ImageMagick Escapes

There is a copious amount of extra data assocated with images (metadata), beyond the actual image pixels. This metadata can be useful, either for display, or for various calculations, or in modifying the behaviour of later image processing operations.

You can utilize percent escapes in a number of options, for example in -format or in montage -label, to print various properties and other settings associated with an image.

Percent Escape Handling

If you request a percent escape such as %[key] the setting is looked for in the following order until the first match has been found...

1. Handle special prefixes such as 'artifact:' 'option:' 'exif:', or 'fx:'. This includes and calculations and or globs of those prefixes such as 'exif:*' or 'artifact:*' (see below).
2. If key contains a glob pattern (but no known prefix) search free-form properties table.
3. If key is a special image 'attribute' name (see list above) return the associated or calculated image attribute.
4. Search for setting as a free-form 'property'
5. Search for setting as a free-form 'artifact'
6. Search for setting as a free-form 'option'
7. Replace escape with empty string, and perhaps produce a warning.

Remember, all long name forms of percent escapes are handled in a is case insensitive manner.

As of IM v6.8.0-5 you can now access the Artifact and Option free-form string tables directly, allowing you to override the above sequence, and avoid accessing an attribute or property of the same name.

   %[artifact:setting]
   %[option:setting]

Single Letter Attribute Percent Escapes

Here are common single letter escapes (short form) is used to report the most common attributes and properties of an image, such as: the image filename filename, type, width, height.

Here is a sample command and its output for an image with filename bird.miff and whose width is 512 and height is 480.

-> identify -format "%m:%f %wx%h" bird.miff
MIFF:bird.miff 512x480

Note that all single letter percent escapes can also be used using long form (from IM version 6.7.6-9, see next). For example %[f] is equivelent to the %f short form.

WARNING: short form percent escapes are NOT performed when the percent is after a number. For example, 10%x10 does not expand the %x as a percent escape. If you specifically want to expand the 'x', use the long form which overrides this special case. EG: 10%[x]10.

Also be warned that calculated attributes can take some time to generate, especially for large images.

Long Form Attribute Percent Escapes

In addition to the above specific and calculated attributes are recognized when enclosed in braces (long form):

Properties

All other long forms of percent escapes (not single letter long form) are handled in a case insensitive manner. Such escapes will will attempt to look up that name specific data sources.

The primary search space (if not a specific attribute listed above) is a free-form property string. Such strings are associated and saved with images, and are typically set using either the -set CLI option (or API equivelent), or from special convenience options (such as -label, -comment, -caption).

These convenience options are globally saved (as 'global options' so thay can be set before images are read), and later are transfered to the property of individual images, only when they are read in. At that time any internal percent escape present is then handled.

To change a property of an image already in memory, you need to use -set.

Note that properties, like attributes (and profiles), are saved with images when write, if the image file format allows.

Artifacts and Options

The previous percent escapes are associated with the primary Attributes and Properties. Which is the original and primary focus of such percent escapes.

However there are many operational settings that are used by various ImageMagick operators that can be useful to set and later access. These consist of per-image Artifacts, and Global options (associated with a list of images, typically the current image list).

Note that the major difference between an artifact and a property is that artifacts, being an internal operational setting, is not saved with images (if such is possible).

For example when you use -define 'distort:viewport=100x100' you are in fact generating a global option, which the -distort operator will use to modify its behaviour (distorted output image 'view').

An Option is essentually a Artifact that has been stored globally as part of a list of images (specifically a 'Wand' of images). As such they are identical, in that a Option, is simply a global Artifact for all the associated images.

As such you can use -set 'option:distort:viewport' '100x100' to achieve the same result of setting a Artifact for the disort operation to use.

Internal Handling of a Global Option...

The Core library ('MagickCore') does not generally directly understand Global Options. As such, continuing the previous example, the DistortImages() function only looks up an artifact to discover if a 'viewport' has been provided to it.

How Global Options are used when a library function requests an Artifact is one of the key differences between IMv6 and IMv7.

In ImageMagick version 6... before each operator, any global Options are copied to per-image Artifacts, for every image in the current image list. This allows various operators to find its operational 'defines' or Artifacts.

In ImageMagick version 7... sets a link back to the global options data, so that if a specific per-image Artifact is not found , then it will look for a equivelent global Option for that image list. directly. This saves coping these free-form options into artifacts repeatally, and means you can now separally define a global option for a list, and a individual overriding artifact for a specific image in that list.

Note that many API's that do not use Wands (PerlMagick for example using arrays of images rather than a Wand). In these API's you will not have Global Options, only per-image Artifacts.

In summery a Global Option, if available, is equivelent to a per-image Artifact.

Glob-Pattern Listing of Properties, Artifacts and Options

The setting can contain a glob pattern. As such you can now list all free-form string properties, artifacts, and options, (but not specific image attributes) using...

  convert ... \
          -print "__Properties__\n%[*]" \
          -print "__Artifacts__\n%[artifact:*]" \
          -print "__Options__\n%[option:*]" \
          ...

The format of glob patterns are very specific and as such is generally only used to list specific settings, such as when debugging, rather than being used for image processing use.

Calculated Percent Escape Prefixes

There are some special prefixes (before the first ':') which performs calculations based on the user provided string that follows that prefix. For example you can do a numerical calculation use %[fx:...] to evaluate the given FX expressions:

   %[fx:expression]

Use pixel: to evaluate a pixel color as defined by the FX expression:

   %[pixel:expression]

Specific Profile Percent Escape Prefixes

You can also use the following special formatting syntax to print EXIF mage meta-data that was included in the image read in:

   %[EXIF:tag]

Choose tag from the following:

Surround the format specification with quotation marks to prevent your shell from misinterpreting any spaces and square brackets.

The following special formatting syntax can be used to print IPTC information contained in the file:

   %[IPTC:dataset:record]

Select dataset and record from the following: