code: def _blend_layers(self, imagecontent, z_x_y):
          
          (z, x, y) = z_x_y
          result = self._tile_image(imagecontent)
          # Paste each layer
          for (layer, opacity) in self._layers:
              try:
                  # Prepare tile of overlay, if available
                  overlay = self._tile_image(layer.tile((z, x, y)))
              except (IOError, DownloadError, ExtractionError)as e:
                  logger.warn(e)
                  continue
              # Extract alpha mask
              overlay = overlay.convert("RGBA")
              r, g, b, a = overlay.split()
              overlay = Image.merge("RGB", (r, g, b))
              a = ImageEnhance.Brightness(a).enhance(opacity)
              overlay.putalpha(a)
              mask = Image.merge("L", (a,))
              result.paste(overlay, (0, 0), mask)
          # Read result
          return self._image_tile(result)
docs: Merge tiles of all layers into the specified tile path