code: def accept(self, path, ext, operator):
      if operator == "resize":
          return "NG"
      elif operator == "save":
          return "OK"
      else:
          if ext == ".png":
              f = path if hasattr(path, "read") else open(path, "rb")
  
              r = png.Reader(file=f)
              width, height, pixels, metadata = r.asDirect()
              f.seek(0)
              bit_depth = metadata.get("bitdepth")
  
              if bit_depth not in [8, 16]:
                  return "NG"
              else:
                  return "Recommended"
          else:
              return "NG"
  
file_path: nnabla/python/src/nnabla/utils/image_utils/backend_events/pypng_backend.py
imports: from __future__ import absolute_import, division
  import numpy
  import png
  from nnabla.logger import logger
  from .common import upscale_pixel_intensity, check_type_and_cast_if_necessary, _imread_before, _imread_after, _imsave_before
  from .image_utils_backend import ImageUtilsBackend
label_desc: Use image node_id 5 for calling the PngBackend obj's underlying member method code with example usage: obj.accept(path, ext, operator) and returns: str, str, str, str, str