Rotationally invariant bar filter. Eight bar filters, each successively rotated 2π/8 degrees, are used to detect the cytoplasmic membrane through a convolution with the DAB stained image. The Gaussian-based bar filter is centered on a 25 × 25 pixel base, with (σx, σy) = (5,1), thus creating an elongated bar. The filters as a set are rotationally invariant as only the maximum response from the convolution is chosen. A view of these filters as an image where higher values are depicted brighter is shown.