 The following layers are obtained by filtering the image with gradually bigger masks, taking into account the discrete nature of integral images and the specific filter structure.This results in filters of size 9×9, 15×15, 21×21, 27×27... Non-maximum suppression in a 3×3×3 neighborhood is applied to localize interest points in the image and over scales.The maxima of the determinant of the Hessian matrix are then interpolated in scale and image space with the method proposed by Brown, et al.