Rotation change performance with SURF and SIFT

Question

Hi,

Above is the matching percentage graph i got when i changed the image to different angles and try to compare SURF and SIFT.

I want to know what is reason for shape i got in the curve. It would be a great help if anyone could explain this to me. Thanks.

Answer 1

The shape is natural, lower performance on the synthetic rotation caused by aliasing, which creates new points and shifts position of old ones. By aliasing I mean stepped diagonal (non-vertical and non-horizontal lines). See 2nd rectangle here for example. Add 'fake' antialiasing to rotated rectangle

Aliasing is caused by interpolation algorithm (I suppose that you use linear interpolation without antialiasing). When you rotate image by 90, 180 or 270 degrees, you are not creating new diagonal lines, so image remains untouched.

What is weird, it is anti-peak with 2 times less keypoints matched - it have to be no more than 70-80% from maximum...For example, the same picture I got for the SIFT from the ASIFT source - they have due to bug in code a little bit shifted next level of octave which leads to errors in keypoints location. SIFTs and SURF performance degrade little bit with rotation, but it have not to be so much. Or second possibility, SIFT and SURF are OK, but interpolation is really bad.