code: def chunk_pHYs(self, pos, length):
      # pixels per unit
      s = ImageFile._safe_read(self.fp, length)
      px, py = i32(s), i32(s[4:])
      unit = i8(s[8])
      if unit == 1:  # meter
          dpi = int(px * 0.0254 + 0.5), int(py * 0.0254 + 0.5)
          self.im_info["dpi"] = dpi
      elif unit == 0:
          self.im_info["aspect"] = px, py
      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 12 for calling the PngStream obj's underlying member method code with example usage: obj.chunk_pHYs(pos, length) and returns: s