Comparison between two images in MATLAB

Question

I need to calculate a comparison between two images in MATLAB that I know have different sizes (for instance size(A)=100x100 and size(B)=50x50 ).

I want to compare with vl_dsift, but dense sift isn't size invariant, so I searched on the internet. I already know that I have to specify the size of image and here lies my question.

I calculate the descriptors but for what value should I replace "a" in the next line for each image A and B?

[f,d]=vl_dsift(I,'size',a)

Answer 1

First calculate keypoint descriptors in both images ( d in vl_dsift ). The dimension of d will be 128 x number of keypoints (which may be different in both the images). Now find matching between all the descriptors of both the images. Sort the distances. If first (say) 10 elements are smaller than a threshold, then the images are a good match. Here you can find, how to match two sift descriptors.

The number 10 and the threshold will have to be decided by trial and error.