PNG Proposed eXIf chunk, draft 2017-02-13

File: png-proposed-eXIf-chunk-2017-02-13.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

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

A copy of this version is archived at


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.


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 (Exchangeable Image File) profile

The four-byte chunk type field contains the decimal values
    101 88 73 102 (ASCII "eXIf")

The data segment of 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, length, and the "Exif ID code" described in 4.7.2(C), i.e., "Exif", NULL, padding byte, are not included. It begins with either "II" or "MM", depending upon the byte order used.

There are no ordering constraints upon the position of the eXIf chunk 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. Multiple eXIf chunks are not forbidden.

The eXIf chunk contains metadata concerning the original image data. If the image has been edited subsequent to creation of the Exif profile, this data might no longer apply to the PNG image data. It is beyond the scope of this specification to resolve potential conflicts between data in the eXIf chunk and in other PNG chunks. It is recommended that unless a decoder has independent knowledge of the validity of the Exif data, the data should be considered to be of historical value only.

While encoders may choose to update them, there is no expectation that any thumbnails present in the Exif profile have (or have not) been updated if the main image was changed.

Image editing applications should consider Paragraph E.3 of the Exif Specification (CIPA DC-008, Exchangeable image file format for digital still cameras), which discusses requirements for updating Exif data when the image is changed. Encoders should follow those requirements, but decoders should not assume that it has been accomplished.

See ISO 12234 (the XMP standard):

CIPA DC-010-2012, Exif 2.3 Metadata for XMP. Available at:

CIPA DC-008-translation-2016, Exchangeable image file format for digital still cameras: Exif Version 2.31,

Description of the Exif file format, (based on Exif Version 2.1), TsuruZoh Tachibanaya, May 28, 1999,

eXIf Recommendations for Decoders

Decoders should check the first two bytes of data to ensure that they are "II" or "MM". All other values are reserved for possible future definition.

eXIf Security considerations

The Exif specification does not contain a requirement that tag "value offset" pointers actually point to a valid address within the file (see Paragraph 4.6.2). Although this seems to be an implicit requirement, decoders should be prepared to encounter invalid pointers and deal with them appropriately.


Contributors to discussion on the png-mng-misc list


End of PNG eXIf Chunk Proposal. Expires 11 September 2017.