PNG Proposed eXIf chunk, draft 20170115a

File: png-proposed-eXIf-chunk-20170115a.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 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 "eXIf" (an unregistered name
   that can be used in test implementations is "exIf"). Caution: the third
   letter in the chunk name is an uppercase "i", not a lowercase "L".

PNG exIf (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 "eXIf")

   The eXIf chunk contains an Exif profile in the format specified in
   "4.7.2 Interoperability Structure of APP1 in Compressed Data" of [CIPA
   DC-008-2016] except that the JPEG APP1 marker and length (the first
   four bytes) are not included.

   The eXIf chunk data segment contains "length" bytes where "length" is
   determined by the chunk "length" field and starting with the "Exif ID
   code" described in 4.7.2(C).

   The encoder shall set byte 5 (described as 'padding') to 0; however the
   decoder shall not check or interpret the byte in any way.

   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.

   See ISO 12234 (the XMP standard:
   https://www.iso.org/obp/ui/#iso:std:iso:12234:-3:ed-1:v1:en) and CIPA
   DC-010-2012, Exif 2.3 Metadata for XMP. Available at:
   http://www.cipa.jp/std/documents/e/DC-010-2012_E.pdf

  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 contains metadata concerning the original image data.
   This data may not apply or may no longer apply to the PNG image data.
   Where conflicting information is present in other chunks in the stream
   that data shall be assumed to be correct unless it can be determined to
   be incorrect. Information relevant only to the original JPEG or TIFF
   encoding shall be ignored.

Contributors

  Contributors to discussion on the png-mng-misc list

     * John Bowler
     * Adam Costello
     * Glenn Randers-Pehrson
     * Nancy M. Randers-Pehrson
     * Cosmin Truta
     * 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/