I'm new to SIFT and was surprised by how much the features changed after resizing an image. Here's the images I used for testing:
After running SIFT on those 3 images, I was surprised to find out that the original image was more "similar" to the cropped one than the resized one (I tested with both bag of words + cosine distance and the distance algorithm described in the SIFT paper).
Is that normal and expected? What would be good approach if I wanted the original and resized images to score as more similar than the cropped one? Would normalizing image size before applying SIFT help?
Edit: After drawing the SIFT keypoints on each image, I'm now seeing that the resized image seems to have less keypoints in a common than the original and cropped ones.
I am guessing I should probably normalize image dimensions before doing SIFT to avoid this issue.