def filter_types
      types = []
      wrap_with_rewind(@filtered_data) do
        scanline_positions.each do |pos|
          @filtered_data.pos = pos
          byte = @filtered_data.read 1
          types << byte.unpack('C').first
        end
      end
      types
    end
def filter_types
      types = []
      wrap_with_rewind(@filtered_data) do
        scanline_positions.each do |pos|
          @filtered_data.pos = pos
          byte = @filtered_data.read 1
          types << byte.unpack('C').first
        end
      end
      types
    end
Returns an array of each scanline's filter type value.