Doug Kerr
Well-known member
There is a lot of well-deserved interest in the use of IPTC metadata in our image files. There are couple of wrinkles that I thought would be worth mentioning.
Metadata
Metadata means "data about data". In our files, the data itself is the image data, and the metadata tells us various things about it.
In fact, the so-called Exif information is actually Exif metadata; the Exif data it describes is the image data (Exif actually refers to the entire file format we usually encounter for JPEG and TIFF files).
IPTC
IPTC refers to the International Press Telecommunications Council which, in collaboration with the American Newspaper Association, has developed various structures for data interchange in connection with journalistic and kindred enterprises.
What we speak of as IPTC metadata is data that describes a "document" (in our case, a digital image), not in the sense of information about exposure, shutter speed, and the like but rather such information as the identity of the "author", copyright information, the location at which the image was taken, and so forth.
IPTC Metadata
Two different data structures have been standardized by the IPTC for the placement of IPTC metadata items in an image file.
The earlier one, formally called the IPTC IIM (Information Interchange Model) format, somewhat parallels the data structure for Exif metadata. It is now often spoken of as the "legacy IPTC format".
The more modern structure is formally called the IPTC Core XMP Schema format (often IPTC XMP for short). As the name would suggest, it is an adaptation of the XML data structure doctrine to this particular need.
Broadly, there is direct correspondence between most of the "data items" (sometimes called "fields") defined for the two formats, although in many cases there are small differences in the specific item names and in their precise definitions. Thus the item "By-line (Author)" in the IPTC IIM structure is almost directly equivalent to the item "Creator" in the IPTC XMP structure.
Some name ambiguity
In any number of contexts the IPTC IIM data structure is referred to as "IPTC" data, while the IPTC XMP data structure is referred to as "XML" data. You can see how this can lead to some ambiguity.
Duality
It is not only possible but fairly common for a given file to contain IPTC metadata in both IIM and XML forms. When this happens, data items than can essentially exist in both forms are not necessarily present in both forms.
Software behavior
Because of this duality of data structures, programs handling IPTC metadata have a number of behaviors. Some older programs only recognize IPTC IIM metadata. (The OPanda IExif program, a helper for browsers that will display the Exif metadata and the "IPTC" metadata in an image displayed in the browser, only recognizes the IPTC IIM metadata.) Many modern programs recognize both, but handle the duality in different ways.
Some will read both IPTC IIM and IPTC XMP metadata, and if there is data in the "comparable" data items in both forms, will report only one of them, with a label arbitrarily drawn from one structure or another. (Most commonly, the XML data item is given precedence and is reported with the XML label.)
Some specialized programs (like ExifTool) will separately report both, with the proper label in each case.
As to writing IPTC metadata to a file (as in an image editor), many programs give the user the choice to write the IPTC metadata in IIM data, in XML form, or both.
An option sometimes available is to write all data items accommodated in both structures only in the IPTC XMP form, but also to write those items only existing in the IPTC IIM structure in that form (so they are not lost altogether).
My current practice
My current practice for files I am preparing to upload to my host site, including those intended for access from posts in these forums, and those I send to people on disks or e-mail to newspapers and the like, is to place the data in both forms.
Metadata
Metadata means "data about data". In our files, the data itself is the image data, and the metadata tells us various things about it.
In fact, the so-called Exif information is actually Exif metadata; the Exif data it describes is the image data (Exif actually refers to the entire file format we usually encounter for JPEG and TIFF files).
IPTC
IPTC refers to the International Press Telecommunications Council which, in collaboration with the American Newspaper Association, has developed various structures for data interchange in connection with journalistic and kindred enterprises.
What we speak of as IPTC metadata is data that describes a "document" (in our case, a digital image), not in the sense of information about exposure, shutter speed, and the like but rather such information as the identity of the "author", copyright information, the location at which the image was taken, and so forth.
IPTC Metadata
Two different data structures have been standardized by the IPTC for the placement of IPTC metadata items in an image file.
The earlier one, formally called the IPTC IIM (Information Interchange Model) format, somewhat parallels the data structure for Exif metadata. It is now often spoken of as the "legacy IPTC format".
The more modern structure is formally called the IPTC Core XMP Schema format (often IPTC XMP for short). As the name would suggest, it is an adaptation of the XML data structure doctrine to this particular need.
Broadly, there is direct correspondence between most of the "data items" (sometimes called "fields") defined for the two formats, although in many cases there are small differences in the specific item names and in their precise definitions. Thus the item "By-line (Author)" in the IPTC IIM structure is almost directly equivalent to the item "Creator" in the IPTC XMP structure.
Some name ambiguity
In any number of contexts the IPTC IIM data structure is referred to as "IPTC" data, while the IPTC XMP data structure is referred to as "XML" data. You can see how this can lead to some ambiguity.
Duality
It is not only possible but fairly common for a given file to contain IPTC metadata in both IIM and XML forms. When this happens, data items than can essentially exist in both forms are not necessarily present in both forms.
Software behavior
Because of this duality of data structures, programs handling IPTC metadata have a number of behaviors. Some older programs only recognize IPTC IIM metadata. (The OPanda IExif program, a helper for browsers that will display the Exif metadata and the "IPTC" metadata in an image displayed in the browser, only recognizes the IPTC IIM metadata.) Many modern programs recognize both, but handle the duality in different ways.
Some will read both IPTC IIM and IPTC XMP metadata, and if there is data in the "comparable" data items in both forms, will report only one of them, with a label arbitrarily drawn from one structure or another. (Most commonly, the XML data item is given precedence and is reported with the XML label.)
Some specialized programs (like ExifTool) will separately report both, with the proper label in each case.
As to writing IPTC metadata to a file (as in an image editor), many programs give the user the choice to write the IPTC metadata in IIM data, in XML form, or both.
An option sometimes available is to write all data items accommodated in both structures only in the IPTC XMP form, but also to write those items only existing in the IPTC IIM structure in that form (so they are not lost altogether).
My current practice
My current practice for files I am preparing to upload to my host site, including those intended for access from posts in these forums, and those I send to people on disks or e-mail to newspapers and the like, is to place the data in both forms.
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