def compute_targets(self, image_group, annotations_group):
        """ Compute target outputs for the network using images and their annotations.
        """
        # get the max image shape
        max_shape = tuple(max(image.shape[x] for image in image_group) for x in range(3))
        anchors   = self.generate_anchors(max_shape)

        batches = self.compute_anchor_targets(
            anchors,
            image_group,
            annotations_group,
            self.num_classes()
        )

        return list(batches)
 Compute target outputs for the network using images and their annotations.
        
keras_retinanet/preprocessing/generator.py