boost::gil::pixel_bit_size

template<typename PixelRef>
struct pixel_bit_size
    : std::integral_constant<int, 0>

using image_t = bit_aligned_image5_type<16, 16, 16, 8, 8, devicen_layout_t<5>>::type;
const int size = pixel_bit_size<image_t::view_t::reference>::value;