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Question about FAST Detector

asked 2014-10-28 14:37:53 -0500

Phinatic gravatar image

updated 2014-10-28 15:11:02 -0500

EDIT: This question has been answered. I have tried to close this, but my lack of reputation does not allow it. I welcome anyone with the permission to close this to do so.

Hi,

I wanted to examine which points in the image were picked up by using the default FAST Detector. I currently use EmguCV to do so, though I think my settings are the same as in this guide (I am calling the default constructor, tried different threshold values and non-max suppression on). I went through the wikipedia article and the guide I pointed to earlier, and tested it on one part of my image. This was the result .

From my understanding, FAST determines whether or not there is a corner based on the following circle of grayscale values (taken from the image, but for clarity):

It is a corner when at least N contiguous pixels are either threshold T brighter or darker than the center point. In this case I think N=12 (as default), and (at first) picked T = 10. This means that this segment of the picture should fail to mark as a corner, since the middle-top and left-middle pixels both have a value of 219, but the middle-bottom and right-middle pixel have 41.

Varying the threshold T however, I did find that this spot does not get detected when T >= 85. This is probably because the top-right and bottom-left pixels have an intensity of 161 (76+85). Could anyone help me explain this behavior? Why does it find the spot at T < 85? Is the default N < 12? Or am I missing some step here?

Thanks in advance.

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There is the Fast-n algorithm, as the author (Edward Rosten http://fastcorner.wordpress.com/2010/09/09/fast-faq/) mentions. I have seen Fast-9 quite often, and I wouldn't be surprised if the OpenCV default implementation is FAST-9.

Doombot gravatar imageDoombot ( 2014-10-28 14:56:05 -0500 )edit

Thank you.

Phinatic gravatar imagePhinatic ( 2014-10-28 15:09:28 -0500 )edit

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answered 2014-10-28 14:58:24 -0500

Moster gravatar image

It tests for 9 contiguous pixels that are brighter or darker. That is the standard.

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Thank you. I'm not sure how I did not stumble upon it reading documentation though. Could you give me a reference to where I could find that the default specifies N=9?

Phinatic gravatar imagePhinatic ( 2014-10-28 15:09:17 -0500 )edit

I found both pages on my own too- I'm not sure how I missed line 360. Will keep in mind that the first ones are usually defaults, Thank you for all the support and have a nice day!

Phinatic gravatar imagePhinatic ( 2014-10-28 15:38:34 -0500 )edit
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Asked: 2014-10-28 14:37:53 -0500

Seen: 138 times

Last updated: Oct 28 '14