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Initial proposals were in the png-mng-misc mailing list, January 2017.
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It is proposed to add the following section to the document "Extensions to the PNG 1.2 Specification, Version 1.4.0"
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, and padding byte, are not included. Therefore, it begins with either "II" or "MM", depending upon the byte order used, and it contains a TIFF header plus a 0th IFD (Image File Directory) and a 1st IFD; the 0th IFD contains pointers to an Exif IFD and/or a GPS IFD; and the 1st IFD contains a thumbnail image. Any IFD except for the 0th IFD may be omitted. Any gap preceding an IFD is filled with bytes of unspecified content.
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. Only one eXIf chunk is allowed in a PNG datastream.
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.
While the PNG specification allows the chunk size to be as large as 2^31-1 bytes, encoders should be aware that, if the Exif profile is converted to JPEG, the total length of the Exif data (the sum of the lengths of the Exif attributes IFD, the GPS IFD, the thumbnail IFD, and the TIFF header) must not exceed 64 kbytes, because it must fit into a JPEG APP1 marker.
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: http://www.cipa.jp/std/documents/e/DC-010-2012_E.pdf
CIPA DC-008-translation-2016, Exchangeable image file format for digital still cameras: Exif Version 2.31, http://www.cipa.jp/std/documents/e/DC-008-Translation-2016-E.pdf
Description of the Exif file format, (based on Exif Version 2.1), TsuruZoh Tachibanaya, May 28, 1999, https://www.media.mit.edu/pia/Research/deepview/exif.html
The first two bytes of data are either "II" for little-endian (Intel) or "MM" for big-endian (Motorola) byte order. Decoders should check the first four bytes to ensure that they have the following decimal values:
73 73 42 0 (ASCII "II", 16-bit integer 42)or
77 77 0 42 (ASCII "MM", 16-bit integer 42)All other values are reserved for possible future definition.
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.