Writing Sore Thumb filters

A Sore Thumb filter is a class derived from sorethumb.filters.Filter. To implement a working Filter class, you need only implement the __call__ method which accepts a PIL image and returns an image (the same object or a new instance), or None if no changes are to be made.

Here’s how the built-in ThumbnailFilter is implemented:

class ThumbnailFilter(Filter):

    def __init__(self, width, height):
        Filter.__init__(self, width, height)
        self.new_size = (width, height)

    def __call__(self, img):
        img = img.thumbnail(self.new_size, Image.ANTIALIAS)
        return img

If the filter accepts parameters in the constructor, it is important to call the base Filter.__init__ with the same parameters. This is because Sore Thumb uses the parameters of the filters to create a folder that is unique for that the combination of filters and parameters used.