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I think the Gaussian weighting of SIFT descriptor is wrong

asked 2013-07-31 21:36:49 -0500

grtwa gravatar image

updated 2013-07-31 21:40:17 -0500

In David G. Lowe's paper, the Sigma of Gaussian weighting function is one half the width of the descriptor window. If the descriptor window is 32x32 pixels with 4x4 subregions, I think the Sigma should be 8 rather than 2. But in sift.cpp, the code is "float exp_scale = -1.f/(d * d * 0.5f)", which means the Sigma is 2. Is there anything wrong?

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Hmm even if it is wrong, your calculation is off also right? You say half of descriptor windows which is 32x32pixels. This means that sigma in this case should be 16 right? I do not know how half of 32 would result in 8.

StevenPuttemans gravatar imageStevenPuttemans ( 2013-08-01 02:37:51 -0500 )edit

Sorry, it is not exact. I think 32x32 means (2xwidth+1)x(2xwidth+1), so width is 15.5 and Sigma is 7.75.

grtwa gravatar imagegrtwa ( 2013-08-01 03:01:37 -0500 )edit
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Hmm, have you checked the vlfeat-implementation? http://www.vlfeat.org/overview/sift.html#tut.sift.ubc

Guanta gravatar imageGuanta ( 2013-08-01 05:51:52 -0500 )edit

I see. Similar with vlfeat,"c_rot" and "r_rot" have been normalized by "hist_width", so the sigma is "d/2". Thanks for Guanta and StevenPuttemans' help.

grtwa gravatar imagegrtwa ( 2013-08-01 09:45:20 -0500 )edit

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answered 2013-08-09 04:37:46 -0500

grtwa gravatar image

OpenCV is correct. Similar with vlfeat,"c_rot" and "r_rot" have been normalized by "hist_width", so the sigma is "d/2".

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Sorry, I have not 50 points to accept my own answer.

grtwa gravatar imagegrtwa ( 2013-08-09 04:43:28 -0500 )edit
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Then help him out and accept his answer :) Also closes this topic down :)

StevenPuttemans gravatar imageStevenPuttemans ( 2013-08-09 05:02:50 -0500 )edit
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added some karma points, now you can accept it ;)

berak gravatar imageberak ( 2013-08-09 05:33:53 -0500 )edit

have done, thanks :)

grtwa gravatar imagegrtwa ( 2013-08-11 23:33:22 -0500 )edit
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Asked: 2013-07-31 21:36:49 -0500

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Last updated: Aug 09 '13