def set_scale(self):
        """generate the lookup which translates y-coordinate to fft-bin"""

        f_min = float(self.lower_freq)
        f_max = float(self.higher_freq)
        y_min = f_min
        y_max = f_max
        for y in range(self.image_height):
            freq = y_min + y / (self.image_height - 1.0) * (y_max - y_min)
            fft_bin = freq / f_max * (self.fft_size / 2 + 1)
            if fft_bin < self.fft_size / 2:
                alpha = fft_bin - int(fft_bin)
                self.y_to_bin.append((int(fft_bin), alpha * 255))
generate the lookup which translates y-coordinate to fft-bin