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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 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
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.