I'm looking to write custom metadata on to images(mostly jpegs, but could be others too). So far I haven't been able to do that through PIL preferably (I'm on centos 5 & I couldn't get pyexiv installed) I understand that I can update some pre-defined tags, but I need to create custom fields/tags! Can that be done?

This data would be created by users, so I wouldn't know what those tags are before hand or what they contain. I need to allow them to create tags/subtags & then write data for them. For example, someone may want to create this metadata on a particular image :

Category : Human

Physical :
    skin_type : smooth
    complexion : fair
    eye_color: blue
    beard: yes
    beard_color: brown
    age: mid

Location :
    city: london
    terrain: grass
    buildings: old

I also found that upon saving a jpeg through the PIL JpegImagePlugin, all previous metadata is overwritten with new data that you don't get to edit? Is that a bug?

Cheers, S

The python pyexiv2 module can read/write metadata.

I think there is a limited set of valid EXIF tags. I don't know how, or if it is possible to create your own custom tags. However, you could use the Exif.Photo.UserComment tag, and fill it with JSON:

import pyexiv2
import json

metadata = pyexiv2.ImageMetadata(filename)
metadata.read()
userdata={'Category':'Human',
          'Physical': {
              'skin_type':'smooth',
              'complexion':'fair'
              },
          'Location': {
              'city': 'london'
              }
          }
metadata['Exif.Photo.UserComment']=json.dumps(userdata)
metadata.write()

And to read it back:

import pprint
filename='/tmp/image.jpg'
metadata = pyexiv2.ImageMetadata(filename)
metadata.read()
userdata=json.loads(metadata['Exif.Photo.UserComment'].value)
pprint.pprint(userdata)

yields

{u'Category': u'Human',
 u'Location': {u'city': u'london'},
 u'Physical': {u'complexion': u'fair', u'skin_type': u'smooth'}}