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.
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/.
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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 "e X I f")
When present, the eXIf chunk 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.
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.
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 "e X I f")
When present, the eXIf chunk 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
69: the Mode byte (ASCII "E") plus the remaining
length-1 bytes in the chunk, starting with
"xif", make up a raw uncompressed EXIF profile
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).
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.
(same as in the uxIf proposal)