jxl_has_jbrd.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
#
# Minimal script to check whether a JPEG-XL file contains JPEG
# lossless reconstruction data, in a more robust way than
# grepping the output of jxlinfo.
#
# This parses the ISO BMFF structure directly, so has no dependencies.
#
# Usage:
#   jxl_has_jbrd.py «filename»
#
# An exit status of 0 means the file has JPEG reconstruction data.
# Any other exit status means it doesn't (or isn't a JXL).
# If multiple filenames are given, the exit status is 0 only if
# ALL files have JPEG reconstruction data.
#
import io
import struct

JXL_CONTAINER_SIG = b'\0\0\0\x0cJXL \r\n\x87\n'

def jxl_has_jbrd(f):
  if f.read(len(JXL_CONTAINER_SIG)) != JXL_CONTAINER_SIG:
    return False # Not a JXL container
  while True:
    boxheader = f.read(16) # size(4) + type(4) [+ bigsize(8)]
    if len(boxheader) < 8:
      return False # End of file
    if boxheader[4:8] == b'jbrd':
      return True
    (size,) = struct.unpack('>I', boxheader[:4])
    if size == 1:
      # Real size is uint64 following the box type
      if len(boxheader) != 16:
        return False # Truncated input
      (size,) = struct.unpack('>Q', boxheader[8:])
    if size < 8:
      return False # Either this is the last box (size==0) or size is invalid
    # Skip to the next box
    f.seek(size - len(boxheader), io.SEEK_CUR)

if __name__ == '__main__':
  from sys import argv, exit
  status = 0
  for fname in argv[1:]:
    with open(fname, 'rb') as f:
      if jxl_has_jbrd(f):
        #print('+' + fname)
        pass
      else:
        #print('-' + fname)
        status = 1
        break
  exit(status)