+ /** Do any useful work that would speed up a subsequent call to ::image().
+ * This method may be called in a different thread to image().
+ * @return log2 of any scaling down that will be applied to the image.
+ */
+ virtual int prepare (boost::optional<dcp::Size> = boost::optional<dcp::Size>()) const { return 0; }
+ virtual size_t memory_used () const = 0;