Custom sorl.thumbnail engine to perform specific crops

cropping_pil.engine.py

from sorl.thumbnail.engines import pil_engine
from sorl.thumbnail import parsers

class CropperEngine(pil_engine.Engine):
        """
        A custom sorl.thumbnail engine (using PIL) that first crops an image according to 4 pixel/percentage
        values in the source image, then scales that crop down to the size specified in the geometry. This is 
        in contrast to sorl.thumbnails default engine which first scales the image down to the specified geometry
        and applies the crop afterward. 
        """
	def create(self, image, geometry, options):
		image = self.orientation(image, geometry, options)
		image = self.colorspace(image, geometry, options)
		image = self.crop(image, geometry, options)
		image = self.scale(image, geometry, options)
		return image	
	
	def _crop_parse(self, crop, xy_image, xy_window):
                """ Taken from sorl.thumbnail.parsers.crop_parse and (roughly!) adapted here """
		crops = crop.split(' ')
		if len(crops) != 4:
			raise parsers.ThumbnailParseError('Unrecognized crop option: %s' % crop) 
		x1, y1, x2, y2 = crops	
			
		def get_offset(crop, epsilon):
			m = parsers.bgpos_pat.match(crop)
			if not m:
				raise parsers.ThumbnailParseError('Unrecognized crop option: %s' % crop) 
			value = int(m.group('value')) # we only take ints in the regexp
			unit = m.group('unit')
			if unit == '%':
				value = epsilon * value / 100.0	    
			return int(max(0, min(value, epsilon)))
		x1 = get_offset(x1, xy_image[0])
		y1 = get_offset(y1, xy_image[1])
		x2 = get_offset(x2, xy_image[0])
		y2 = get_offset(y2, xy_image[1])
		return x1, y1, x2, y2
		
	def crop(self, image, geometry, options):
		crop = options['crop']
		if not crop or crop == 'noop':
			return image
		x_image, y_image = self.get_image_size(image)
		x1,y1,x2,y2 = self._crop_parse(crop, (x_image, y_image), geometry)
		return self._crop(image, x1, y1, x2, y2)
		
	def _crop(self, image, x1, y1, x2, y2):
		return image.crop((x1, y1, x2, y2))

test.py

def get_crop():
    """
    An example of how to use the custom cropping engine inline.
    x1, y1, x2, y2 are percentage based values of where to apply the crop (they can also be pixel based)
    """
    img = ... // The full-size source image
    x1 = 10
    y1 = 10
    x2 = 80
    y2 = 50
    use_cropper = True

    if self.use_cropper:
        # Swap engines
        old = default.engine
        default.engine = CropperEngine()
        crop = get_thumbnail(
                    self.source, 
                    "%sx%s" % (crop_width, crop_height),
                    crop="%s%% %s%% %s%% %s%%" % (int(x1), int(y1), int(x2), int(y2)))
        # Replace normal engine
        default.engine = old
        return crop