PNG Proposed eXIf chunk, draft 20170114a

File: png-proposed-eXIf-chunk-20170114a.html

Status of this Memo

   This document is an informal draft of the PNG development group.

   Comments on this document can be sent to the PNG specification
   maintainers at png-mng-misc@lists.sourceforge.net [1].

   Distribution of this memo is unlimited.

   Initial proposals were in the png-mng-misc mailing list, January 2017.

   At present, the latest version of this document is available on the
   World Wide Web from
   ftp://ftp.simplesystems.org/pub/libpng/png-group/documents/ [2].

Notices

   Copyright © 2017 Glenn Randers-Pehrson

   Permission is granted to copy and distribute this document for any
   purpose and without charge, provided that the copyright notice and this
   notice are preserved, and that any substantive changes or deletions
   from the original are clearly marked.

Abstract

   This document describes two variants of a special-purpose chunk type
   that has been proposed for use in various PNG (Portable Network
   Graphics) and related multi-image applications. It has not yet been
   approved for registration by the PNG developers, and therefore is
   experimental and its format is subject to change. The proposed chunk is
   (with an unregistered name or names that can be used in test
   implementations shown in parentheses): eXIf (exIf, uxIf, zxIf).

PNG uxIf (raw EXIF) proposal

   It is proposed to add the following section to the document "Extensions
   to the PNG 1.2 Specification, Version 1.4.0"

  3.7. eXIf EXIF profile

   The four-byte chunk type field contains the decimal values
    101 88 73 102 (ASCII "e X I f")

   When present, the eXIf chunk contains contains EXIF metadata that is
   compatible with specifications issued by CIPA, "Exchangeable image file
   format for digital still cameras" [CIPA DC-008-2016] or its successors.
   See https://www.media.mit.edu/pia/Research/deepview/exif.html

   The eXIf chunk contains an EXIF profile in a raw format:
         Data: length bytes where "length" is determined
               by the chunk "length" field, containing a
               raw EXIF profile.

   At most one eXIF chunk shall occur in a PNG datastream. There are no
   ordering constraints upon the position of the eXIf beyond those imposed
   by the PNG specification, i.e., if present, the eXIf chunk may appear
   anywhere between the IHDR and IEND chunks except between IDAT chunks.

  Exif Security considerations

   It is proposed to add the following two clauses to Section 7 (Security
   Considerations) of the "Extensions to the PNG 1.2 Specification,
   Version 1.4.0" document:

   The eXIf chunk may contain data such as image resolution and dimensions
   that are redundant with existing PNG chunks. Applications must be
   prepared to handle such redundant and perhaps conflicting information.

   The eXIf chunk may contain a "thumbnail" or a "preview" of the image.
   If the image has been redacted or otherwise edited, authors may be
   surprised to learn that they have leaked private information by
   publishing an unredacted image.

PNG zxIf proposal

   It is proposed to add the following section to the document "Extensions
   to the PNG 1.2 Specification, Version 1.4.0"

  3.7. eXIf EXIF profile

   The four-byte chunk type field contains the decimal values
    101 88 73 102 (ASCII "e X I f")

   When present, the eXIf chunk contains contains EXIF metadata that is
   compatible with specifications issued by CIPA, "Exchangeable image file
   format for digital still cameras" [CIPA DC-008-2016] or its successors.
   See https://www.media.mit.edu/pia/Research/deepview/exif.html

   The eXIf chunk contains an optional "mode" byte followed by an EXIF
   profile in a raw format or in a zlib-encoded compressed format:
         Mode: 1 byte
               0: the remaining length-1 bytes of data make up a
                  zlib-encoded EXIF profile written in accordance
                  with chapter 5, "Deflate/Inflate Compression"
                  of the PNG specification version 1.2 (which
                  is Clause 10.1, "Compression method 0" of the
                  ISO PNG specification).
              69: (ASCII "E") the remaining length-1 bytes of
                  data, combined with the byte "E", make up a raw
                  uncompressed EXIF profile
              Other values of Mode are reserved for other
                  compression methods which might be defined
                  in a future version of this specification.

         Data: the remaining length-1 bytes where "length" is
               determined by the chunk "length" field

   At most one eXIF chunk shall occur in a PNG datastream. There are no
   ordering constraints upon the position of the eXIf beyond those imposed
   by the PNG specification, i.e., if present, the eXIf chunk may appear
   anywhere between the IHDR and IEND chunks except between IDAT chunks.

  Exif Security considerations

   (same as in the uxIf proposal)

Contributors

  Contributors to discussion on the png-mng-misc list

     * John Bowler
     * Adam Costello
     * Cosmin Truta
     * Glenn Randers-Pehrson
     * Willem van Schaik

  Editor

     * Glenn Randers-Pehrson, glennrp@users.sourceforge.net

   End of PNG eXIf Chunk Proposal. Expires 14 August 2017.

References

   1. mailto:png-mng-misc@lists.sourceforge.net
   2. ftp://ftp.simplesystems.org/pub/libpng/png-group/documents/