of SIFT give you different results. The algorithm has a number of parameters that can be tweaked.

David Lowe has a good implementation of SIFT on his website:
http://www.cs.ubc.ca/~lowe/keypoints/

I used David Lowe's SIFT demo as part of this project.