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

Question

Philip Meier 2019-7-30

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/474106-what-exactly-is-the-scale-attribute-that-is-calculated-by-detectsurffeatures

评论： Philip Meier 2019-8-12

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?

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Jalaj Gambhir 2019-8-5

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/474106-what-exactly-is-the-scale-attribute-that-is-calculated-by-detectsurffeatures#answer_386199

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.

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Philip Meier 2019-8-12

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()?

请先登录，再进行评论。