function disk(radius)
    x = collect(-radius:radius)
    y = x'
    d = Int.((x .^ 2 .+ y .^2)  .<= radius ^ 2)
end

function custom_filter(buf)
    radius = 5
    d = disk(radius)
    values = buf[d .== 1]

    buf = centered(buf)
    mid = buf[0, 0]
    G = sum(values .>= mid) - 1
    S = sum(values .< mid)
    N = length(values) - 1
    return (G - S) / N
end

function min_max(img, mask)
    img = maximum(img) - img
    filtered = imfilter(img, Kernel.gaussian(2))
    filtered[mask .== 0] = 0
    mm = mapwindow(custom_filter, filtered, (11,11), Fill(0))
    return mm
end