vlad-vector

I have a folder of images. I want to compute VLAD features from each image. I loop over each image, load it, and obtain the SIFT descriptors as follows: repo = '/media/data/images/'; filelist = dir([repo '*.jpg']); sift_descr = {} for i = 1:size(filelist, 1) I = imread([repo filelist(i).name]) ; I = single(rgb2gray(I)) ; [f,d] = vl_sift(I) ; sift_descr{i} = d end However, VLAD requires the matrix of descriptors to be 2D. See here . What is the correct way to process my SIFT descriptors, before VLAD encoding? Thank you. First, you need to obtain a dictionary of visual words, or to be more