Comments on this document can be sent to the PNG specification maintainers at
png-mng-misc@lists.sourceforge.net.
Distribution of this memo is unlimited.
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 optional "mode" byte followed by an Exif profile in a raw format or in a zlib-encoded compressed format. It begins with either a compression_method byte, "I" or "M", depending upon the byte order used and whether the data is zlib-compressed or uncompressed.
The Exif profile is 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.
Mode: 1 byte
0: the remainder of the data segment (chunk_length - 1
bytes) 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). When uncompressed, the
first two bytes are "II" or "MM", depending upon
the byte order used within the profile.
73: the Mode byte (ASCII "I") plus the remainder
of the data segment, beginning with another "I",
make up a raw uncompressed Exif profile in
little-endian ("Intel", LSB first) byte order.
77: the Mode byte (ASCII "M") plus the remainder
of the data segment, beginning with another "M",
make up a raw uncompressed Exif profile in
big-endian ("Motorola", MSB first) byte order.
Other values of Mode are reserved for other
compression methods which might be defined
in a future version of this specification.
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
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-2012, Exchangeable image file format for
digital still cameras: Exif Version 2.3. Available at:
http://www.cipa.jp/std/documents/e/DC-008-2012_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
It is proposed to add the following clause 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.