code: def chunk_tRNS(self, pos, length):
      # transparency
      s = ImageFile._safe_read(self.fp, length)
      if self.im_mode == "P":
          if _simple_palette.match(s):
              # tRNS contains only one full-transparent entry,
              # other entries are full opaque
              i = s.find(b"\0")
              if i >= 0:
                  self.im_info["transparency"] = i
          else:
              # otherwise, we have a byte string with one alpha value
              # for each palette entry
              self.im_info["transparency"] = s
      elif self.im_mode in ("1", "L", "I"):
          self.im_info["transparency"] = i16(s)
      elif self.im_mode == "RGB":
          self.im_info["transparency"] = i16(s), i16(s[2:]), i16(s[4:])
      return s
  
imports: import logging
  import re
  import struct
  import zlib
  from .None import Image, ImageFile, ImagePalette
  from ._binary import i8, i16be, i32be, o16be, o32be
  from ._util import py3
label_desc: Use image node_id 8 for calling the PngStream obj's underlying member method code with example usage: obj.chunk_tRNS(pos, length) and returns: s