What is the purpose of feature_mat?

[xyg_coder]

Hello, I am now reading the code. It's so elegant. And I have one question,

`
unsigned long* feature_mat;

features = cvCreateSeq( 0, sizeof(CvSeq), sizeof(struct feature), storage );
for( o = 0; o < octvs; o++ )
{
feature_mat = calloc( dog_pyr[o][0]->height * dog_pyr[o][0]->width, sizeof(unsigned long) );
for( i = 1; i <= intvls; i++ )
for(r = SIFT_IMG_BORDER; r < dog_pyr[o][0]->height-SIFT_IMG_BORDER; r++)
for(c = SIFT_IMG_BORDER; c < dog_pyr[o][0]->width-SIFT_IMG_BORDER; c++)
/* perform preliminary check on contrast */
if( ABS( pixval32f( dog_pyr[o][i], r, c ) ) > prelim_contr_thr )
if( is_extremum( dog_pyr, o, i, r, c ) )
{
feat = interp_extremum(dog_pyr, o, i, r, c, intvls, contr_thr);
if( feat )
{
ddata = feat_detection_data( feat );
if( ! is_too_edge_like( dog_pyr[ddata->octv][ddata->intvl],
ddata->r, ddata->c, curv_thr ) )
{
if( ddata->intvl > sizeof(unsigned long) )
cvSeqPush( features, feat );
else if( (feature_mat[dog_pyr[o][0]->width * ddata->r + ddata->c] & (1 << ddata->intvl-1)) == 0 )
{
cvSeqPush( features, feat );
feature_mat[dog_pyr[o][0]->width * ddata->r + ddata->c] += 1 << ddata->intvl-1;
}
}
else
free( ddata );
free( feat );
}
}
free( feature_mat );
}
`

What's feature_mat used for?
Thank you so much.