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Editing Metadata using Exiftool and Unicode

When it comes to editing image metadata, no program or API gets close to ExifTool in terms of robustness, feature count and support. ExifTool has been written in Perl and is provided within a .exe wrapper for the Windows platform. This executable can be controlled through command line parameters and a parameter (arg)  file.

Editing metadata by passing command line parameters and attribute values generally works well, as long as certain restrictions are not violated. All parameters must be correctly escaped and may only contain ANSI characters that are supported by the current user’s codepage. The maximum length of the command line is limited, and therefore entering long text might be difficult in some cases.

Unfortunately it is impossible to enter any characters that are stored in Unicode encodings such as UTF-8, UTF-16 or UCS-2. This is due to a restriction in the Perl interpreter binary, which only accepts and forwards 8 bit character sets. Technically, UTF-8 could be passed as an 8 bit character set, however due to the command line string handling of Windows, only text encoded with the current System ANSI codepage can be passed using the CreateProcess() API.

Fortunately there are a few workarounds for passing Unicode data to ExifTool:

  • Use the -E option to write special characters as HTML character entities. Examples: ä – ä Umlaut, 
 – line break, 

 – line break (Windows), 立 – Chinese character 立, etc. In case the text already contains some HTML- entities, you would have to escape them first. With this approach you will be restricted to the maximum length of the command line, which is between 2047 and 8191 characters (MSDN). When using the Win32 API CreateProcess(), the maximum argument length is 32000 characters (MSDN).
  • Write the data directly to stdin and specifyon the command line which attribute is supposed to store the data. This is a very fast approach if you need to write only a single attribute in any encoding, including binary data. No escaping is necessary in this case. Multi-value attributes and line breaks are supported.
  • Write the text or binary data for each attribute into a separate temporary file before calling ExifTool. Make sure you remove every file after ExifTool finishes processing. Similar to the above, you can write any kind of data without escaping, and as an additional benefit you can write parameters in different encodings at the same time. To pass a file as a parameter, use the “attribute<=filename” syntax (needs to be escaped with double quotes on the command line).
  • Write all attributes of the same encoding into an arg file. Then call ExifTool using the -@ parameter. You could also add processing instructions to the file, as well as the names of the files to be processed.  Another benefit is that size restrictions of the command line do not apply, and therefore an arbitrary number of files and attributes can be processed. All you have to make sure is that you only use a single encoding (probably UTF-8) for all attributes in the file. Since every assignment has to fit into a single line, multi value attributes and text with line breaks has to be escaped. In case you have to write any text that contains line breaks, you have to escape them with a $/ and change the = operator to <=. Also note that you have to escape all $ characters by $$.
  • Use the argfile approach from above, but do not use a temporary file. Instead, write the contents of that file directly to stdin. You can use the -@ - syntax for this. The main advantage compared to using an arg file is that you do not need to worry about cleaning up.
  • Combine any of the above ways of passing data.

Background

Image metadata is frequentlystored within an EXIF header. Unfortunately, EXIF has a limited concept of character sets used to encode information. According to the specification, only ASCII encoded text can be stored in text attributes. Actually many applications will also read and and write text in other 8-Bit encodings, with Latin1 being a very common denominator. For any text that is written in the current ANSI encoding of the user’s system, the user will most likely be able to retrieve the saved text without any loss in information. This however is only the case as long as the meta data is viewed on a system with the same code page. Depending on the characters used to encode the attribute, a system with a different default ANSI code page is likely to show wrong characters or even complete nonsense. Except for a few attributes such as UserComment, EXIF does not forsee the usage of Unicode.

IPTC fortunately does offer storage of Unicode text. It is possible to flag all IPTC text as UTF-8. Therefore all Unicode characters can be safely stored within IPTC, as long as the reading and writing applications adhere to the specification. Storing UTF-8 encoded text is supported by ExifTool.

XMP supports UTF-8 by default, so whenever writing XMP data, one of the above techniques should be used.

Link to ExifTool: http://www.sno.phy.queensu.ca/~phil/exiftool/

ExifTool forum at CPAN: http://www.cpanforum.com/dist/Image-ExifTool

  1. Phil Harvey
    August 20th, 2009 at 15:20 | #1

    Just FYI: As of exiftool version 7.87 you can now enter special characters as HTML character entities with the -E option.

  2. August 20th, 2009 at 16:58 | #2

    Post has been updated. Thanks Phil!

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