How to compute SIFT descriptors over MSER ellipsoids in VLFeat?

Question

I have a grayscale image (one value per pixel between 0 and 255) and I want to detect keypoints using MSER algorithm (i.e. ellipsoids) and describe those keypoints using SIFT descriptors. I am using the functions in the VLFeat library.

First I get the centroids and the ellipsoids using the functions vl_mser_get_regions (returns a vector of values indicating the position of the centroid in the image) and vl_mser_get_ell (returns a vector of dimension 9 describing the ellipsoid). Then, I want to apply the function vl_sift_calc_raw_descriptor over each one of the previous ellipsoids to obtain a SIFT descriptor per ellipsoid.

I show an example of what I have done until yet (omitting the non-transcendent code):

/*********** MSER *************/
vl_mser_process (filter, image) ; //mser filter creation

number_regions         = vl_mser_get_regions_num (filter) ; //number of ellipsoids
regions                = vl_mser_get_regions     (filter) ; //centroids of the ellipsoids

vl_mser_ell_fit (filt) ;

dimension_ellipsoids     = vl_mser_get_ell_dof (filter) ; //dimension of the vector that describes the ellipsoids: 9
number_frames            = vl_mser_get_ell_num (filter) ; //number of ellipsoids
frames                   = vl_mser_get_ell     (filter) ; //ellipsoids



/*********** SIFT *************/
sift_filter = vl_sift_new (image_width, image_height octaves, levels, omin) ; //sift filter creation

for(int i=0; i < number_frames; i++){ //iterate over each ellipsoid

    // HERE is the problem, I do not what input parameters I have 
    //to pass to this function except the first one (they must surely
    //be obtained from the ellipsoids vector)
    vl_sift_calc_raw_descriptor (   sift_filter,
            vl_sift_pix const *     grad,
            vl_sift_pix *           descr,
            int                     width,
            int                     height,
            double                  x,
            double                  y,
            double                  sigma,
            double                  angle0 ) 
}

The problem is that I do not know how to use the 9-dim vector describing each ellipsoid in the vl_sift_calc_raw_descriptor function.