Custom Exif Tags

Question

I am using exiv2 to manipulate metadata in a jpeg file. I need to write more information related to image processing into the metadata. Is is possible to create Custom Exif

Answer 1

From http://www.exif.org/Exif2-2.PDF:

D. Tags Relating to User Information

MakerNote
A tag for manufacturers of Exif writers to record any desired information. The contents are up to the manufacturer, but this tag should not be used for any other than its intended purpose.
Tag = 37500 (927C.H)
Type = UNDEFINED
Count = Any
Default = none

UserComment
A tag for Exif users to write keywords or comments on the image besides those in ImageDescription, and without the character code limitations of the ImageDescription tag.
Tag = 37510 (9286.H)
Type = UNDEFINED
Count = Any
Default = none

exiv2 supports MakerNote tags: http://dev.exiv2.org/projects/exiv2/wiki/How_to_add_support_for_a_new_makernote

If you don't want to do this, you can use UserComment: http://www.exiv2.org/doc/exifcomment_8cpp-example.html

Answer 2

A better way to have custom metadata is by using Adobe XMP with a custom namespace.

Answer 3

Based on my research EXIF tags are conformed to a standard set of tags as described here.

Within the standards document it states (in reference to EXIF tags),

A registration system is used for character codes to avoid duplication. When a character code is registered, a standard document is indicated in the reference column to indicate the character format specification. If a character code is used for which there is no clear specification like Shift-JIS in Japan, Undefined is designated.

Therefore its my understanding that there is a standard set of tags (the registered tags) otherwise when you query it using a tool such as exiftool it will show up as 'Undefined'.

So in order to have a tag that is recognizable by tools or other entities, you'd need to register your tag to become part of the standardized tags.