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Initial proposals were in the png-mng-misc mailing list, January 2017.
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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".
The words "must", "required", "should", "recommended", "may", and "optional" in this document are to be interpreted as described in RFC-2119 which is consistent with their plain English meanings. The word "can" carries the same force as "may".
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.
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. There are no size constraints upon the eXIf chunk size beyond those imposed by the PNG specification, i.e. maximum 2^31-1 bytes. 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 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. It is beyond the scope of this specification to resolve potential conflicts between data in the eXIf chunk and in other PNG chunks.
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 little-endian integer 42)or
77 77 0 42 (ASCII "MM", 16-bit big-endian integer 42)All other values are reserved for possible future definition.
While the PNG specification allows the chunk size to be as large as 2^31-1 bytes, decoders should be aware that, if the Exif profile is written to a JPEG datastream, 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) may need to be adjusted to not exceed 2^16-2 bytes, so it can 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.
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.
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). Offsets are measured in bytes from the beginning of the TIFF header (the "II" or "MM"). Although this seems to be an implicit requirement, decoders should be prepared to encounter invalid pointers and deal with them appropriately.
1. Safe-to-copy ("eXIf") versus unsafe-to-copy ("eXIF"): The Exif profile (and thus the eXIf chunk) may contain not only material that is safe-to-copy, such as camera, lens, exposure and GPS information, but also unsafe-to-copy material about the image data, such as image orientation, width, height, gamma, and JPEG-specific compression information. We resolved that by including a statement that such unsafe-to-copy material should be regarded as historical in nature only, and made the chunk safe-to-copy to reduce the risk of losing valuable information.
We did explore other approaches involving saving only the safe-to-copy material and unsafe-to-copy material in different PNG chunks, or otherwise dissecting the Exif profile into multiple eXIf/eXIF chunks, but those do not meet our objective of providing a simple way of preserving the Exif profile existing in other image formats. See for example the Exif text keyword proposal of December 2000, at pmt.sourceforge.io/exif.
2. The eXIf chunk contains TIFF-like and JPEG-like features that seem inappropriate for use in a PNG file. In particular, the IFD structure seems to be of little use. However, we retain IFD and other TIFF-like features for compatibility with workflows involving conversion to and from JPEG/Exif or TIFF/Exif formats and with existing tools for manipulating an Exif profile.