Structured Information

Reading
Writing
Copying
Deleting
Field Names
Serialization
User-Defined

# Structured Information

ExifTool has the ability to read and write XMP structures through the use of either structured or flattened tags. The ability to write via structured input was added in ExifTool version 8.44; older versions wrote using flattened tags only.

To illustrate the concept of a flattened tag, the `XMP-exif:Flash` structure contains `Fired` and `Mode` fields (among others). The flattened tags corresponding to these structure fields are `XMP-exif:FlashFired` and `XMP-exif:FlashMode`. In the XMP Tags documentation, flattened tags are indicated by an underline (`_`) after the Writable type.

This page describes various techniques used to read and write XMP structures using both structured and flattened tags.

## Reading

When reading, structures are flattened by default, and ExifTool returns one "flattened" tag for each field in the structure:

>     > exiftool -xmp:all a.xmp
>     XMP Toolkit                     : Image::ExifTool 8.44
>     Flash Fired                     : True
>     Flash Mode                      : On
>     Flash Return                    : Return not detected

But the `-struct` option may be used to give structured output. In this mode structures are returned instead of separate "flattened" tags:

>     > exiftool -struct -xmp:all a.xmp
>     XMP Toolkit                     : Image::ExifTool 8.44
>     Flash                           : {Fired=True,Mode=On,Return=Return not detected}

(Note: As illustrated in the example above, structures are serialized for console output by the ExifTool application. However, via the API with the `Struct` option, they are returned as Perl HASH references.)

The `-struct` option may also be combined with the JSON (`-j`), PHP (`-php`) or XML (`-X`) output formats to provide a structured format which may be more compatible with other applications.

## Writing

When writing, flattened tags and structures may be used interchangeably. For example, the following commands all have the same effect.

>     exiftool -flashmode=on -flashreturn=not -flashfired=true a.xmp
>     exiftool -xmp:flash="{mode=on,fired=true}" -flashreturn=not a.xmp
>     exiftool -xmp:flash="{mode=on,fired=true,return=not}" a.xmp

(Note: Structures must be serialized when writing via the command-line application, in the same format as when reading with the `-struct` option.)

An advantage of writing in structured form is that it can be easier to achieve the desired hierarchy with complex structures or when there are multiple structures in a list. For example, this command adds a new hierarchical keyword to the XMP-mwg-kw:HierarchicalKeywords list:

>     exiftool -hierarchicalkeywords+="{keyword=cat,children={keyword=Siamese}}" a.jpg

But the flattened tags may be more convenient for adding or replacing a single field in an existing structure because writing as a structure would requires that entire structure be replaced. For example, the following command adds a new second-level keyword to an existing HierarchicalKeywords structure:

>     exiftool -hierarchicalkeywords2+="Persian" a.jpg

Tricky: There is one drawback when using this technique to add new fields to existing structures in lists: New fields are added to the first structure which doesn't already contain the corresponding field. So before adding a new field to a arbitrary structure, dummy fields must first be added to all earlier structures in the list which are missing this field. However, the alternative of adding a new field by writing structured information also has its drawbacks. Here, although a specific structure in a list can easily be targeted through any unique combination of field values, the drawback is that the entire structure must be replaced (see Deleting / Replacing below).

The flattened tag names may also be used to write structures at any level in a complex hierarchy. The following example writes a third-level structure inside a HierarchicalKeywords structure:

>     exiftool -hierarchicalkeywords2Children='{Keyword=Tabby,Applied=true}' a.jpg

(Note: Containing structures are created as necessary. In this case, the HierarchicalKeywords and top-level KeywordInfo structures would be created if they didn't already exist.)

The order of structure fields is not significant, so they may be read in a different order then written, unlike arrays which maintain the same order. To give a predictable output, fields in structured information are sorted in alphabetical order of field name by ExifTool when reading and writing.

If there are errors converting some fields of the input structure, other fields are still written and a warning is issued (but only one warning per structure is reported). This also applies when copying structured information except that the `-v3` option must be used to see the warnings when copying.

Programmers: Structured information is written and read as Perl HASH references via the ExifTool API, but it may also be written as a serialized string. The following two techniques are equivalent:

>     # as a HASH reference
>     $exifTool->SetNewValue('XMP:Flash' => { mode=>'on', fired=>'true', return=>'not' });

>     # as a serialized string
>     $exifTool->SetNewValue('XMP:Flash' => '{mode=on,fired=true,return=not}');

## Copying

By default, tags are copied as structures, but flattened tag names may still be copied by specifying them explicitly. Copying as structures allows the hierarchy of complex structures to be preserved.

