Automatic thumbnail format for Sorl - see http://gregbrown.co.nz/code/sorl-thumbnail-auto-format/

sorl_backends.py

#!/usr/bin/python

from sorl.thumbnail.conf import settings
from sorl.thumbnail.base import ThumbnailBackend


FORMAT_DICT = {
    'png': 'PNG',
    'jpeg': 'JPEG',
    'jpg': 'JPEG',
}


class AutoFormatBackend(ThumbnailBackend):
    def get_thumbnail(self, file_, geometry_string, **options):
        """
        Sets the format option (if not explicitly set) to the same format as the
        original file.
        """
        
        if not options.get('format'):
            ext = str(file_).split('.')[-1].lower()
            options['format'] = FORMAT_DICT.get(ext, settings.THUMBNAIL_FORMAT)
        
        return super(AutoFormatBackend, self).get_thumbnail(file_, geometry_string, **options)

usage.md

Usage

Place sorl_backends.py on your path, and add the following to your django settings.

THUMBNAIL_BACKEND = 'sorl_backends.AutoFormatBackend'

# this is still used if the format cannot be automatically inferred (eg for a .gif image)
THUMBNAIL_FORMAT = 'JPEG'