code: def set_bpp(self, bpp):
          
          self.bpp = bpp
          self.maxc = 2 ** self.bpp - 1
          self._set_dtype()
          self.calc_cmap()
          self.recalc(callback=False)
docs: Set the bit depth (per band) for the output.
          A typical "32-bit RGBA" image would be 8; a 48-bit image would
          be 16, etc.