>     # this copies all XMP information as structures
>     # (flattened tags are not copied by default...)
>     exiftool -tagsfromfile src.jpg -xmp:all dst.jpg

>     # ... but flattened tags may be copied individually.  Here the
>     # first level hierarchical keywords are copied to the Subject tag
>     # (this may be done in the same command as one that copies structures)
>     exiftool -tagsfromfile src.jpg "-subject<hierarchicalkeywords1" dst.jpg

Note that when copying a specific structure, only the top-level structures may be specified:

>     # this copies the complete keyword hierarchy
>     exiftool -tagsfromfile src.jpg -keywordinfo dst.jpg

>     # WRONG because HierarchicalKeywords is NOT a top-level structure!
>     exiftool -tagsfromfile src.jpg -hierarchicalkeywords dst.jpg

The copy-as-structure feature may be disabled with `--struct` on the command line, or by setting the Struct option to 0 via the API. When this is done, only flattened tags are copied, and structures may not be specified. Conversely, if the structure option is enabled (by setting the Struct option to 1 via the API, or with `-struct` on the command line), only structures are copied, and flattened tags may not be specified.

(Note: This method of copying as flattened tags was the default behaviour with ExifTool 8.43 and earlier. Copying as structures has been the default since the ability to write structured information was introduced in version 8.44, but an enhancement in version 8.82 allowed flattened tags to be copied explicitly without the need to disable the Struct option.)

## Deleting / Replacing

A complete structure is deleted by specifying one or more matching fields. For example, the following command deletes all HierarchicalKeywords structures which have the Keyword "`Terrier`" at the second level:

>     exiftool -hierarchicalkeywords-="{Children={Keyword=Terrier}}" a.jpg

Structure fields may also be deleted individually using the flattened tag names. The following command deletes only the matching fields from the second-level of all HierarchicalKeywords structures:

>     exiftool -hierarchicalkeywords2-="Terrier" a.jpg

When deleting and adding back items in lists in the same command, new items are inserted at the point in the list where the first item was removed, or at the end of the list if no items were deleted. This applies to lists of structures as well as simple lists of string values, and provides a mechanism to replace a specific structure or field.

## Field Names

Structure field names use a format very similar to tag names in ExifTool. The following table lists some similarities and differences between tag names and structure field names:

> Feature | Example | Tag Names | Field Names
> --- | --- | --- | ---
> Case Insensitivity | Title, title, TITLE | Yes | Yes
> Alternate Language Suffix | Title-de | Yes | Yes
> Numerical Value Suffix | Mode# | Yes | Yes
> Group Name Prefix | XMP-dc:Title | Yes | No†

> † Except that group name prefixes are allowed in structures which support arbitrary XMP fields (ie. Region Extensions)

## Serialization

Structures are serialized when reading or writing from the command line. However, serialization is not done when reading via the API, and is optional when writing via the API. The serialization algorithm is as follows:

1. Escape the following characters in string values (structure field values and list items) by adding a leading pipe symbol (`|`):
   - pipe symbols (`|`) and commas (`,`) anywhere in the string
   - closing curly brackets (`}`) anywhere in structure field values
   - closing square brackets (`]`) anywhere in list items
   - an opening curly (`{`) or square (`[`) bracket, or whitespace character (SPACE, TAB, CR or LF) if it appears at the beginning of the string
   (Note: Any other character may be escaped by adding a leading pipe symbol without effect.)
2. Enclose structures in curly brackets. Use an equal sign (`=`) to separate field names from their corresponding values, and a comma between structure fields.
3. Enclose lists in square brackets, with a comma between list items.
4. Optional whitespace padding may be added anywhere except inside a structure field name, or inside or after a string value, and an optional comma may be added after the last field in a structure.

For example, with a command like this:

>     exiftool "-RegionInfo<=INFILE" a.xmp

the INFILE below may be used to write structured information to XMP:RegionInfo.

>     {
>       AppliedToDimensions =
>       {
>          W = 4288,
>          H = 2848,
>          Unit = pixel,
>       },
>       RegionList =
>       [
>         {
>           Area =
>           {
>             W = 0.15, H = 0.17, X = 0.3, Y = 0.4,
>             Unit = normalized,
>           },
>           Description = A Physics Icon {relatively speaking|},
>           Name = Albert Einstein,
>           Type = Face,
>           Extensions = {
>             XMP-xmpRights:UsageTerms = copyright Phil Harvey,
>             XMP-xmpRights:UsageTerms-fr = droit d'auteur Phil Harvey,
>           },
>           SeeAlso = dc:subject,
>         },
>         {
>           Area =
>           {
>             W = 0.06, H = 0.09, X = 0.5, Y = 0.6,
>             Unit = normalized,
>           },
>           Description = this is a test|, what did you expect?,
>           Type = Focus,
>           FocusUsage = EvaluatedUsed,
>         }
>       ],
>     }

In this example, white space has been added in all allowed locations for demonstration purposes and to improve readability. Also, optional commas have been added after the last field of each structure. (Note that a comma may NOT be added after the last item in a list because this would be interpreted as an additional list item of a zero-length string.)

## User-Defined Structures

User-defined XMP structure tags may be created via the ExifTool config file. See the NewXMPxxxStruct tag definition in the XMP-xxx examples of the sample config file for more details.

Last revised Mar. 10, 2012

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