Good feature descriptor/matcher for matching photo of different size

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Hi All,

First of all, sorry if answer to this question is too obvious/easy. I have been playing around opencv for couple weeks, but without any background in computer vision, i am hitting a road block now and am hoping your expertise can point me to the right direction :)

i am trying to match two identical images but with different size (dimension), or one might be a cropped version of the other. There is no rotation variation.

Here are the steps that my program follows

1. extract feature (key point)
2. compute descriptor from key point
3. matching descriptors of the two images

I have tried using SURF for step 1 and 2 and a brute force L2 matcher for step3. It works quite well, but since it is patented, I am trying to find a alternative. I have tried a few different combination of feature detector and descriptor extractor, but the result is not good. Is there a good combination of "feature detector/descriptor extractor/matcher"choice that might worth a try?

Thanks so much