Regarding Standard Oxford Format for vlfeat sift

Question

One of my upper classmates has given me a data set for experimenting with vlfeat's SIFT, however, her extracted SIFT data for the frame part contains 5 dimensions. An example is given below:

192
9494
262.08 749.211 0.00295391 -0.00030945 0.00583025 0 0 0 45 84 107 86 8 10 49 31 21 32 37 46 50 11 23 49 60 29 30 24 17 4 15 67 25 28 47 13 11 27 9 0 40 117 99 27 3 117 117 39 19 11 18 16 32 8 27 50 117 102 20 23 18 2 10 36 45 47 84 37 16 36 31 9 50 112 52 12 9 117 36 6 4 3 15 54 117 9 3 2 31 94 101 92 23 0 20 47 36 38 14 1 0 34 19 39 52 27 0 0 31 6 14 18 29 24 13 11 11 12 10 3 1 4 25 29 5 0 5 6 3 12 29 35 2 93 73 61 50 123 118 100 109 58 44 79 122 120 108 103 87 92 61 28 33 55 107 123 123 37 73 60 32 93 123 123 89 118 118 77 66 118 118 63 96 118 94 60 27 41 74 108 118 107 81 107 118 118 43 73 64 118 118 118 56 45 38 27 58
432.424 57.2287 0.00285143 -0.00048992 0.00292525 10 12 19 26 88 43 14 10 3 4 44 50 125 74 0 1 2 4 47 34 17 3 0 0 3 3 8 6 1 0 0 1 11 12 14 17 43 37 10 6 35 36 125 77 47 10 5 13 2 7 125 125 125 29 0 2 1 3 11 15 33 5 1 0 36 14 7 8 102 64 37 27 41 8 2 2 55 53 103 125 4 2 2 5 125 125 41 28 1 3 4 7 32 11 3 1 46 29 6 7 125 57 3 3 49 11 0 1 90 34 19 31 10 3 3 6 122 33 10 9 0 2 11 10 7 2 2 1 35 64 129 129 129 93 48 44 24 55 129 117 129 71 41 19 44 65 76 58 129 129 129 89 42 48 57 96 129 129 90 55 133 118 58 42 58 42 133 133 133 62 24 17 18 12 133 133 133 133 133 125 78 33 17 29 133 133 82 45 23 11 13 44
... // the list keeps on going for all keypoints.

This file is simply descriptors' data of an image. There are a few things I need to know:

• what are the first two values '192' and '9494'?
• what is the 5th value for the keypoint? vlfeat's sift normally gives out 4 values for key point's frame.

So I asked her what is this 5th dimension, and she pointed me to search for "standard oxford format" for sift feature.

The thing is I tried to search around regarding this standard oxford format and sift feature, but I got no luck in finding it at all. If somebody knows anything regarding this, could he please point me to the right direction?

Answer 1

192 represents the descriptor length ,9494 represent the Number of key-points you have in the file. The other line consists of [WORD_ID] [X] [Y] [A] [B] [C] X and Y is the feature centroid and A, B, C define the parameters of the ellipse in the following equation A*(x-X)^2 + 2*B*(x-X)(y-Y) + C(y-Y)^2 = 1

You can check the official website for the formate Here If you are using VLfeat package you can read here how to read the file in Oxford format. If you are very curious how the file formate is read in VLfeat vl_ubcread function. Here is the code.