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What exactly is the scale attribute that is calculated by detectSURFFeatures()?

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Philip Meier
Philip Meier on 30 Jul 2019
Commented: Philip Meier on 12 Aug 2019
Title says it all. I'm aware that a higher scale means a "larger" feature, but I'm confused by the number of different scales. Consider this snippet, which is a slightly modified version of the example in the documentation to detect as much points as possible:
K>> I = imread('cameraman.tif');
K>> points = detectSURFFeatures(I, 'MetricThreshold',0);
K>> length(unique(points.Scale))
ans =
54
With default parameters detectSURFFeatures() uses 3 octaves and 4 scale levels. If I understood the SURF algorithm correctly this should limit the number of different scales to 3*4=12. Can someone help me out here?

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Answers (1)

Jalaj Gambhir
Jalaj Gambhir on 5 Aug 2019
Hi,
You are confusing the 'scales' here. Each octave has fixed number of scale levels. detectSURFFeatures() does not return that value. This function returns the scale that is dependent on size of the image. You can verify this by resizing the image and then checking the unique scales returned. It would be different from 54.

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Philip Meier
Philip Meier on 12 Aug 2019
Now I'm even more confused. So this "scale" has nothing to do with the term scale and its meaning used in the original paper? To quote from the documententation of the SURFPoints class:
"Scale: Specifies scale at which the interest points were detected. This value must be greater than or equal to 1.6."
To me this sounded alot like the definition from the paper, which should be independent of the image size. If that is not the case, could you answer the original question: What exactly is the scale attribute that is calculated by detectSURFFeatures()?

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