/usr/bin/pngfix

Usage: /usr/bin/pngfix {[options] png-file}
  Tests, optimizes and optionally fixes the zlib header in PNG files.
  Optionally, when fixing, strips ancillary chunks from the file.

OPTIONS
  OPERATION
      By default files are just checked for readability with a summary of the
      of zlib issues founds for each compressed chunk and the IDAT stream in
      the file.
    --optimize (-o):
      Find the smallest deflate window size for the compressed data.
    --strip=[none|crc|unsafe|unused|transform|color|all]:
        none (default):   Retain all chunks.
        crc:    Remove chunks with a bad CRC.
        unsafe: Remove chunks that may be unsafe to retain if the image data
                is modified.  This is set automatically if --max is given but
                may be cancelled by a later --strip=none.
        unused: Remove chunks not used by libpng when decoding an image.
                This retains any chunks that might be used by libpng image
                transformations.
        transform: unused+bKGD.
        color:  transform+iCCP and cHRM.
        all:    color+gAMA and sRGB.
      Only ancillary chunks are ever removed.  In addition the tRNS and sBIT
      chunks are never removed as they affect exact interpretation of the
      image pixel values.  The following known chunks are treated specially
      by the above options:
        gAMA, sRGB [all]: These specify the gamma encoding used for the pixel
            values.
        cHRM, iCCP [color]: These specify how colors are encoded.  iCCP also
            specifies the exact encoding of a pixel value; however, in
            practice most programs will ignore it.
        bKGD [transform]: This is used by libpng transforms.    --max=<number>:
      Use IDAT chunks sized <number>.  If no number is given the IDAT
      chunks will be the maximum size permitted; 2^31-1 bytes.  If the option
      is omitted the original chunk sizes will not be changed.  When the
      option is given --strip=unsafe is set automatically. This may be
      cancelled if you know that all unknown unsafe-to-copy chunks really are
      safe to copy across an IDAT size change.  This is true of all chunks
      that have ever been formally proposed as PNG extensions.
  MESSAGES
      By default the program only outputs summaries for each file.
    --quiet (-q):
      Do not output the summaries except for files that cannot be read. With
      two --quiets these are not output either.
    --errors (-e):
      Output errors from libpng and the program (except too-far-back).
    --warnings (-w):
      Output warnings from libpng.
  OUTPUT
      By default nothing is written.
    --out=<file>:
      Write the optimized/corrected version of the next PNG to <file>.  This
      overrides the following two options
    --suffix=<suffix>:
      Set --out=<name><suffix> for all following files unless overridden on
      a per-file basis by explicit --out.
    --prefix=<prefix>:
      Set --out=<prefix><name> for all the following files unless overridden
      on a per-file basis by explicit --out.
      These two options can be used together to produce a suffix and prefix.
  INTERNAL OPTIONS

EXIT CODES
  *** SUBJECT TO CHANGE ***
  The program exit code is value in the range 0..127 holding a bit mask of
  the following codes.  Notice that the results for each file are combined
  together - check one file at a time to get a meaningful error code!
    0x01: The zlib too-far-back error existed in at least one chunk.
    0x02: At least one chunk had a CRC error.
    0x04: A chunk length was incorrect.
    0x08: The file was truncated.
  Errors less than 16 are potentially recoverable, for a single file if the
  exit code is less than 16 the file could be read (with corrections if a
  non-zero code is returned).
    0x10: The file could not be read, even with corrections.
    0x20: The output file could not be written.
    0x40: An unexpected, potentially internal, error occurred.
  If the command line arguments are incorrect the program exits with exit
  255.  Some older operating systems only support 7-bit exit codes, on those
  systems it is suggested that this program is first tested by supplying
  invalid arguments.

DESCRIPTION
  pngfix:
  checks each PNG file on the command line for errors.  By default errors are
  not output and the program just returns an exit code and prints a summary.
  With the --quiet (-q) option the summaries are suppressed too and the
  program only outputs unexpected errors (internal errors and file open
  errors).
  Various known problems in PNG files are fixed while the file is being read
  The exit code says what problems were fixed.  In particular the zlib error:

        "invalid distance too far back"

  caused by an incorrect optimization of a zlib stream is fixed in any
  compressed chunk in which it is encountered.  An integrity problem of the
  PNG stream caused by a bug in libpng which wrote an incorrect chunk length
  is also fixed.  Chunk CRC errors are automatically fixed up.

  Setting one of the "OUTPUT" options causes the possibly modified file to
  be written to a new file.

  Notice that some PNG files with the zlib optimization problem can still be
  read by libpng under some circumstances.  This program will still detect
  and, if requested, correct the error.

  The program will reliably process all files on the command line unless
  either an invalid argument causes the usage message (this message) to be
  produced or the program crashes.

  The summary lines describe issues encountered with the zlib compressed
  stream of a chunk.  They have the following format, which is SUBJECT TO
  CHANGE in the future:

     chunk reason comp-level p1 p2 p3 p4 file

  p1 through p4 vary according to the 'reason'.  There are always 8 space
  separated fields.  Reasons specific formats are:

     chunk ERR status code read-errno write-errno message file
     chunk SKP comp-level file-bits zlib-rc compressed message file
     chunk ??? comp-level file-bits ok-bits compressed uncompress file

  The various fields are

$1 chunk:      The chunk type of a chunk in the file or 'HEAD' if a problem
               is reported by libpng at the start of the IDAT stream.
$2 reason:     One of:
          CHK: A zlib header checksum was detected and fixed.
          TFB: The zlib too far back error was detected and fixed.
          OK : No errors were detected in the zlib stream and optimization
               was not requested, or was not possible.
          OPT: The zlib stream window bits value could be improved (and was).
          SKP: The chunk was skipped because of a zlib issue (zlib-rc) with
               explanation 'message'
          ERR: The read of the file was aborted.  The parameters explain why.
$3 status:     For 'ERR' the accumulated status code from 'EXIT CODES' above.
               This is printed as a 2 digit hexadecimal value
   comp-level: The recorded compression level (FLEVEL) of a zlib stream
               expressed as a string {supfast,stdfast,default,maximum}
$4 code:       The file exit code; where stop was called, as a fairly terse
               string {warning,libpng,zlib,invalid,read,write,unexpected}.
   file-bits:  The zlib window bits recorded in the file.
$5 read-errno: A system errno value from a read translated by strerror(3).
   zlib-rc:    A zlib return code as a string (see zlib.h).
   ok-bits:    The smallest zlib window bits value that works.
$6 write-errno:A system errno value from a write translated by strerror(3).
   compressed: The count of compressed bytes in the zlib stream, when the
               reason is 'SKP'; this is a count of the bytes read from the
               stream when the fatal error was encountered.
$7 message:    An error message (spaces replaced by _, as in all parameters),
   uncompress: The count of bytes from uncompressing the zlib stream; this
               may not be the same as the number of bytes in the image.
$8 file:       The name of the file (this may contain spaces).