dense sift and vlfeat

Question

I want to ask two questions about dense sift(dsift) and vlfeat:

1. Any material that details dsift? I have seen many that said "dense SIFT is the SIFT's application to dense grids". But what does this mean? Can it be described in a more detailed manner? I read the source code dsift.c and dsift.h in vlfeat and the technique details about dsift . But there are many things I cannot understand. Existing papers usually focus on the application of dsift.
2. I use vlfeat in my C program and it works fine. But when I custom the parameters with vl_dsift_set_geometry , it goes wrong. Because I do not know how dsift works, I do not know how to set binSizeX/Y and numBinX/Y properly. I read in a paper "patch size 76". Does patch refer to a 4*4 grid? I somewhat got confused by the terms bin , patch and grid . Well, my question is, with patch size being 76, how to set binSizeX/Y and numBinX/Y?(image size 256*256)?

Answer 1

In SIFT, the first step is to detect key points. Key points detection is performed at multiple scale. The next step is to describe the key point to generate the descriptor. The distribution of the key points over the image is not uniform, depending on the detected key points. In dense sift features, there is no key points detection, based on a grid at specific points, sift features will be detected at specific scale. This is not useful if you are matching objects that may appear at different scales.

There is the phow version which is a combination between dense sift and sift. Instead of detecting sift at pre-specified locations and pre-specified scales, sift features are detected at pre-specified locations but different scales. In phow, all sift features detected at the same point ( different scales) will be combined together to construct a single feature at the location